The design and fabrication of an optical nanoelectromechanical switch based in a III-V material system

THE DESIGN AND FABRICATION OF AN OPTICAL

NANOELECTROMECHANICAL SWITCH BASED IN A III-V

MATERIAL SYSTEM

by

Reginald Bryant

Submitted to the Department of Electrical Engineering and Computer Science in Partial Fulfillment of the Requirements for the Degree of Masters of Science in

Electrical Engineering

at the

Massachusetts Institute of Technology

September 2001

C 2001 Reginald Bryant All rights reserved

The author hereby grants to MIT permission to reproduce and to distribute publicly paper and, electronic copies of this thesis document in whole or in part.

A u th o r: ... ... ,..c... Department of Electrical Engineering and Computer Science

August 17, 2001

A ccep ted by:... ... ... . ..

Leslie Kolodziejski

/ofessor

of Electrical Engineering IT hepSfupervisor

Certified by:... z... ... Arthur C. Smith Chairman, Department Committee on Graduate Students

MASSACH USETTS INSTITU TE

OF TECHNOLOGY

BARKER

THE DESIGN AND FABRICATION OF AN OPTICAL

NANOELECTROMECHANICAL SWITCH BASED IN A III-V

MATERIAL SYSTEM

by

Reginald Bryant

Submitted to the Department of Electrical Engineering and Computer Science on August 17, 2001 in Partial Fulfillment of the Requirements for the Degree of

Masters of Science in Electrical Engineering

ABSTRACT

As part of ongoing research efforts towards the fruition of all-optical signal processing, the possibility of a tunable wavelength filter using a photonic crystal is currently being explored. The examination, critical analysis and development of a nanoelectromechanical (NEM) optical switch will serve as a prelude to the successful realization of a wavelength tunable microcavity photonic crystal in a III-V material system. The NEM switch can be catalogued as an extremely useful component. The device will employ nanoelectromechanical systems (NEMS) technology. NEMS, evolving from a flourishing, yet still developing, microelectromechanical systems (MEMS) technology, incorporates some of the same founding physics principles of MEMS while offering a reduction in size. Also, the optoelectromechanical device will be fabricated in a material system not commonly used for electromechanical operation. Once developed, new fabrication sequences could lead to more complex nanoelectromechanical designs in a similar system. Lastly, since the optical switch will be fabricated in a material system commonly used for optical devices, it can be easily integrated into an optical network.

In this thesis the design and fabrication of the NEM optical switch structure will be discussed.

Thesis Supervisor: Leslie A. Kolodziejski

TABLE OF CONTENTS List of Figures ... 4 List of Equations ... 6 Acknowledgments ... 7 Chapter 1: Introduction ... 9 1.1 Motivation ... 10 1.2 Principle Results ... 12 1.3 Organization of Thesis ... 13

Chapter 2: Design Theory ... 14

2.1 Optical Theory ... 14

2.2 Electromechanical Theory ... 21

Chapter 3: Hardware Profile ... 32

3.1 Process Equipment ... 32

3.2 Quality Monitoring Equipment ... 37

Chapter 4: Device Fabrication ... 39

4.1 Process Overview ... 42

4.2 Process Detail...42

Chapter 5: Work In Progress ... 53

5.1 The Nanoelectromechanical Switch Design ... 53

5.2 Proposed Process Flow ... 53

Chapter 6: Conclusion ... 57

Bibliography ... 58

LIST OF FIGURES

Number Page

1. NEMS Switch Pictorial Diagram...8

2. NEMS Structure I: Optical Simulation Diagram... 17

3. NEMS Structure I: Transmission vs. Cantilever Deflection Graph... 18

4. NEMS Structure II: Optical Simulation Diagram... 19

5. NEMS Structure III: Optical Simulation Diagram ... 20

6. Square Dielectric Waveguide Dispersion Relation... 21

7. 1D Cantilever Lumped Model... 23

8. Exaggerated 2D Distributed Cantilever Model ... 24

9. 2D Distributed Cantilever Model Initial and Boundary C ondition D iagram ... 28

10. NEMS I and NEMS II voltage-length relation graph:... 29

11. NEMS I and NEMS II voltage-length relation graph... 29

12. MEMS I, II & III voltage-length relation graph... 30

13. MEMS IV, V, VI voltage-length relation graph ... 30

14. (14A) Photolitography (Waveguide and Cantilever)... 39

(1 4B) Liftoff (Nickel Mask) ... 39

(14C) RIE (S102 Hard Mask)... 39

(1 4D) Wet Etch (Nickel Removal)... 39

(14F) Photolithography (Trench Definition) ... 40

(14G ) R IE (A lG aA s)... 40

(1 4H ) W et Etch (Si02 Rem oval) ... 40

(141) Photolitohography and Liftoff (Front Side Contacts)... 40

(14J) Oxidation of AlGaAs/Cleaving/Lapback/ Backside Contact D efinition ... 40

(14K) Photolithography (Trench Redefinition)... 41

(14L) W et E tch and Release ... 41

15. (1 5A) SEM Image: S1813 Photoresist Waveguide Pattern... 44

(15B) SEM Image: S1813 Photoresist Waveguide Pattem... 44

16. (16A) SEM Image: S1813 Photoresist Waveguide Pattern... 45

(16B) SEM Image: S1813 Photoresist Waveguide Pattern... 45

17. (1 7A) SEM Im age: Liftoff Nickel Pattern ... 46

(17B) SEM Im age: Liftoff Nickel Pattern... 46

18. SEM Image: Etched Si02 with Metal Mask ... 47

19. (19A) SEM Image: S1813 Photoresist Mask/GaAs Etch... 48

(19B) SEM Image: S1813 Photoresist Mask/GaAs Etch... 48

20. (20A) SEM Image: S1813 Photoresist Mask/GaAs Etch... 50

(20B) SEM Image: S1813 Photoresist Mask/GaAs Etch... 50

21. (21A) SEM Image: S1813 Photoresist Mask/GaAs Etch... 51

LIST OF EQUATIONS

Number Page

1 M axw ell's E quations... 14

2 Mixed Dielectric Master Equation... 15

3 Pull-in Voltage Relation... 24

4 Pull-in G ap R elation... 24

5 2D Distributed Cantilever Model Differential Equation... 25

6 2D Distributed Cantilever Model Normalized Differential Equation ... 26

7 2D Distributed Cantilever Model Finite Difference Equation... 26

8 2D Distributed Cantilever Matrix Equation... 27

ACKNOWLEDGMENTS

I wish to express gratitude to Professors Kolodziejski, Joannopoulos, Smith and Ippen for their insight and expertise in the completion of this manuscript. In addition, I would also like to extend my sincere appreciation to my coworkers Alexei Erchak, Solomon Assefa, Gale Petrich, and Michelle Povinell for their assistance, answers, and active involvement to make this project a success. Also, I would like thank the faculty and staff of the Experimental Materials Laboratory and the Nanostructures Laboratory for their valuable input. Finally, I would like to extend my sincere apprecitation to Aisha who gave me endless support and encouragement.

IFigure 1: NEMS Switch Pictorial iaga I

CHAPTER 1: INTRODUCTION

This thesis will discuss some of the ideas necessary for employing a nanoelectromechanical system photonic band gap (NEMS PBG) filter by presenting a NEMS optical switch design. A nanoelectromechanical system involves the movement of nanometer sized materials by the electronic pressure from two oppositely charged plates. A photonic band gap crystal is a structure that incorporates periodic changes in a material's refractive index to control the propagation of photons. The photonic bandgap crystal inhibits electromagnetic waves within a certain frequency range from propagating. The symmetry of a PBG crystal can be broken to allow a single frequency to propagate through the device. This symmetry breaking effectively creates a cavity inside the crystal in order to confine a specific frequency or mode. The dependence upon cavity thickness on wavelength is an exponential relationship. So, precise control of cavity optical behavior within a PBG crystal is required in order to isolate a given wavelength. Due to chemical processing advances in material systems with a high refractive index contrast (i.e.

III-V

material system); high quality PBG filters are being engineered. The exponential dependence of cavity thickness on wavelength and excellent wavelength selectivity make PBG type filters a viable candidate for active tuning via cavity modulations [1,2].

Figure 1 represents a diagram of the NEMS optical switch's topology. The first section will be a fiber mode matched multimode waveguide designed for optimum optical signal transfer from fiber to chip. Section two will be a tapered waveguide that will adiabatically transfer the energy of the multimode waveguide to a single mode waveguide. Sections one and two will rest on an oxide of a lower index, that will aid in the confinement of the optical signal to the gallium arsenide waveguide

(III-V

material). The third section will be the electromagnetic actuated single mode waveguide that will be suspended in air. The fourth section will be a single moded receiving waveguide that will also rest on an oxide. The fifth section will transfer the signal to a waveguide width that will be suitable for output fiber coupling. The sixth section will be used to couple the optical signal out to a fiber. It can be readily resolved that such a planar switch scheme could easily be implemented in an array formation.

MOTIVATION

Everyone from industry to academia is looking to optics and photonics for solutions to a score of technological challenges. Everything ranging from packet switching, wavelength division multiplexing, filtering and signal routing to large-scale data storage, computing and systems integration has been thought of in terms of its predilection towards optical operation. One of the major themes associated with these efforts is to make active photonic and optical device. A simple contrast of current active electronic devices and the future idealized active optical/photonic device lends credits to the future viability of optical technology. Electronics are susceptible to operational noises, require heat sinks for large scaled integration, and will soon be unable to meet demand for

ever increasing high-speed applications. The ideal active optical/photonic device will not be susceptible to operational noises nor require heat sinks since, unlike electrons, photons are virtually massless and do not experience energy dissipation or momentum shifts due to particle-to-particle interactions and periodic columbic forces. Also, the only limit in operation speed is the speed of

light.

In an attempt to fashion practical active optical devices after the idealized optical device, a wide variety of methodologies are currently being explored. Some of these devices operate under acoustical modulation, non-linear perturbations, chemical/material interactions, and electrical/material interactions. However, most of these types of devices do not operate like idealized optical devices. For instance, the liquid crystal optical device consists of randomly arranged micro-liquid crystal droplets in a polymer film. Actuation is accomplished by applying an electric field to the material to promote constructive wave propagation through the material. The actuation speeds associated with switching between wavelengths are on the order of milliseconds. Liquid crystal filters have a respectable tuning range on the order of nanometers. However, this tuning range comes at the expense of high tuning voltages. Typical actuation voltages can reach as high as 300 volts [3]. Carrier based devices take advantage of a material's index of refraction variation due to free carrier injection. Changes in a material's refractive index are achieved by the plasma effect due to injected carriers. Although narrow bandwidths can be tuned at speeds in the nanosecond regime, the wavelength tuning range is on the order of angstroms. Also, such devices based on the plasma effect have the disadvantage of being strongly attenuated and exhibit large insertion losses while operating at high currents [4]. Thermo-optic devices exploit index changes of a material associated with joule

heating. Although the range over which wavelength tuning is wide, these types of devices suffer in switching speeds and power requirements. Thermal heating is typically a slow process and hard to control. These effects lead to a slow response in thermo-optical based devices. Typical tuning range per watt is 22. This means that one watt of power is dissipated into a material to get 22 nm of dynamic tuning range [5]. Electromechanical devices take advantage of the physical force generated by two oppositely charged plates. Such notable devices that use this technology include Fabry Perot filters, optical fiber switches, and movable mirrors. Typical silicon based MEMS devices are known to operate at high voltages and have a voltage-speed inverse relationship. However, these devices have ultra-low current and power requirements. Since a capacitive force interaction actuates the MEMS devices, virtually no current flows through these devices [6][7]. The NEMS-PBG filter possesses the same characteristics of MEMS devices while having advantages over their electromechanical operation. Unlike most other MEMS devices, the proposed NEMS PBG filter can achieve faster tuning speeds and lower actuation voltages than typical silicon based MEMS devices due to the reduction in size and the fact that the filter is based in a

III-V

material system, which is more susceptible to capacitive force and its reduction in dimension.

PRINCIPLE RESULTS

Presented in this thesis are the design and fabrication groundwork accomplishments necessary for the development of the NEMS switch. The results of empirical simulations for both electromechanical and optical operation of the NEMS switch are surveyed with logical conclusions drawn as to how a real NEMS device would perform. The intuition behind the simulations will also

be discussed. As a proof of concept, a scaled up MEMS switch was also simulated with the same tools in order to compare and contrast the predicted and actual data as well as aid in the future simulations of the NEMS device. Presented in this thesis is a synopsis of various processing steps that are required for the fabrication of the MEMS device. All of the results presented are directly related to the fabrication sequence associated with the NEMS device.

ORGANIZATION OF THESIS

This thesis is organized as follows. The second chapter is a two-part theoretical discussion about the optical operation of NEMS switch, and the electromechanical operation of the NEMS switch. In this theoretical analysis, various simulation results presented that illustrate the physics behind the device. In the third chapter, there is a discussion of the various pieces of equipment used in the fabrication of the device. This discussion focuses on the general operation of the equipment. The fourth chapter is an in-depth account of the processing issues surrounding the fabrication of a microelectromechanical switch. This section discusses the processing chemistry associated with the fabrication of this device, and further elaborates upon the operation of the equipment presented in chapter two. The fifth chapter muses over the progressive work of the nanoelectromechanical switch. This section will incorporate the results presented in the prior section to describe the issues associated with this device's fabrication. Chapter 3, chapter 4, and chapter 5 will be presented in a sequential manner, closely mirroring the actual process sequence of the test device and actual device. Chapter 6 will summarize the material that is present in this thesis.

CHAPTER 2: DESIGN THEORY

The switching scheme that this thesis will attempt to explore and improve upon is the mechanical tuning of an index guide rectangular slab. The basic idea behind the NEMS switch is to suspend a gallium arsenide waveguide in air and operate it with an actuation voltage applied to it. There is a coupled optical and electromechanical problem present. For the sake of clarity, it will be assumed in the first section that the waveguide can be actuated from a position that is aligned to a receiving waveguide to a position that is in intimate contact with a gallium arsenide substrate. The electromechanical issues that govern the waveguide's actuation will be addressed in the following section.

OPTICAL THEORY [8,9,10]

In collaboration with Prof.

Joannopoulos'

research group at MIT, 3D energy modeling simulations were carried out to determine the relationship between optical transmission and the deflection of the cantilever.

1 8B

V-B=O VxE +---= _C 0

a

V -D = 4rfp

VxH-I--=-J

Two methods were used in the simulation of the optical switch structure. The two methods were frequency-domain and time-domain simulations. Both methods solves for full vectorial solutions to Maxwell's equations (1) to extract relevant information about the structure. The frequency-domain method solves Maxwell's equations to resolve the eigenstates and eigenvalues of the switch's electromagnetic (EM) modes by using a plane wave basis with the appropriate boundary conditions. The time-domain method examines the structure by using initial conditions and a time evolving propagating signal. This method uses the time evolving signal to excite the modes in the waveguide. The frequency-domain method examines the propagation of electromagnetic fields within a given structure by solving the master equation given by:

V X

V

x

H(r)

=

H(r)

(2)

(e(r)

C

)

This eigenvalue equation represents a condense version of Maxwell's equations where the field intensities are small enough to be considered linear; and the material system that is being examined is nonmagnetic, linear, isotropic, and low-loss. This equation determines the radial macroscopic magnetic field H(r) for a given switch's geometry, c(r), at a specific frequency, o. From this information, the electric field and the allowable modes for a given structure can be determined. The time-domain method is used to examine temporal characteristics of the device such as transmission and resonance decay-time. The time-domain method usually determines the effect that peripheral components (i.e. substrate, and oxide) have on the switch device.

The simulation results of three NEMS structures are presented. Each NEMS structure was

electromagnetically analyzed in a 3D program. The structures were normalized to a length a and

situated in a normalized cell filled with a refractive indexed material of one (i.e. air). The length of

these cells were set in such a manner as to make the transversing EM fields independent of that

dimension. The cross section of the normalized cells was chosen to obtain a realistic account of

how the device would function. The flux plane of the output waveguides in the NEMS plane was

chosen to be slightly larger than the cross section of the output waveguide in order to measure

optical fields that are weakly confined to the waveguide. A typical flux measurement is taken at a

(1/5)a

Flux Plane

4

g

(2/5)ci

9C Input Waveguide Output Waveguide

4oc

6o

Figure 2: NEMS Switch Structurc I: Optical Simulation Diagram

The first NEMS structure (NEMS I) that was simulated is depicted in figure 2. Two waveguides of a-normalized width and (2/5)L-normalized thickness were initially aligned with a

(1/5)cc-normalized separation for the on state. The input waveguide and output waveguide are both

separated from the substrate by a 4a-normalized height. For the off state, when the two waveguides are misaligned, the input waveguide becomes lowered into intimate contact with the substrate. The index of refraction of the waveguides and the substrate was set at 3.37 to reflect the material properties of gallium arsenide operated at a wavelength of 1.55 nm. Both the on and off state were simulated within a normalized cell of roughly 9c-height and 6a- width. A time-domain simulation was carried out to determine the on/off contrast. The structure was excited with normalized frequencies from 0.3c/a -0.34c/a, where c is the speed of light and a is unit of measure associated with the structure. For this simulation cx is taken to be the width of the waveguides. Figure 3 illustrates the normalized optical transmission as a function of the waveguide offset. Although a 10

-~ .- lip. --

-dB contrast could be obtained, usable optical switches require larger on/off contrast: a contrast of 40-60 dB. 0.6 Waveguide/Cantilever Separation= 100 nm 0.5 4 Gap =26M nm 0.4 - GaAs Caniikwr

o

Thickness = 200 nm In

c

0.3 - Width = 500 nm M 4D Output Wamguide

E

oThickness = 200 nm n 5 0.2 - Width =200 nm .- 0.1 -0 0 0.2 0.4 0.6 0.8 1 1.2 1.4

offset (micron)

Figure 3: NEMS Structure I: Transmission vs. Cantilever Deflection Graph

The next NEMS structure (NEMS II) to be simulated is depicted in figure 4. Again, two waveguides of a-normalized width and (2/5)a-normalized thickness were initially aligned with

a (1/5)cx-normalized separation for the on state. The input waveguide is separated from the

substrate by 6a-normalized height while the output waveguide is separated from the substrate by a

4a-normalized height. For the off state, when the two waveguides are misaligned, the input waveguide is aligned to a step discontinuity in the substrate. The index of refraction of the waveguides and the substrate was set at 3.37 to reflect the material properties of gallium arsenide operated at a wavelength of 1.55 nm. Both the on and off state were simulated within a normalized cell of roughly 9a-height and 6a- width. The normalized flux from the output waveguide in the on

state was 0.95 with a 0.01 deviation for frequencies from 0.3c/a - 0.34c/A. The normalized flux

from the output waveguide in the off state was 0.0005 with a 0.0001 deviation within a 0.3c/a -0.315 c/cc frequency range. For x= 500 nanometers, the NEMS structure pictured in figure 4 is simulated to operate at 1.55 micrometers with an on/off contrast of roughly 63 dB. Although these simulated results seemed promising, such a NEMS structure could lead to more complex fabrication sequences than the first proposed design scheme.

(1/5)a

4

1*-9a

A

6ax Input Waveguide

6cc

Output Waveguide

t

4a

Figure 4: NEMS Switch Stnicture II: Optical Simulation Diagram

Flux Plane

(2/5)a

V

(1/2)cc

Li11

put Waveguide _OUtPi

Optical Sink Waveguide

-+

IL

(1/2)a

Flux Plane

Optim

Low oss-talk distance

Figure 5: NEMS Switch Structure III: Optical Simulation Diagram

The NEMS structure that is currently being simulated (NEMS III) is depicted in figure 5. Two waveguides of

a

x a normalized cross sectional area are aligned with a (1/2)a normalized separation. An additional waveguide of the same dimensions sits parallel to the output waveguide displaced by a perpendicular distance for optimum low cross-talk. The index of refraction of all three waveguides is set at 3.37. The normalized cell size of the simulation is about 11C-normalized

height and 16c-normalized width. It is predicted that such a simulation should yield on/off contrasts on the order of the second NEMS structure described previously. The advantage of such a design scheme lies within the design fabrication sequence of the device. The processing of this structure is of a more planar nature as opposed to the previous design that was more of a vertical nature. Figure 6 depicts the dispersion relation of the square waveguides present in this design

L

11cc

16cc

scheme. The guided modes are shown as nonlinear lines, or bands. The continuum of radiative modes is shown as the shaded region to the left of the bands; these modes will not be guided.

0.4 0.35- 0-3-0 02 5- S0-2- 0.15- 0.1-0.05

0-7

1-

I

-0 0.050-1 0.15 0.2 0.25 0 3 0.35 0.4 0.45 0.5 wave vectorj2nr/a)

Figure 6: Square Dielectric Waveguide Dispersion Relation Graph

ELECTROMECHANICAL THEORY [11,12]

Figure 1 illustrates the optical switch with a front side contact for horizontal deflection and a backside substrate contact for vertical deflection. In order for the optical switch to have a strong on/off contrast, the cantilever must be deflected by an appreciable amount. However, if this requisite deflection distance exists outside the operation regime of the cantilever, the device will malfunction. The major cause of the malfunction is linked to the thin layer of water molecules accumulated on the electrode surfaces during operation, which fuses the electrodes together by adhesive forces[12]. The deflection regime of the cantilever is directly related to the height, or

separation, from the substrate. The greater the separation distances between the cantilever and the substrate, the greater the deflection regime of the cantilever. The voltage required to deflect the cantilever is inversely related to the cantilever's length and directly related to the separation from the substrate. If the device is long and close to the substrate, the deflection voltage will be low. Also, the cantilever must have dimensions that are admissible for release during the drying process of fabrication. When the device dries, the cantilever and the substrate could come into contact with each other due to an adhesion force. If this adhesion force exceeds the restoring elastic forces of the cantilever, it could result in an irreversible adhesion of the surfaces. The critical length of the cantilever determines the point at which the adhesion force exceeds the restoration force. The critical length follows a direct relationship with the height of the cantilever and an inverse relationship with the thickness. Various release schemes will be explored in chapter 4. The cantilever must be deflected from alignment with the receiving waveguide as much as required to allow for maximum signal attenuation. For the vertical displacement case, the cantilever must be fabricated at a height that permits such deflections. This height will set the critical length of the cantilever and the critical length will determine the amount of voltage that must be applied to the structure for deflection. There will be several design tradeoffs associated with the inception of this device. For the horizontal displacement case, the cantilever's deflection distance is set by the topology. Once optical simulations resolve the rough dimensions of the optical switch, the mechanical portion of the design can be developed.

The basic concept of electrostatic pull-in can be intuitively understood by first examining the one dimensional spring model (Figure 7) where the cantilever is approximated by two conductive plates biased by a voltage, V.

Keff

Pex

+

* 9 P, V

Figure 7: 1D Lumped Cantilever Model

The two conductive plates are initially separated by a distance g.. The only degree of freedom for this illustration is the gap spacing, g, which is a function of horizontal displacement. The applied voltage V becomes a distributed electrostatic load that creates an electrostatic pressure, P. = e.V2_/2g2_{. After perturbation by the electrostatic pressure, Pe, the two plates are then separated by a}

smaller distance g. In the small-deflection regime, the effective spring constant, Kef, is derived from the requisite amount of pressure, P.,, required to lower the top plate to the lower plate transversing the initial distance g. Therefore, the spring constant is defined as: Kf = Pt,,/g. Pex, represents the built in pressure that the structure exerts upward. As the voltage increases, the gap decreases until

an instability condition is reached. Using the pressure-balance equation, P(g) = K., (g. - g) + Pnet

equilibrium,

dP(g)/dg

<0, the critical pull-in voltage, Vp1 (3), and the critical pull-in gap, gpj (4),where the instability condition is met can be derived as:

8 Kef go + ex VP = 2c P1 27ev g9p = ± +

3

Kj)

(3)

(4)

The normalized relationship between gap distance and voltage is nonlinear. The stable region of operation for this device exists for a gap greater than 2/3 of the initial position. Instability is reached when the gap is lowered below the 2/3 of its initial position. At this point, the cantilever snaps down to the bottom electrode.

The two-dimensional distributed model can be extended from the one-dimensional model

Tip

g(x

presented. The 2D model accounts for the fact that the actual structure has a non-igid, position-dependent gap as illustrated in Figure 8. The position-position-dependent cantilever is discretely divided into smaller 1D cantilevers. Each follows the same constitutive relationships described previously with natural boundary conditions. In Figure 8, V represents the voltage applied to the structure; g. represents the initial gap existing between the top and bottom electrodes without an electrical bias; g(x) represents the position dependent bending of the cantilever under an electrical bias; the tip represents the last 1D cantilever from the base of the structure.

Much like the one-dimensional case, a similar pull-in voltage, Vp1, verses pull-in gap, gp1, relationship

can be derived using a numerical fiite-difference method, which solves the 2D differential equation for the cantilever structure in the small deflection regime. Since the fabricated cantilever will be actuated on a small vertical deflection scale in comparison to the lateral dimensions, this will prove to be a reasonable approximation.

E I

g

=6 -

60V2W

I+

0.65-

(5)

OXg

2

g 2

E

(wide beams)

E is Young's Modulus,

E = (1- v2) v is Poisson's Ratio,

E

(narrow beams)

s. is permittivity of vacuum,

t is the beam thickness,

__ 3w is the beam width,

2

g is the gap,

The 2D differential equation (5) takes into account an electrostatic distributed load with a position-dependent gap plus a fringing field correction in the case of beams. By representing the force per unit area as that of a locally parallel plate capacitor, this model assumes only small-angle bending, and neglects any non-uniformity in electric field due to curvature. Equation (5) resolved into a normalized equation (6) shown below.

g

g"=

a+bg

(6)

a=- 6,

2EI

b=--.65

62

2EI

By using variational theory, the gap displacement denoted as g can be substituted by (g+s), where P is the small variation about an initial gap g. After equation (6) is

2h g" bh+

ah/

4

_bh

4 ₄

.,-2

_-,

+s, 6i6+

' 2 -4ein

+

i = 2 + -h g "

gI

g _i

g

9 i

(7)

substituted with (g+s) and the finite difference form of the derivatives, equation (7) can be derived. Recasting this equation into a matrix equation yields equation (8).

* 0 -. *- 1 1 -4 6+ 2h'4 g" bh2 gi gi _ 0 1 -4 0 0 1

0

0

0

0

0

0

0 0 1 -4 +

2h4g"" bh~

_ i+1 gi+1 _ -4 1 0 0 0 0 1 -4 -4 1 0 0 0 0 0 0 0 -4 1 0 0 0 0 ci2 E si 1+1 8.+2

ah

4 _bh4 2 + --g, g, ah4 _bh4

-4 f 2 +g,+ ,+₁ g,+₁ (8)

The Matlab script, used in the beam simulation, solves this equation iteratively until the

s's

are minimized. The specific boundary and initial conditions for the cantilever is depicted in figure 9. The series of graphs in Figures 9A - 9G shows the variable cantilever length dependence of the pull-in voltage. For the horizontal deflection case, it is assumed that the behavior of the cantilever closely mirrors that of the vertical deflection. So, the same Matlab script was used to analyze these structures.

g 1 2 { 4 x=0 H = L/(d-1) Step Size Initial Conditions:

Uniform gap distance along cantilever:

Boundary Conditions:

Cantilever fixed at one end:

There is no curvature of cantilever at fixed end: The cantilever has no momentum at free end: The cantilever has no shearing at free end:

N-4 N-3 N 2 N-1 N * x=L g(x,t=0) = g. g(x=0,t) =g g'(x=0,t) =g,) g"(x=L,t) =0 g.'(x=L) = 0 g3 =g g2 = g4 gN-3 -2gN2 + gN-1 0 -gN-4 +2gN-3 -2g'N + = 0

Figure 9: 2D Distributed Cantilever Model Initial and Boundary Condition Diagram

g(x)

--- --- --- - --- - - - - N M

l

-:NEMS 11 PUL L-IN-1.3053:

--- -- ---- ----.- .--- --- - - --mi 2 30 35 40 *Mbt* Caw4orLtw4wnf) 45 50 Length Cantilever Width w) Pull in Displaceme Vertical Electrode Figure 10: NEMS I and NEMS II voltage-length relation graph

Figure 10 shows the electromechanical penalty of employing the NEMS II design; a 200 nm thick, 500 nm wide cantilever with an electrode separation of 3000 nm; as opposed to NEMS I design; a 200 nm thick, 500 nm wide cantilever with an electrode separation of 2000 nm. Figure 11 shows the advantage of using NEMS III design; a 200 nm thick, 200 nm wide cantilevers with an

GMllwnrasnId Canhlevr Sam2O Model :: -: ::--- --- - --- --- --- :--- --- - - NEMS I (Horzontal Deflection)

:::--:::---- NEMS III

NEMSI (H-orizzoatalDe *ction):!LL.k-IN O.

0.8269-NEMS I .0 ULL-IN 0.834 --- --- -- - ... --- -- -- --- - --- -- --- --- ---

--a

t 25$ V 35 40 45 5 Cantilever Gap (g) Pull.i_in e*Displacemnent Thickness (t) Horizontal Electrode , Lngh

0 Figure 11: NEMS I and NEMS Im voltage-length relation graph

electrode separation of 2000 nm; as opposed to horizontally deflecting the NEMS I structure.

A

comparison of the electromechanical response of the vertical deflection in figure 10 to horizontal deflection in figure 11 suggests that it requires less voltage to deflect NEMS III structure from left to right than it takes for NEMS I structure to be deflected up and down. There have even been

I

reports showing that a curved electrode in the horizontal deflection setup can further reduce the voltage required for actuation.

'r MEMS -- ----

MEMS---NENS 1114 MEMS-- SLL II --- ----] jimjjjj. jpjj MEMSV '

MSLULli 445 MEMS IIIvau.mn~m- MEMS M

MEMS III: RULL IN 41.5456 HENS VI: PULLINq 1.5456

.. .. .. .. -- -- - I

-Figure 12: MEMS 1, 11, & III voltage-length relation graph Figure 13: MEMS IV, V, & VI voltage-length relation graph

Data points from MEMS structures I-VI will be used to ascertain the mechanical properties of GaAs, which will aid in accurately predicting the behavior of the NEMS structures. Figure 12 represents the simulated pull-in voltage dependence of the first set of scaled-up MEMS structures. MEMS I is a 200 nm thick, 1000 nm wide structure with an electrode separation of 2500 nm, which is deflected vertically. MEMS II is a 200 nm thick, 1000 nm wide structure with an electrode separation of 2000 nm, which is deflected horizontally. MEMS III is a 200 nm thick, 1000 nrm wide structure with an electrode separation of 4000 nm, which is deflected horizontally. These structures will be fabricated upon the same sample with various lengths. Figure 13 represents the simulated

pulling voltage dependence of the second set of scaled-up MEMS structures. MEMS IV is a 200 nm thick, 2000 nm wide structure with an electrode separation of 2500 nm, which is deflected vertically. MEMS V is a 200 nm thick, 2000 nm wide structure with an electrode separation of 2000 nm,

which is deflected horizontally. MEMS VI is a 200 nm thick, 1000 nm wide structure with an

electrode separation of 4000 nm, which is deflected horizontally. These structures will also be

fabricated upon the same sample with various lengths.

With an initial survey of the data presented in the optical theory section and the electromechanical

theory section, we can see that maximum on/off contrasts cannot be achieved solely due to the

pull-in gap condition. The deflection of the cantilever cannot go beyond 920 nm, which limits the

transmission coefficient of the "off' state. In order to achieve excellent on/off contrasts and

reasonable actuation voltages, the gap spacing and cantilever length must be varied. Calculations for

CHAPTER 3: HARDWARE PROFILE

PROCESS EQUIPMENT [1,13]

The Riber Instruments S.A. 32P gas source molecular beam epitaxy (GSMBE) growth system is the machine that was chosen to grow the heterostructures to be processed. The molecular beams are formed from themal evaporated Group III elements: gallium and aluminum. Group V elements, arsenic and phosphor, are provided by thermal cracking in their gaseous hydride forms. The GSMBE system is always under ultrahigh vacuum conditions (<IxiG'1 torr) and is equipped with effusion cells containing extremely pure material sources, which yield high-purity heterostructure layers. With growth rates at a few anstroms per second and beam switch speeds that operate at a fraction of a second, the heterostructures can be made with abrupt step interfaces. The Riber system consists of five major chambers: the introduction chamber, the transfer chamber, the buffer chamber, the Auger electron spectroscopy chamber and the reactor chamber(s). A robotic arm that extends from the transfer chamber translates the epi-ready wafers between the various chambers. The introduction chamber is the loading area where the wafer is placed in the system. This is the only chamber that is not constantly maintained under ultra-high vacuum. After the wafer is loaded, it is then baked at 210'C to remove moisture on the substrate. The wafer is then robotically maneuvered through the transfer chamber into the buffer chamber. The buffer chamber holds the sample until it is ready to be transferred into the growth chamber via a transfer rod. The substrate manipulator allows the sample to be rotated in order to face the material effusion cells. After the sample is then transferred to the Ill-V reactor chamber, it is heated up to the growth

temperature. The native oxide on the substrate surface is desorbed, which occurs at 5800C. During the growth of the heterostructure a reflection high-energy electron diffraction (RHEED) system monitors the growth. After the growth, the wafer is cycled back through the chambers in reverse order. The Auger electron spectroscopy chamber is used to examine the chemical make-up of the surface following the growth.

The Plasmatherm Inc. Series 700 Wafer'Batch reactive ion etching/plasma enhanced chemical vapor deposition (RIE/PECVD) system is a two-chamber stainless steel system which has one chamber dedicated to deposition processes and the other used for etching processes. The Plasmatherm system has three components: the radio-frequency (RF) power system, the vacuum system, and the gas flow control system, that are controlled by recipes programmed in the onboard computer interface. In each of the chambers, the RF power system is used to ignite plasmas, which supplies the necessary energy for chemical and physical reactions to occur. Plasmas formed in the chambers are driven at 13.56 MHz with power ranging from 10-50 W for PECVD processes and 100-500W for RIE processes. A network of capacitors and inductors controls as well as isolates the RF power supply from the chamber. The Plasmatherm system allows users to adjust the RF signal by either adjusting the capacitive load of the network or tuning the RF signal itself. The pumping system for the Plasmatherm has two pumps: roughing pump, and turbo pump. The mechanical pump consists of two pumps: roughing pump and blower. The roughing pump is used to pump the chamber from atmosphere pressures to 2 Torr. The blower is used to assist the roughing pump to achieve pressure ranges from 2 Torr- 20 mTorr. The turbo pump is used to reach low pressures ranging from 20 mTorr - 1x106 _{Torr. Pressures in higher regimes are monitored by a thermal}

pressure gauge, which convectively measures pressure by the heat loss by a filament in a

voltage-controlled circuit. The system uses baratron capacitace monometer to monitor the chamber

pressure. This is the pressure that is displayed on the computer monitor. The thermal pressure

gauges are just used in the the pumping system to control the opening and closing of pressure

valves. Pressures in the low-pressure regime are monitored with an ion gauge, which determines the

pressure by the thermionic emission of electrons from a filament in a voltage controlled circuit.

Each chamber has four separate mass flow controlled (MFC) gas lines, one purge gas line and one

vent line. The first MFC line, or channel, in the RIE chamber has four gases: boron trichloride

(BCb3), silane (SF), carbon tetrafluoride (CF4), and halocarbon 23 CHF3. The second channel of the

RIE chamber has two gases: silicon tetrachloride (SiCl4), and methane (CH4). The third channel of

the RIE chamber has chlorine (Cl2), and hydrogen (H2) as its two input gases. The fourth MFC line

has three gases: helium (He), argon (Ar), and oxygen (02). The purge lines in both the RIE and

PECVD chambers are primarily used to purge the process gases from the chamber. In the RIE chamber, the purge line is solely dedicated to nitrogen while the PECVD chamber has its purge line

shared by a CF4/0 2 mixture and Nitrogen. It should be noted that the MFC's are not accurately

calibrated for the respective gases that flow through them. For instance, a 5% SiH4 95% He gas

mixture passes through a MFC that is calibrated for 2% SiH4 98% N2 gas mixture. Such a caveat

does not necessarily affect reproducibility since once the relationship between gas flow and the MFC

reading is established, successive runs will yield reproducible results, albeit not accurate. The system

is typically defaulted to idle under low pressure. Gas lines, which are used during a recipe, are always

atmosphere pressure in order to load the sample(s). During a process recipe, the chambers are initially pumped down to a base pressure of 5x10-5 before any process gases are introduced. At the end of the process run, the chambers are again vented to atmosphere in order to remove the sample(s). Finally, the gas lines are again evacuated with gas source cylinders closed.

Photolithography is a multi-step process that requires the use of a spinner, a few ovens/hotplates, aligner, and glass beakers. Each of these steps is performed under a controlled setting to retain reproducibility. The spinner is used to apply a uniformly thick layer of photoresist on a sample. A sample is held to the spinner chuck by vacuum while drops of resist is statically applied to the surface. The spinner then spins the resist to equally distribute it across the sample's surface. The resist thickness is set primarily by the resist viscosity and secondarily by the spinner's rotational speed. Prior to spin coating, a singeing oven or hotplate is used to remove moisture from the sample surface by evaporation. Singeing aids in the adhesion of applied resist films. Following spin coating, soft baking densifies the applied resists. In the event that the resist is used as an etch mask, postbaking is required to stabilize and harden the resist after it has been developed. This step is usually skipped in the event the resist will be used for lift-off. Alignment was done on the Karl Suss MJB-3 aligner. Alignment is done manually with microscope optics and stages. The mask with the desired pattern is aligned to the sample. The sample and mask are placed in hard contact. The sample is then exposed with a narrow ultraviolet light band of wavelengths centered around 320 nm, 365 nm or 405 nm. After exposure, the sample is then developed and rinsed with a manual method of dipping baskets and beakers. One beaker contains a dilute developer corresponding to the specific photoresist that was used and the other beaker contains deionized (DI) water used for

rinsing the development agent from the sample. Both development time and exposure time is critical to the outcome of the sample.

The electron beam evaporator deposits metals on sample surfaces by heating a metal filled crucible with a tightly focused electron beam. The beam sweeps out a scalable area upon the metal surface. Samples are loaded onto a metal stage with clips. These loaded metal stages are then placed above the metal filled crucible to be evaporated. While the e-beam evaporation chamber is pumped to the process pressure and the electron beam is ramped to the process intensity, a shutter isolates the loaded samples from being deposited upon. Once the e-beam and the chamber pressure become stabilized, the shutter opens and metal vapors condense on the samples forming a thin film.

Liftoff is accomplished in a two-beaker process. One beaker contains a specific reacting agent that removes the desired resist. This beaker can sometimes be heated and/or placed in an ultrasonic bath to aid in the resist removal process. After the sample is introduced and removed from this solution, it is then placed in a rinsing liquid to remove the residue of the reacting agent.

A tube furnace is used to carry out the oxidation process. The setup consists of a carrier gas source, H20 gas source and the oxidation furnace. The oxidation chamber is a quartz tube partially

surrounded by a single-zone furnace. The oxidation furnace is heated to the oxidation temperature with an empty sample boat, sample holder. After the furnace's temperature stabilizes, the sample boat is pulled from the furnace and then loaded with a sample. The loaded sample boat is immediately pushed back into the furnace. The oxidation furnace is again stabilized to the oxidation temperature with the loaded sample. During this stabilization period, the furnace is continuously purged with N2to ensure that the growth chamber remains dry. After the furnace is stabilized for 15

minutes, steam is flown through the furnace by nitrogen flowing through a bubbler. At the end of

the oxidation, the steam flow is discontinued and dry N2 is allowed to flow through the system. The

chamber is cooled down to 2000C before the sample is removed from the system.

Wet etching is accomplished in a two-stage solution emerging sequence. Two Teflon dishes

are prepared with a reactive species etchant solution in one and DI water in the other.

The plasma asher is a PECVD/RIE machine that is dedicated to etching photoresist. The asher

chamber is preloaded with a sample to be stripped and an oxygen and helium gas mixture is allowed

to flow through the chamber. This gas mixture is then sparked with an RF signal to produce plasma

that in turn etches the sample free of resist.

QUALITY MONITORING EQUIPMENT [10]

The thickness and refractive index of very thin transparent films is measured by an

ellipsometer. Deposition rates and growth rates of films can be ascertained using the ellipsometer.

The ellipsometer's curve-fitting algorithm works under the premise that the material under

investigation is of high quality. The quality of the film, i.e. the density of the film, can be assessed by

second order means like visually measuring the thickness of the film in a scanning electron

microscope (SEM) and then using this thickness to curve fit the film's index of refraction with the

ellipsometer. Since the ellipsometer uses the index of refraction of high quality films, data fits that

deviate widely from preset values can be considered low quality films.

One of the fundamental quality monitoring pieces of equipment is the standard microscope.

patterning alignment, it also helped in troubleshooting the parameters associated with photolithography like exposure time, development time, and resist thickness.

In order to resolve more detailed visual information about micro/nano structures, a scanning electron microscope (SEM) was used. The SEM uses a focused beam of electrons to scan across a sample in order to resolve its image. The beam of electrons is focused by a series of magnetic field producing coils. These coils essentially act as lenses. Once the electrons from the beam strike the sample's surface, they backscatter in a fashion that illustrates the features on the sample. The intensity of backscattered electrons versus position are displayed on a CRT screen. In operation mode, the SEM system is kept at high vacuum while operating. The electron microscope was used to inspect the vertical profile of patterned features. One of the patterned samples was cleaved perpendicular to the features of interest, making an atomically smooth cross section that aided in analyzing the sample's profile. The cleaved sample was then placed upon a wedge shaped holder, and tilted 20 degrees. In order to prevent charging frm the electron beam, samples must be coated with a thin layer of conductive material. This conductive material is usually applied to the surface of a sample through a cold plasma process that doesn't destroy the sample's contours.

CHAPTER 4: DEVICE FABRICATION

Figure 14A:Photolithography(Waveguides and Cantilever) Figure 14B:Lift-off (Nickel Etch Mask)

Figure 14C: RIE (SiO2Hard Mask) _{Figure 14D: Wet Etch (Nickel Removal)}

C tact Metal Photoresist SiO2 A

I

.GaAs InGaP GaAs substrate -

39

-t?~1 I

Figure 14F: Photolithography(rrench Definition)

Figure 14H: Wet Etch (SiO2 Removal)

C Cotact Metal Photoresist Sic)2 GaAs A1. Al..1GaAs InGaP GaAs substrate

Figure 14G: RIE (AlGaAs) I

Figure 14K Photolithography (trench Redefinition)

Figure 14L: Wet Etch and Release

O (:ontact Metal Ni Photoresist SiO2 GaAs AlOAli GaAs TnGaP GaAs substrate

PROCESS OVERVIEW

The first step in the development of the NEMS switch was first to design and fabricate a scaled up model of the switch. By doing so, basic fabrication issues such as release methods, etching times, and liftoff techniques can be worked out using only optical techniques with fast turn around periods.

The first issue to be resolved was a comparable scale up of the switch's thickness, length, and substrate separation that would give some insight on what can be expected during the fabrication of a nanometer sized switch. Since the design goal of the nanometer switch calls for a length around 20 microns, a width of 500 nanometers, and a thickness of 200 nanometers, a 1 and 2 micron switch needs to have these same scaled parameters.

PROCESS DETAIL [1, 13, 14, 15]

Figure 14A illustrates the grown heterostructure on an n-doped gallium arsenide substrate. The heterostructure consisted of the indium gallium phosphide (InGaP), aluminum gallium arsenide (AlGaAs), and gallium arsenide (GaAs). The n-doped GaAs substrate will serve as an electronically active material, that acts as an electrode. The InGaP layer has a few functions. One of the functions will be to act as an etch stop layer during the wet etch of AlGaAs, which will be described later in the processing chain of events. It also serves as an insulating layer, separating the ground electrode from the cantilever once it is electrostatically pulled onto the substrate. The AlGaAs layer will be the

sacrificial material used to release the cantilever. Once the AlGaAs layer is oxidized to form aluminum oxide (ALO) it acts as an optical isolation layer for the waveguides that rest on top of it. The GaAs layer was patterned to form the coupling waveguides and switch. This GaAs was grown to be p-type.

Figure 14A illustrates a plasma enhanced chemical vapor deposited (PECVD) layer of silicon dioxide (SiO2) upon the grown heterostructure. Since SiO2 is a strong material that can

withstand heavy ion bombardment, it is used as a hard mask for reactive ion etching profiles in the underlying GaAs and AlGaAs layers. Equation (9) shows the chemical reaction that takes place.

SiH

4

+

2N

2

0 - SiO

2

+2H

2

+4N

2

(9)

SiH4 and N20 are combined to form SiO2 with the byproducts of hydrogen (H2) and nitrogen (N2).

The SiO2 was deposited at 900 mTorr with 400 sccm of SiH4 and 550 sccm of N20 with a 300 watt

plasma at 13.56MHz. The substrate was heated to 300'C to provide the energy for the chemical reaction upon the sample's surface. Details of the PECVD process can be found in the appendix. By measuring the thickness of the SiO2 film at various times, a deposition rate of 14.48 nm/min was

ascertained. One of the major benefits of PECVD is its built in compressive stress, which reduces the tendency of the SiO2 films from developing pinholes and cracking during deposition.

Figure 15A illustrates the photolithography definition of the waveguide and switch topology. The main objective of this photolithography step is to have an inverted nickel mask after the resist is lifted off. Due to the higher resolution possible, a positive resist was used instead of a negative resist

Figure 15A: SEM image: S1813 Photoresist Waveguide Pattem Figure 15B: SEM image: S1813 Photoresist Waveguide

Patten

for lift-off. The sample was baked at 200'C for at least 30 minutes to separate water residue from the sample surface. A thin layer of Hexamethyldisilazane (HMDS) is spun on the sample at 4000 revolutions per minute for 30 seconds to improve the adhesion of the photoresist upon the SiO2

surface. Adhesion is accomplished by the strong bond formed between the silylation portion of HMDS and the oxide surface while the methyls bond with the photoresist. Shipley's 1813 photoresist was spun on the sample in the same manner as the HMDS, at 4000 revolutions per minute for 30 seconds, yielding a 1.3-micron thick layer. After the spinning of resist, the sample experienced edge beading. The photoresist that accumulates upon the edges of the sample can be up to 20-30 times thicker than that on the rest of the chip. This thickness could lead to poor sample/mask contact during exposure. The application of acetone to the edges of the sample alleviated this problem. The sample was soft-baked at 90C for 30 minutes to densify and remove the solvents in the resist. The sample was aligned to a chrome on glass optical mask and exposed for 21 seconds in hard contact with the mask. After the exposure, the sample was developed in a MF-319 developer for 45 seconds and then immediately submerged into a DI water rinse for 45 seconds.

Since this photoresist will be used for liftoff, the sample was not postbaked. The optical microscope and the scanning electron microscope (SEM) were used to examine the quality of the photolithography process. Details of the photolithography process can be found in the appendix.

Figure 16A- SEM image: S1813 Photoresist Waveguide Pattem Figure 16B: SEM image: S1813 Photoresist Waveguide Pattem

Figures 16A and 16B shows a photolithography defined 2 micron trench. After a series of trials, it was concluded that Shipley's 1813 was not able to resolve one micron images due to the limitations of the contact aligner. Currently, photolithography using Shipley's 1805 positive resist, a thinner resist, is being developed to resolve one micron images. A 20-nanometer layer of nickel was deposited upon the sample in order to invert the resist pattern by a liftoff process.

Liftoff is accomplished by first spraying the sample with acetone several times. Then the sample is placed in a Teflon dish of acetone for a five minute soak. The Teflon dish is then placed in an ultrasonic bath to further aid in lifting off the resist. After the sample is taken from the

acetone solution, it is then placed into a methanol solution for a few minutes. The sample is

extracted from the methanol solution and then sprayed with methanol. The sample was then

inspected under the optical microscope and scanning electron microscope. Details of the nickel

liftoff process can be found in the appendix. Figure 17A and 17B shows the results of a 2-micron

nickel inverted waveguide.

Figure 17A- SEM image: Liftoff Nickel Pattem Figure 17B: SEM image: Liftoff Nickel Pattern

Ideally the nickel should not have the jagged edges. The reason for the jagged edges is due to the photoresist's positive sloping walls and wavy profile as seen in figures 15B. There are several solutions that are currently being explored to achieve better lift-off patterning. Surface treatments such as chlorobenzene and toulene can give the photoresist negative sloping sidewalls by hardening the top layer of resist, increasing the dissolution rate of the resist's surface. These surface-hardening agents can be applied to the samples before or after they are exposed.

Figure 18 represents the etch profile of SiO

2 with a nickel mask. SiO2 will serve as an etch

mask to etch deep recesses into the underlining GaAs and AlGaAs. The SiO2 was etched with

halocarbon 23 (CHF3) at a flow rate of 15 sccm, a pressure of 15 mTorr, and a power of 300 watts. Details of the reactive ion etching of SiO2 can be found in the appendix. The directional nature of

RIE

combined with CHF3's polymer sidewall passivation layer provides SiO2 with a highly

anisotropic etch. During etching, CHF3 deposits a heavy polymer on the underlying GaAs layer,

Figure 18: SEM image: Etched SiO2 with Metal Mask

which, unlike SiO2, cannot be removed by ion bombardment. SiO2 is highly selective over GaAs in

After the nickel pattern was transferred onto the SiG2, the nickel was stripped off with a nickel etchant. The sample is submerged in a wet nickel etchant solution for approximately five minutes. The sample is then removed from the solution and rinsed with acetone and methanol.

The SiO2 mask is used to transfer the pattern into the GaAs and AlGaAs layer. The upper

GaAs layer was completely etched while the AlGaAs layer was partially etched. Again the reactive ion etcher was used to perform this task. GaAs and AlGaAs were etched with 30 sccm of boron trichloride (BCl3), and 20 sccm of silicon tetrachloride (SiCl) at a pressure of

041

J-, .~,

Figure 19A- SEM image: S1813 Photoresist Mask/GaAs Etch Figure 19B: SEM image: S1813 Photoresist Mask/GaAs Etch

30 mTorr and incident RF power of 250 watts. This recipe was predicted to etch GaAs at 180-220 nm/min and AlGaAs at a slightly lower rate due to its aluminum content. A total thickness

of 1450 nm was reactive ion etched: 200 nm of GaAs and 1250 nm of AlGaAs. Although BCl3/SiCl4 is highly anisotropic, it has a

poor

selectivity between masking materials

(i.e. photoresist).

By using a 100+ nm thick SiO2 layer as a mask, there should not be any degradation of the pattern to

be etched.

The third photolithography step defined a trench to aid in the release of the cantilever. The sample was baked at 2000C for at least 30 minutes. A thin layer of HMDS is spun on the sample at 4000 revolutions per minute at 30 seconds. Shipley's 1813 photoresist was spun on the sample at 4000 revolutions per minute for 30 seconds, yielding a 1.3-micron thick layer. Shipley's 1813 photoresist provided adequate resolution for the trench features ranging between 20 to 90 microns. The sample was soft-baked at 900C for 30 minutes. The trench features on the chrome/glass optical mask was aligned to the waveguide topology and exposed for 21 seconds in hard contact. After the exposure, the sample was developed in a MF-319 developer for 45 seconds and then immediately submerged into a DI water rinse for 45 seconds. A few samples were baked after the resist was developed. Both set of samples were used to test the effects of baked resist etches versus non-baked resist etches. The optical microscope and the scanning electron microscope (SEM) were used to examine the quality of the photolithography. Figures 19A and 19B represent the etch profile of the baked photolithography defined trench: it was etched with BC, at a flow rate of 15 sccm, a pressure of 20 mTorr, and a power of 250 watts. Figures 20A and 20B represent the etch profile of the unbaked photolithography defined trench: it was etched with BC, at a flow rate of 15 sccm, a pressure of 20 mTorr, and a power of 250 watts. Details of the GaAs etch can be found in the