Silicon Micromaching: Bulk
5.2 ISOTROPIC AND ORIENTATION-DEPENDENT WET ETCHING
Wet chemical etching is widely used in semiconductor processing. It is used for lapping and polishing to give an optically flat and damage-free surface and to remove contami-nation that results from wafer handling and storing. Most importantly, it is used in the fabrication of discrete devices and integrated circuits (ICs) of relatively large dimensions to delineate patterns and to open windows in insulating materials. The basic mechanisms for wet chemical etching of electronic materials were described in Section 2.4. It was also mentioned that most of the wet-etching processes are isotropic, that is, unaffected by crystallographic orientation.
However, some wet etchants are orientation-dependent, that is, they have the property of dissolving a given crystal plane of a semiconductor much faster than other planes (see Table 5.1). In diamond and zinc-blende lattices, the (111) plane is more closely packed than the (100) plane and, hence, for any given etchant, the etch-rate is expected to be slower.
A commonly used orientation-dependent etch for silicon consists of a mixture of potas-sium hydroxide (KOH) in water and isopropyl alcohol. The etch-rate is about 2.1 um/min for the (110) plane, 1.4 urn/min for the (100) plane, and only 0.003 um/min for the (111) plane at 80 °C; therefore, the ratio of the etch rates for the (100) and (110) planes to the (111) plane are very high at 400:1 and 600:1, respectively.
Table 5.1 Anisotropic etching characteristics of different wet etchants for single-crystalline silicon
Etchant
Temperature Etch-rate (jim/hour) of (°C) Si(100) Si(110) Si(111)
80 ' Appendix M provides a list of all the worked examples provided in this book.
ISOTROPIC AND ORIENTATION-DEPENDENT WET ETCHING 119 (111)
- - Resist
Figure 5.1 Anisotropic etching of (100) crystal silicon
Figure 5.1 shows orientation-dependent etching of (100)-oriented silicon through pat-terned silicon dioxide (SiO2), which acts as a mask. Precise V-grooves, in which the edges are (111) planes at an angle of approximately 55° from the (100) surface3, can be realised by the etching. If the etching time is short, or the window in the mask is sufficiently large, U-shaped grooves could also be realised. The width of the bottom surface, w, is given by
w = WQ — 2h coth(55°) or w = WQ — 1.4h (5.1) where WQ is the width of the window on the wafer surface and h is the etched depth.
If (110)-oriented silicon is used, essentially straight walled grooves with sides of (111) planes can be formed as shown in Figure 5.1.
Worked Example E5.1: Mechanical Velcro Objective:
The objective is to apply isotropic and anisotropic wet etching to fabricate a dense regular array of microstructures that act as surface adhesives (Han et al. 1992). The principle of bonding is that of a button snap, or a zipper, but in a two-dimensional configuration.
The bonding principle is shown by a schematic cross section in Figure 5.2. When two surfaces fabricated with identical microstructures are placed in contact, the structures self-align and mate. Under the application of adequate external pressure, the tabs of the structures deform and spring back, resulting in the interlocking of the two surfaces.
Thus, the structures behave like the well-known 'Velcro' material.
(a) (b) (c)
Figure 5.2 Basic steps involved in bonding together silicon 'Velcro' ' The value of 55° is important to remember.
120 SILICON MICROMACHINING: BULK Process Flow:
1. A 120-nm SiO2 layer is grown at 1000°C in dry oxygen on (100) silicon wafers.
The oxide is patterned using optical lithography into an array of 10 um2 rectangular islands, with one edge aligned 45° to the (110) flat (see Figure 5.3(a)).
2. After photoresist stripping, the wafer is immersed in an anisotropic etch bath that consists of aqueous KOH (33-45 percent, 84 °C, 4 min) and isopropyl alcohol. The etching results in a truncated pyramid with exposed (212) planes, which are the fastest etching surfaces. The (212) planes intercept the (100) base plane at an angle of 48°
(See Figure 5.3(b)).
3. After stripping the masking oxide and cleaning the samples with a conventional chemical sequence, a thick SiO2 layer (~1.0 to 1.5 um) is grown at 1000 °C in wet oxygen. The oxide is patterned by a second mask that consists of an array of Greek crosses, each approximately 18-um wide, aligned to the original array (see Figure 5.3(c)).
4. The oxide crosses act as a mask for a second etch in KOH (~3 min), which removes some of the underlying silicon. Finally, the microstructures are completed by etching the wafer for two minutes in an isotropic etching bath (15:5:1 HNO3:CH3CO2H:HF). This step provides the vertical clearance for the interlocking mating structures and the lateral undercut necessary to produce the four overhanging arms. Although the isotropic silicon etch also attacks the oxide, the selectivity is sufficiently large so as not to cause a significant problem (see Figure 5.4(d)).
Resist SiO2
(a)
SiO2
(b) Si0
Si substrate
Si substrate
Si substrate
Si substrate (d)
Figure 5.3 Process flow for the fabrication of silicon microvelcro
ISOTROPIC AND ORIENTATION-DEPENDENT WET ETCHING 80
60
-40
20
-0
300
200
0 200 400 600 800 Insertion pressure (kPa)
(a)
Figure 5.4 (a) Damaged area against insertion pressure and (b) tensile strength against area damaged (Han et al. 1992)
Mechanical Testing:
Patterned samples, nominally (8 x 8) mm2, were interlocked by applying a load to the upper substrate; the insertion pressure is monitored by placing the entire assembly on an electronic force scale. The bond strength of the mating structures is then characterised by direct measurements of the tensile load needed to induce failure. Bond strength is determined by applying a tensile load through a pulley and measuring the force necessary for separation. Separation of the samples (failure) is always accompanied by damaged areas only on some regions of the mating surfaces, implying that the samples are only interlocked over these damaged regions. The fraction of the damaged area is found to be proportional to the insertion pressure (Figure 5.4(a)). Also, the tensile load necessary to induce failure is proportional to the fraction of the damaged area (Figure 5.4(b)).
Extrapolation of the straight line plot of the area damaged against the tensile strength to 100 percent interlocking yields a tensile strength of approximately 1.0 MPa.
Failure Analysis:
The analysis assumes a simple cantilever model as shown in Figure 5.5.
In the figure, Fn is the interaction force between the tabs and / is the length of the tab. The bending stress, a, is given by
a(x) = M(x)y
(5.2) where x is measured from the edge of the tab that is attached to the substrate, the bending moment M(x) is given by Fn(l - x) Iz is the moment of inertia (bh3/12) of the rectangular cross-sectional area of width b and thickness h about the centroidal axis (z-axis), and y is the distance from the neutral plane. The maximum bending stress amax
occurs when x = 0 and y = ±h/2 and is given by
6/y
bh2
(5.3)
122 SILICON MICROMACHINING: BULK
t t t t t t t t
I I I I I I I I
Figure 5.5 Simple cantilever model of the failure mode of silicon microvelcro (Han et al.
1992)
Similarly, the maximum shearing stress rmax occurs at the neutral plane >' = 0:
3Fn
(5.4)
amax is higher than Tmax with the ratio of Tmax/amax equal to h/4l for the design used.
Visual examination of the tested samples indicates that failure is accompanied by damage to the edge of the tab and is consistent with the failure occurring when amax
exceeds the yield point ayp. From Figure 5.5, we have
_ ext
-rn — . . \-J-J)
4 sm a
where Fext is the tensile load (i.e. force per unit surface area) applied to the sample.
Using Equation (5.3), we get
" max — • , , i . "• * exi —
2bh2sma
which, when we include friction with a static coefficient /z, becomes 2amMbh2 sin a
ext ~ 3d2l(\ + Mcota)
The tensile strength (failure load) of the structure can be found by substituting design values for b, h, l, a, and /z (0.5) and by substituting ayp (6 x 105 kPa for the oxide) for amax in Equation (5.7) to obtain a value of Fext equal to 1.1 MPa, which is in agreement with the value obtained from the extrapolation of data in Figure 5.4(b). This finding confirms that the failure mechanism is that of bending stress, which exceeds the oxide yield point at the tab edge.
ISOTROPIC AND ORIENTATION-DEPENDENT WET ETCHING 123 Worked Example E5.2: Undoped Silicon Cantilever Beams
Objective:
To fabricate a cantilever beam oriented in the (100) direction on (100) silicon wafers (Choit and Smits 1993).
Process Flow:
1. A layer of SiO2 that is 0.5 um thick is grown on a (100) n-type silicon wafer.
The wafer is spin-coated with a layer of positive photoresist. The masks needed to fabricate the cantilevers are shown in Figure 5.6(a, b). Cross-hatched areas represent opaque regions of the masks. For mask 1 (Figure 5.6 (a)),
w1 = 2(wb + and (5.8)
where lb, wb, and tb are the length, width, and thickness of the beam, respectively.
l1 and w1 are shown in Figure 5.6(a). For mask 2 (Figure 5.6(b)), we have
= wb + and l2 = (5.9)
where d is a small parameter that corrects design errors and mask misalignment, l2
and w2 are shown in Figure 5.6(b). The two masks have essentially the same pattern, except that mask 2 has a smaller beam width than mask 1. The wafer is patterned with mask 1. The wafer is oriented in such a way that the length of the cantilever beam is in the (010) direction of the wafer, as shown in Figure 5.6(c).
2. The wafer is then immersed in a bath of buffered oxide etch (BOE) to remove the SiO2 in the areas that are not covered by photoresist, and this is followed by dissolving the resist in an acetone bath. A transverse cross section of the beam region after the resist has been removed is shown in Figure 5.7(a). The wafer is now ready to be bulk-etched in sodium hydroxide (NaOH) at 55 °C.
3. Etching will take place in regions where the (100) planes of silicon are exposed.
Lateral etching of silicon directly underneath the SiO2 passivation layer will also occur; the lateral planes that are etched are the (100) equivalent planes. These planes are normal to the substrates. The rate of downward etching is the same as that of lateral etching; this will result in walls that are almost completely vertical (see Figure 5.7(b)). Planes are formed at the clamped end of the cantilever beam (111).
Mask 1
IT
JL
Mask 2
--H h---W-)
(a) (b)
Figure 5.6 Masks required to fabricate the cantilever (100) Si wafer
(c)
124 SILICON MICROMACHINING: BULK
H—M
dwbd
•H H K- w, 'b+V2 d wb d
K--H /
---Hh*-HK--(a) (d)
Figure 5.7 Four process steps to make and release the cantilever
4. The wafer is then etched in BOE to remove all the SiO2 and is then cleaned and oxidised to grow a fresh layer of SiO2 that is 1 urn thick. The wafer is spin-coated with a layer of positive photoresist and patterned with mask 2. After the unprotected oxide is etched away in BOE, the resist is removed in acetone. The wafer is then etched in NaOH at 55 °C until the bulk silicon is completely under-etched in the areas that are directly underneath the beam. Figure 5.7(c, d) shows the evolution of the silicon cantilevers etched in this way at different stages of the final etching in NaOH.