Paper presented at the "SPIE symposium on smart materials, structures and MEMS "
               held at Bangalore Dec. 11 - 14 1996, Proc. of SPIE, 3321, 287-297 (1996)

This paper describes the fabrication techniques and characterization of silicon dioxide micromechanical components with a layer of porous aluminum oxide which results in novel properties. An aqueous SOL process has been developed to obtain a layer of porous Aluminum oxide on the silicon dioxide. The micro-porous surface, so realized, can be used as sensitive moisture and gas detectors. Various parts fabricated in silicon dioxide are cantilever, cross beam, spiral spring and resonator, coated with porous aluminum oxide, and micro-probes, coated with Chromium-gold for electrical contacts.

This paper also demonstrates the use of the trapezoidal pit etched in silicon during micromachining as a radiation concentrator. The results of derivation of the concentrator efficiency clearly shows the advantage of the reflections from the trapezoidal cavity.

Keywords: silicon micromachining, microfabrication, MEMS, radiation concentrator

1.  INTRODUCTION

Since the micro-parts are made of silicon dioxide, various other materials could be deposited on top of silicon dioxide by thin film and thick film techniques. Metals are usually deposited by thin film techniques like vacuum evaporation or sputtering. Dielectrics, ferroelectric and piezoelectric materials are coated by both thin film as well as thick film techniques. We have deposited aluminum oxide by the thick film SOL technique. The SOL process has resulted in porous aluminum oxide on the silicon dioxide. The micro-porous surface, so realized, can be used as sensitive moisture and gas detectors.

The trapezoidal pit etched in silicon during the micromachining is bound by <111>planes. It appeared that the same can be exploited for use as a radiation concentrator with the focus at the tip of a cantilever or the center platform of the cross beam, as both these structures have been used extensively in microcalorimetry. Small thermal capacity of this micro-device enables its use as a high sensitivity radiation detector. This paper gives results of a radiation concentrator which is based on double reflections from the <111>side walls. High collection efficiency so obtained clearly shows the advantage of the reflections from the trapezoidal cavity.

2 EXPERIMENTAL

Various steps used in fabrication of micromachined structures in oxidized silicon are given below;

2.1 Oxidation of silicon
 

Oxidation of silicon is done in a furnace at 1150C in steam ambient. Silicon wafers are stacked into a slotted quartz boat and pushed in to the center of a quartz tube. Oxygen is bubbled through a quartz bubbler filled with deionized water and maintained at 95C. The thickness of the oxide is approximately 1 micron for a 3 hour oxidation. All micro-parts are made using the 1 micron oxide wafers.

2.2 Deposition of metal/refractory films

2.2.1 Thin film techniques:

Metal, dielectric, magnetic and other films are obtained by vacuum deposition, sputtering, chemical vapor deposition. In the present study only metal systems like Titanium-Gold or Chromium-Gold have been deposited using e-beam evaporation in a Leybold Heraus Ultra High Vacuum system capable of 10^-8 torr pressure. A thickness monitor is used to control the thickness of adhesive layers (Titanium or Chromium) to about 300A and that of gold to about 1000A

2.2.2 Thick film techniques:

Metal and metal-alloy thick films are obtained by electroplating or electroless plating. Dielectric, ferroelectric, piezoelectric thick films are obtained by sol-gel and other thick film techniques. In the present work, the aluminum oxide films on silicon dioxide are obtained by an aqueous SOL process which belongs to the family of SOLGEL processes.

Aqueous Solgel Process :

Porous materials have been deposited on these silicon dioxide structures to result in novel properties. Taking advantage of the fact that these micro-parts are made of Silicon dioxide, various ceramics can be coated by SOLGEL process using spin coating method, and sintered at higher temperatures like 500-1200C. During the sintering process, of which the details are given below, the interface between silicon dioxide and the deposited material forms silicates, thus resulting in excellent adhesion. The micro-porous surfaces, so realized, can be used for sensitive moisture / gas detectors.

Nanocrystalline particles of bohemite(AlOOH) were synthesized by the reaction of water on aluminum metal pieces. To enhance the reaction process, aluminum metal pieces were coated by very thin layer of mercury by dipping in 0.001N HgCl₂ solution. Bohemite particles thus obtained were then suspended in water to form a stable sol by adding small quantity of acetic acid as an electrolyte and peptizing at 80C. This aqueous sol was then directly used for spin coating (2000 rpm for 20sec) Bohemite particles were converted to alumina by post annealing at high temperature (600-1200C). During the annealing process the particles grow in size.

Atomic Force Microscope (AFM) scan over the surface reveal the particle sizes to be 500-2000A for the samples heated at different temperatures in the range of 500-1100C. It was also found that, above 1000C, it reacts with silicon dioxide to form a glassy phase of aluminum silicate.

2.3 Photolithography and window etching

Photolithography is a technique of transferring the desired pattern onto the oxidized silicon wafer. In this study, the photomasks have been made by an Electron Beam Lithography (EBL) attachment to the Jeol Scanning Electron Microscope (SEM). Chrome plates are coated with electron resist and the desired pattern is exposed by the electron beam. The plates are developed and chromium is etched in NaOH - Potassium Ferricyanide etchant. The electron resist is stripped in Trichloroethylene (TCE). The photomasks so made are used to expose a positive resist coated oxidized silicon wafer by ultraviolet (UV) lamp of a mask aligner. The photoresist is developed, post baked and the oxide is etched in buffered hydrofluoric (HF) acid. This leaves oxide pattern with bare silicon where the silicon is expected to get etched by the crystallographic <111>stop etchant.

2.4 Crystallographic <111> stop etching (aqueous KOH etchant)

Micromachining pertains to the use of specialized fabrication techniques for the CONTROLLED SELECTIVE ETCHING of silicon and numerous films used in silicon planar processing to obtain 3-dimensional structures having micrometer dimensions needed for silicon sensors and actuators.

2.4.1 Anisotropic Silicon Etchant : KOH + Water + IsoPropanol (KOH)

Anisotropic wet chemical etchants are orientation dependent. These are known to selectively etch <100>and <110>and leave <111>relatively free from attach. The etch rates in <111>direction is typically 50 times slower than in either <100>or <110>directions. This results in vertical <111>sidewalls in <110>orientation and sidewalls with angle of 54.7^o in <100>orientation silicon wafers. This gives rise to well controlled 3-D structures.

Another crystallographic <111>stop etchant, which comprises of pyrocatechol, EthyleneDiamine and water, does give better results but it is more hazardous and hence requires reflux condenser with dry nitrogen bubbling to provide an inert gas cover to keep oxygen away. The chemical work bench with good exhaust is necessary. The etchant is opaque and therefore it is rather difficult to observe the progress or the possible end-point of etching. For this reason, the authors have used KOH etchant to demonstrate various micro-parts in silicon dioxide with metal or aluminum oxide layers.

2.4.2 Etching procedures and techniques

Etching of silicon in specific areas is achieved by defining the geometry of a an etch-stop layer by lithography and then etching the silicon in KOH etchant. There are 3 methods used to realize an etch-stop layer,

a) Layer of silicon dioxide or -nitride, b) heavily diffused boron layer, c) PN junction

Of these, we have used a layer of silicon dioxide (with or without aluminum oxide or Chromium-Gold layer on top of oxide) as the etch stop layer.

2.5 Study of various micromechanical structures

Micromachined structures :

Given below is a list of (some of the) structures which can be realized in silicon, (there are 'new' structures being added to this list almost every day),

Bulk micromachining structures,

a) membranes or diaphragms b) cantilevers c) bridges d) grooves e) holes

Applications benefiting from various structures are

The trapezoidal pit etched in silicon during fabrication of cantilever structure can be used as a radiation concentrator with the focus at the tip of the cantilever or at the center of the cross beam. Small thermal capacity of this micro-device enables its use as a high sensitivity radiation detector.

Assuming that double reflection from the opposite <111>faces reflect beam of collimated light rays to the center platform, we have derived the capture efficiency of the cross beam structure on the pyramidal pit from the reflection ray diagram as shown in figure 1. The detailed derivation is given elsewhere⁷ but only relevant equations and results are given here.

Where q is the distance of the incident ray from the center and p is the distance of the doubly reflected ray form the center. The fraction of radiation received on the platform is given by

The results from equation [2] are given in Table I. For example, A 40 micron square capture area at the center of 100 micron square pyramidal pit is shown to collect approximately 50 percent radiation incident on the device which is 3 times more than that received directly.

Five different structures were fabricated with 5-10 micron minimum features and about 100 micron pyramidal cavity size. These are described below,

4.1 Cantilever

Cantilever has many applications as reed vibrators, bending /deflection by thermal radiation or resistive heating. It is used as AFM or other Near field Microscope tips. The resonant frequency of the cantilevers made by us is in the range of 300 KHz to 3 MHz.

Cantilever shown in Plate 1(a) is the simplest structure to characterize the vibration frequency, deflection of the tip due to point loads etc. with the width, thickness, length of the cantilever. For applications as a accelerometer the tip of the cantilever will have a mass attached to the tip.

4.2 Cross beam

Cross beam has two main applications

(1) As a thermally isolated platform at the center of the cross - this is useful in micro-calorimetry.

(2) With a centrally attached mass it can be used as 3-D accelerometer / gyroscope in space applications.

The cross beam in Plate 1(b) shows a minimum area center platform. In real applications the center platform may have larger area as well as centrally loaded mass.

4.3 Spiral

Spiral has micro-movements in and out of the cavity. These movements are caused either by thermal or point loads at the center. It acts as a very low stiffness spring. The Plate 1(c) shows a spring with rectangular bends. However, smooth spiral springs are preferred in actual applications.

4.4 Resonator

The resonator in Plate 1(d) is a doubly suspended torsion resonator with Q's in excess of 40,000. These can be used to measure viscosity of fluids in resonant mode and as vibration sensors in space vehicles and large ground based antenna structures.

4.5 Micro-probes

The micro-probes have tip dimensions (thickness = 1 micron, width =2-10 microns and length = 10-50 microns) such that these can be used to probe microorganisms, biological specimens, membranes etc. These are metallized to allow measurement of pH, K- Na- Ca ion concentrations, electric potentials and conductivity etc. in the vicinity of the probe tip. These probes are designed for ease in handling (having a length of about 1mm and 1/2 mm width) and for providing a choice of different probe tip dimensions. Optical microphotograph of a single probe is shown in Plate 1(e).

This paper described the fabrication techniques and applications of silicon dioxide micromechanical components with a layer of porous aluminum oxide which results in novel properties. Various parts fabricated are cantilever, cross beam, spiral spring, and resonator on Silicon dioxide coated with aluminum oxide. Micro probes with micron and sub micron tips have been made in silicon dioxide coated with Cr-Au metallization.

This paper also demonstrates the use of the trapezoidal pit etched in silicon during micromachining as a radiation concentrator.

1. S. Middelhoek and S.A. Audet, \em Silicon Sensors,

Academic Press, Chapter 7, pp. 316-330, 1989.
2. K.E. Petersen, "Silicon as a mechanical material",Proc. IEEE, Vol. 70, pp. 420-457, 1982.
3. J.B. Angell, S.C. Tracy and P.W. Barth,

"Silicon Micromechanical Devices",
Scientific American, Vol. 248, pp. 36-47, April 1983.
4. H.J. Rolke and H.F. Schlaak, "Micromechanics and Microsystems Technology",Siemens Review, R and D Special, pp. 20-24, Spring 1990.
5. G. Stix, "Micron Machinantions",Scientific American, Vol. 257, pp. 106-117, Nov. 1992.
6. K.J. Gabriel, "Engineering Microscopic Machines",Scientific American, Vol. 260, pp. 118-121, Sep. 1995.
7. To be published

Serial No.	Fractional Size of the Platform	Direct Light	Reflected onto Platform	Total light on Platform
1	.05	.0025	.0175	.0275
2	.1	.01	.047	.057
3	.15	.0225	.09	.113
4	.2	.04	.146	.186
5	.25	.0625	.215	.277
6	.3	.09	.296	.386
7	.35	.1225	.39	.513
8	.4	.16	.497	.657
9	.45	.2025	.617	.82
10	.5	.25	.75	1

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