Design Variations

A number of variations to the conventional choices for structures and materials have been reported. These include ultra-thin dielectric diaphragms, bossed or corrugated diaphragms, double-walled diaphragms with embedded rigid electrodes, and structures with sense electrodes located external to the sealed cavity.

Ground

Pyrex glass Servo

electrode

Sensing electrode

p⁺⁺ silicon Metal layer Diaphragm

FIGURE 3.12 Lead transfer in an electrostatically servo-controlled pressure sensor. (Reprinted with permission from Wang, Y., and Esashi, M. [1997] “A Novel Electrostatic Servo Capacitive Vacuum Sensor,” Proc., IEEE International Conference on Solid-State Sensors and Actuators [Transducers ’97],2, pp. 1457–1460.)

For achieving very high sensitivities, Equation 3.40 suggests that the area of the diaphragm should be increased and the thickness should be decreased. However, a larger and thinner diaphragm develops high stress under applied pressure, which may cause device failure. Dielectric materials compatible with silicon processing have been explored as alternative structural materials for pressure sensors. In particular, stress-compensated silicon nitride has been the focus of some interest. It is both chemically and mechanically robust, with a yield strength twice that of silicon. High sensitivities were reached by using a large and thin sil-icon nitride diaphragm of 2 mm diameter and 0.3µm thickness [Zhang and Wise, 1994]. Silicon nitride is under high tensile residual stress after LPCVD deposition, which reduces the diaphragm deflection. To compensate this residual stress, a silicon dioxide layer, which contributes compressive stress, was used between two silicon nitride layers (60 nm/180 nm/60 nm). To compensate the non-linear behavior due to large deflections, a 3µm thick boss (p Si) was located in the center of the diaphragm. Its diameter was 60% of the diaphragm; this percentage is generally a good compromise: a greater percentage lowers sen-sitivity, a smaller percentage does not provide the fullest benefit toward linearity. The sensitivity was 10,000 ppm/Pa (5 f F/mTorr), which is 10 greater than the typical value for capacitive devices. The minimum resolution pressure was 0.1 mTorr. The TCO and TCS were, respectively, 910 ppm/K and2900 ppm/K.

The dynamic range was130 Pa.

Introducing corrugation into a pressure sensor diaphragm allows a longer linear travel, and can con-tribute toward larger dynamic range. Corrugations can be created by wet or dry etching. Ding (1992), and Zhang and Wise (1994a) present bossed and corrugated diaphragms under tensile, neutral, and compres-sive stress. Both papers report a square root dependence of deflection versus pressure for unbossed corru-gated pressure sensors. Residual stress in the diaphragm can result in bending when corrugations are present even in the absence of an applied pressure. The use of a boss in the center has been shown to considerably improve this, but can cause a deflection in the opposite direction.

Wang et al. (2000) reported a differential capacitive pressure sensor consisting of a double-layer diaphragm with an embedded electrode. As shown in Figure 3.14, one capacitor is formed between the upper diaphragm and middle electrode, and another between the electrode and lower diaphragm, serving as a capacitive half-bridge. The diaphragms are mechanically connected to each other, and move in the same direction, which is dependent on the pressure difference between the upper and lower surfaces of the entire structure. The advantage of this pressure sensor is that the sense cavity is sealed, but still can generate a sig-nal based on differential pressure. In the reported effort, a diaphragm size of 150 150µm², thickness of 2µm, and capacitor gaps of nominally 1µm were used. With one side of the pressure sensor at atmosphere, the nominal capacitance change is 86 f F for a differential pressure change from80 kPa to 80 kPa.

Most masking steps in the fabrication of pressure sensors are consumed on electrical lead transfer from the inside of the vacuum cavity to the outside as shown in the previous devices. A pressure sensor that elim-inates the problem of the sealed lead transfer by locating the pick-off capacitance outside the sealed cavity is illustrated in Figure 3.15 [Park and Gianchandani, 2000]. A skirt-shaped electrode extends outward from the periphery of the vacuum-sealed cavity, and serves as the element which deflects under pressure.

The stationary electrode is metal patterned on the substrate below this skirt. As the external pressure

Tab contact Glass substrate

SiO₂/Si₃N₄/SiO₂ SiO₂/Si₃N₄/SiO₂

Poly-SiP⁺⁺ Si Ti/Pt/Au Poly-SiP⁺⁺ Si Ti/Pt/Au

External lead for

glass electrode External lead to Si body

Dielectric

FIGURE 3.13 Sealed lead transfer for a capacitive pressure sensor using a sub-surface polysilicon layer. (Reprinted with permission from Chavan, A.V., and Wise, K.D. [1997] “A Batch-Processed Vacuum-Sealed Capacitive Pressure Sensor,”International Conference on Solid-State Sensors and Actuators [Transducers], pp. 1449–1451.)

increases, the center of the diaphragm deflects downwards, and the periphery of the skirt rises, reducing the pick-off capacitance. This deflection continues monotonically as the external pressure increases beyond the value at which the center diaphragm touches the substrate, so this device can be operated in touch mode for expanded dynamic range. To fabricate this, a silicon wafer was first dry etched to the

Differential pressure P

Sealed cavity Silicone gel

d₁− ∆d

C₁+ ∆C C₂− ∆C

d₂+∆d

A B C

FIGURE 3.14 Differential capacitive pressure sensor with double-layer diaphragm and embedded rigid electrode.

(Reprinted with permission from Wang, C.C., Gogoi, B.P., Monk, D.J., and Mastrangelo, C.H. [2000] “Contamination Insensitive Differential Capacitive Pressure Sensors,”Proc., IEEE International Conference on Microelectromechanical Systems, pp. 551–555.)

H G1 R2 R1

T2

T1

Substrate (glass) Electrode

Sealed cavity (volume V)

Deformable skirt or flap

(silicon) T3

G2

₂

Vbias X

Y Z

FIGURE 3.15 Electrostatic attraction between the electrode and skirt opposes the deflection due to external pressure.

desired height of the cavity, and then selectively diffused with boron to define the radius of the pressure sensor. The depth of the boron diffusion determined the eventual thickness of the structural layer. The sil-icon wafer was then flipped over and anodically bonded to a glass wafer that had been inlaid with a Ti/Pt metal that serves as interconnect and provides the bond pads. The undoped Si was finally dissolved in EDP, leaving the pressure sensor on the glass substrate. Fabricated devices are shown in Figure 3.16.

Numerical modeling indicates that for a device with diaphragm radius of 500µm and total radius of 1 mm, cylinder height of 30µm, uniform wall thickness of 5µm, and nominal sense capacitor gap of 5µm, the nominal capacitance is 3.86 pF and the sensitivity is about2900 ppm/kPa in non-touch mode, but drops to270 ppm/kPa in touch mode.

Mastrangelo (1995) described a single crystal silicon pressure diaphragm using epitaxial deposition.

The advantage of this kind of sensor is that it does not require bonding on glass as most p⁺⁺silicon pressure sensors do, and the diaphragm stress condition and material properties are predictable, as single crystal silicon provides more consistent performance than polysilicon.