Presented and published
      Proc. of SPIE, vol. 3903,  pp 223-231

There have been several cantilever-based actuators which provide movement in a direction transverse to the length of the cantilever. Here we describe a novel actuator which gives movement in the same direction as that of the length of the element. This is achieved by a curling up action of a bimetal like 'metal on silicon dioxide' straight element due to large differences in the coefficients of linear expansions and the high temperature which is obtained during the metal deposition.

The cantilever is made from a silicon dioxide patterned in the shape of an acute angled ">" tip which is aligned to the major flat of silicon wafer. The width at the base is 10 um and reduces to 2 um near the tip. During the cavity etching, the silicon etching starts at the tip and slowly moves under the silicon dioxide. In about 120 min etching in KOH/IsoPropanol/Water etchant at 80C, the etching under the tip moves right upto the base of the ">" shaped tip. Furthermore, advantage is taken of the fact that KOH etchant etches thermally grown silicon dioxide at the rate of about 1500 Angstrom/hour. This results in a tapered oxide tip. The tapering in width as well as in thickness help develop a higher temperature at the tip during the metal deposition by filament evaporation. On cooling, the tip bends towards the metal side due to large contraction in metal as compared to that of oxide. We routinely get U-bends and have achieved even multiturn curling.

Keywords: micromachines, MEMS, Anisotropic etching, cantilevers, current sensors, smart materials, actuators

       1.  INTRODUCTION

Machines have always been a part of mans life. Men have tried to utilize machines in their life for as many applications as possible. Silicon has brought a technological revolution in the field of microelectronics. Now for the past decade mechanical sensors have been fabricated in silicon to be used in the field of automotive applications. Silicon sensors have replaced more expensive and less reliable mechanical assemblies. This field of applications wherein mechanical components are fabricated in silicon is called as micro machines or Micro-Electro-Mechanical Systems (MEMS) because of their dimensions. Such mechanical components are also called as micro parts or microstructures. MEMS have been used largely in automotive applications for pressure sensing, temperature sensing and as accelerometers.

These microstructures, made of SiO₂, having dimensions 1-10 micron in width and 50-100 micron in length were suspended over a cavity of 140 micron deep in <100> in silicon. The 100 micron X 100 micron cavity is obtained by a crystallographic etchant KOH. Such microstructures then bend or vibrate in the cavity formed giving a 3D like effect to the devices. Some of the microstructures fabricated in our laboratory ¹ are as follows namely Cantilevers, Cross-bridges, Spirals, Doubly suspended resonators, Micro-probes, Magnetic sensors etc. These microstructures were subjected to different treatments for different applications, like evaporation of metals for thermal applications or sputtered with magnetic materials like Cobalt for magnetic actuator like applications etc.

Applications of MEMS today are very wide. But to begin with mostly simple microstructures like Cantilevers and Resonators were used. Cantilevers were used as the humble devices as Temperature sensors using the bimetallic principle. Resonators were used as the magnetic sensors.

Cantilevers, in our laboratory have been designed, so as to be used as the first moving or bending sensor. Such cantilevers traditionally have provided movement in a direction transverse to the length of the cantilever arm. But here we describe a novel actuator, which gives movement in the same direction as that of the length of the element. The cantilever is made from a silicon dioxide patterned in the shape of an acute angled ">" shape. Current sensitive actuators were planned as applications for such cantilevers. This is achieved by a curling up action of a bimetal like ‘metal on silicon dioxide’ straight element due to large differences in the coefficients of linear expansions and the high temperature which is obtained during the metal deposition. Such bending applications can be used to make current sensitive or temperature sensitive contacts. The cantilevers can be bend forward or backward where it will make contact with immovable metallized contacts. Cantilever-based Actuators can then be integrated with associated signal conditioning electronics for a full fledged device.
 

1.1. ELECTRON BEAM LITHOGRAPHY

A PC based electron beam lithography (EBL) attachment to JEOL JSM-840 scanning electron microscope (SEM) is used to generate patterns of various sizes varying from submicron size all the way to few millimeters. EBL system consists of a SEM whose e-beam is controlled by external XY scan signals generated by D-A converters through a 80486 IBM compatible PC. EBL system accepts patterns generated by DesignCad 2D package. The Beam control hardware and software setup termed as Nanometer Pattern Generation System (NPGS) has been purchased from JC Nabity Systems Ltd., USA. The Beam is switched ON/OFF by an electromagnetic blanker (Beam Blanker Device, BBD) mounted in the column of SEM.

Fabrication of masks on chrome plates as well as Direct write on wafer can be done by such an EBL system. To expose a pattern, the necessary run file has to be defined, which contains detailed information about Beam current, exposure dose, magnification, centre to centre spacing and line spacing. The exposure dose is dependent on the type of e-resist used. And the magnification, centre to centre spacing and line spacing is dependent on the dimensions of the pattern design. A dose of 140 micro Coulomb per sq.cm, at highest possible accelerating voltage, 40KV, was used to minimise lateral beam spread. Beam current of 1000 pA was chosen to achieve a smooth exposed line while keeping the total exposure time in control.

The EBL writing technique was standardised² in terms of SEM parameters like accelerating voltage and beam current, so as to get the resolution patterns namely Rose pattern, Wheel pattern and Spiral, with reducing width towards inside, exposed on silicon substrates, spin coated with 1000 Angstrom thick electron beam resist.

2.  DESIGN


The pattern of current controlled tip is designed using DesignCAD 2D software such that the tip is aligned to the major flat of silicon wafer. The tip is in the shape of an acute angled ">" shape. The width at the base is 10 um and reduces to 2 um near the tip. The length from the base to the tip is 70 um. Four such tips are placed, one at each side of a square of 140 um length. The base tips are extended to 250 um length for providing a probe area in such a fashion that only four contacts are sufficient for all the four tips. The total pattern area is about 800 um by 800 um. Hence the magnification used on SEM is 100X. The necessary run file is then defined using the MRF command.

A photomask, generally a glass plate on which there is a thin layer of metal, chrome is fabricated. To expose the pattern, first the chrome plate is spin coated with e-resist and prebaked. After exposing, the e- resist is developed, post baked and etched in chrome etchant. Now, the chrome mask is ready for further use.
 

3.  PROCESS STEPS OF FABRICATION OF MEMS DEVICES
 
Now the pattern is transferred on Oxidized Silicon Sample using Optical lithography so as to get a large number of samples in a short time. Also photomask can be exposed two times on the same sample from opposite sides so as to increase the yield of devices per samples. Pattern is etched on Oxidized Silicon using BHF solution. And Photo-resist on sample is stripped–off. This pattern in oxide acts further as the negative type of protective mask against the silicon etchant. Please see figure (1) which shows a part of such a pattern exposed and developed on oxidized silicon just before etching.

 
3.1. METALLISATION OF OXIDISED SILICON STRUCTURES

Evaporate Nichrome and Gold successively on the patterned wafers at slightly warmer temperatures. During evaporation a thermal stress gets created between the oxidized silicon and the metals.

 

Figure (2) shows the curl-up action after 45 minutes of etching time.
 
 

 

Figure (3) shows the curl-up action after 60 minutes of etching time.

 
 
 

Figure (4) shows the curl-up action after 90 minutes of etching time.

 
 
 

Figure (5) shows the curl-up action after 120 minutes of etching time.

3.3 UNCURLING OF THE TIPS

Nichrome on the oxide acts as the heating element! When current is passed through the current control tip, it gives controlled heating, which uncurls and straightens the spliced ">" tip structure tip. Such an action is seen in nature - as curled up antennae of butterflies, curled up tongues of reptiles, or curled grasp-and-hold tentacles in creepers. As we are aware that man has tried to look towards nature for mimicking various applications. We foresee applications of such current controlled ">" shaped curled tips in providing extended movements as well as in grasp-and-hold operations.

4.  RESULTS

Resistance measurements were carried out on the tips thus fabricated by using a simple ohm meter on the extended gold contacts. Table 1 shows the I/V Measurements data which indicates that the resistance heater formed by Nichrome is independent of the etch duration. This data matches with the estimated resistance.

5. RESULTS OBSERVED UNDER OPTICAL MICROSCOPE

Any movements in such micromachines have to be seen under microscope due to their size limitations. Uncurling of the tips was observed by flowing 1-15 mA of current through the tip using a small zoom stereo microscope because of localized heating near the tip edge. The tip fused at 22 mA of current in open air conditions due to oxidizing of the Nichrome and gold. Fusing current may increase when tip will be in vacuum or in encapsulated environment.

   6. CONCLUSIONS

Micron-sized current control tips were designed by Design-CAD and chrome masks made by EBL system from NPGS. We have successfully fabricated current-controlled SiO₂ microtips by using micromachining technique. Curling of tip takes place due to bimetallic stresses during fabrication. And uncurling by passing control currents of the order of 1 to 15 mA. Fusing currents of 22 mA (in air) were observed.

     7. REFERNCES

1.  U.D. Vaishnav, P.R. Apte, S.G. Lokhre, V.R. Palkar and S.M. Pattalwar, "Micromechanical components with novel properties",
    Proc. of SPIE, 3321, 287-297 (1996)

2.  S.G. Lokhre, S.C. Purandare, and B.M. Arora, " Diffraction Grating pattern on GaAs by electron beam lithography", DAE Solid
     State Physics Symposium, IACS, Jadhavpur, Calcutta, Dec. 27-31, 1995

Go back to MEMS page

Go back to Apte's web-page