Study on si electrostatic and electroquasistatic micromotors

171

Sensors and Actuators A, 35 (1993) 171-174

Study on Si electrostatic and electroquasistatic J -B Huang,

micromotors

P -S Mao, Q -Y Tong and R -Q Zhang

Mmoelectronrcs Centre, Southeast lJruvers~ty.Nanpng 210018 (Chrna)

(Rewved May 8, 1992, accepted May 19, 1992)

Abstract An electrostatic vanable-capacltance nucromotor and an electroquaslstatic mductlon mxromotor are discussed Both the mlcromotors, wth rotors of 160 pm hameter, have been fabncated by the surface nucromachmmg of polyslhcon In order to reduce the fnctlon m the nucromotors, the appbcatton of a gas-lubncated bearmg 1s proposed An aerodynamic m-pumpmg spiral grooved thrust nucrobeanng wth a groove Hrldth of 4 m and a groove depth of 300 nm has been deslgned and used m the mxromotors

1. Introduction In the field of electrostatic mlcromotors, there are three different types (1) the vanable-capacltance mlcromotor (VCM), which 1s synchronous and has received the most attention to date [ 1,2], (2) the electroquaslstatlc mductlon nucromotor (IM) [3], which 1s asynchronous, and (3) the wobble motor, also called the harmonic motor [4] The pnorlty m this paper 1s gven to the first two types It 1s well known that the study of electrostatic motors, both synchonous and asynchronous, has a surpnsmgly long history The study of the asynchronous motor began as long ago as 1893 [5j The motor operated at 3800 V and 40 Hz wrth a speed of 250 rpm In 1969, a cylmtical electroquaslstatlc mductlon motor [ 51 was developed, with a volume of a few cubic centlmetres, which gave a torque of the order of 5 pN m at 150 V and 30 Hz This was a mmlature motor, the statorrotor gap of which was 55 pm The surface mlcromachmmg of &-related materials has recently been apphed to the construction of electrostatic rmcromotors In general, tlus work has focused on the VCM However, the IM will be an appealing alternative to the VCM m some apphcatlon fields [3] Analyses of both VCM and IM types have been done both quahtatlvely and quantitatively [ 5, 61 The quantitative results [6] show that almost no performance difference exists between a VCM and IM wth smular structural dunenslons

0924-4247/93/%6 00

The mam differences between a VCM and an IM are as follows First, they have different operational prmclples The former 1s based on the vanable St&or-rotor capacitance, while the latter IS based on the charge relaxation m the rotor and stator-rotor gaps The charge relaxation causes the nnage charges induced on the rotor surface to lag behind the potential waves travellmg around the stator electrodes Secondly, there exists a small structural difference between a VCM and an IM Therefore, the fabncatlon processes ~111be a little different The VCM rotor must contam physically salient poles m order to obtam the vatlable stator-rotor capacitance Plananzatlon over this saliency may require undesirable fabncatlon steps The IM rotor can be a smooth uniform conductor However, the conductivity of the rotor matenals may strongly affect the IM performances, so fabncation dlfficultles associated wth VCM rotor saliency may be traded for those associated wth conductlvlty control m the IM

2. Structure and process In the followmg, we descnbe the process used to make a VCM and IM at the same tune Polyslhcon surface mlcromachmmg IS used m the process, m which LPCVD polyslhcon IS used as the structural material, phosphoslhcate glass (PSG) 1s used as the sacficlal layer and &con mtnde as the electrical insulator @ 1993- Elsevw Sequoia All nghts reserved

172

Figure 1 shows the cross-sectional sketches and top views of the VCM (left) and the IM (right) It can be seen from the Figure and the process described below that the stator-rotor gaps of the VCM and IM are different In the former, the gap depends on the hotlzontal process size, that is, the photohthographlc precision In the latter, the gap depends mamly on the vertical process sizes, which are of the thicknesses of the two sacrlficlal layers ( 1st and 2nd PSG layers) It 1s well known that the gap distance plays an Important role m electrostatic nucromotors The control of the vertical process size IS easier than that of the horizontal process size m the IC process, even the control of a vertical size of submlcrons 1s not difficult Therefore, it 1s desn-able to get a small gap (vertical) by usmg a snnple process with low-precision photolithography The VCM and IM m Fig 1 are fabricated by startmg with a slhcon substrate covered with an oxide-mtnde composite layer The 1st LPCVD polyslhcon (300 nm) IS deposited on the top of the composite layer, followed by a heavy phosphorus doping This polyslhcon layer is defined as the ground plane of both the VCM and IM and the bottom stator electrodes of the IM After this step, the 1st PSG layer (1 pr@ is deposited and defined,

which acts as a sacrlficlal layer The next step 1s to deposit the 2nd polyslhcon layer (1 5 pm), which structures the rotors of both the VCM and IM and the stator of the VCM The subsequent doping of the polysdlcon 1s divided mto two steps A heavy phosphorus dopmg IS used for the rotor of the VCM and a light doping for the rotor of the IM The 2nd PSG layer (1 pm) 1s then deposlted and a photohthographlc step 1s used to expose the 1st polyslhcon layer at the centre of the rotors so that the next deposltlon of the 3rd polyslhcon layer will anchor to the 1st polyslhcon layer and the substrate The 3rd polyslhcon layer (1 pm) 1s deposited for the flanged hub of both the VCM and IM and for the top stator electrodes of the IM After this step, all previously deposlted PSG layers are removed m an HF-based etchant The remammg polyslhcon layers form both the VCM and IM Except for the dopmg control of the rotors, the processes of both the VCM and IM are the same and neither of them needs any planarlzatlon process steps Figure 2 (a) and (b) shows SEM photographs of the VCM and IM, respectively, m which the rotor diameters are 160 pm

polyS1

poly-Sl SiaNa sio2

(a)

VCM

Rg 1 Cross-sectmnal and the IM (nght)

It4

sketches and top views of both the VCM (left)

(b) Fig 2 SEM photographs

of the VCM (a) and IM (b)

173

3. Gas microbearing for micromotors The fnctlon of the bearmg m mlcromotors 1s regarded as an essential design problem Most previous designs used a plain beanng [2], m which mechanical contact fnction force 1s large and senous Recently the apphcatlon of gas-lubncated bearmgs to the design of nucromotors has been proposed by the authors [7j Figure 3 1s the structural schematic diagram of a gas-lubricated nucrobeanng It 1s called the aerodynamic m-pumping spiral grooved disc thrust bearing It has a flat beanng surface with an annular band of shallow grooves cut mto It, leaving a plain sealing band Gas 1s dragged inwards along the grooves and a pressure rise 1s caused by the restrlctlon at the inward end of the grooves This pressure nse makes the rotor of the mlcromotors separate from the substrate With this kmd of nucromotor nucrobeanng the fnctlon can be reduced greatly The spiral grooved thrust mlcrobeanng has been realized by a process compatible with the fabncatlon of the rmcromotors The spiral grooves are formed by photolithography m the 1st polyslhcon layer Here the 1st PSG layer IS also used for the planarlzatlon interim layer except for the sacnficlal layer All the other processmg steps are the same as those described above Figure 4 shows the mlcromotor (VCM) wth the m-pumpmg spiral grooved mlcrobearmg (a) and the amphficatlon of

2rd

(4

(b) Fig 4 Mxromotor (VCM) with spnal grooved gas mwrobeanng and amphticatlon of grooves m the mwobe.anng (b)

(a)

the grooves m the nucrobearmg (b), m which the groove width and depth are 4 pm and 300 nm, respectively

poly-si 2nd 1st Si,N4 Siol

’ P/Pm t

_AL@d Fig 3 Structural schematic dlagmm of m-pumpmg spiral grooved thrust mlcrobeanng and radial pressure dlstnbutmn m the mwrobearmg (Pa denotes the amblent pressure)

4. Conclusions The electrostatic variable-capacitance mlcromotor (VCM) and the electroquaslstatlc mductlon nucromotor (IM) can be realized by similar polyslhcon surface nucromachmmg processes The only difference m the processes 1s the dopmg control m their rotors The former needs heavy phosphorus doping, and the latter only light doping The stator-rotor gap m the VCM (side-dnve, without any plananzatlon steps) depends on the horuontal process size, that is, the photohthographic precrslon, wlule the gap m the topandbottom-dnve IM (also urlthout any plananzatlon steps) depends on the vertical process size, the control of which appears to be easier

174

The gas-lubrxated rmcrobearmg can be easily used m the design of the nucromotors so that thar frxtlon can be greatly reduced The reahzatlon process of gas mlcrobearmgs IS fully compatible with the fabncatlon process of both the VCM and IM, without any addltlonal processmg steps An aerodynamic spiral grooved thrust mlcrobearmg has been designed and demonstrated m this paper In fact other rmcrobearmgs, mcludmg aerostatic, Journal and drsc (axlal) gas bearmgs, can also be easily realized v&h mlcromachmmg

degrees in electromc engmeermg from Naqing Institute of Technology (Southeast Umverslty), Naqmg, China, m 1983, 1986 and 1989, respectively Since 1989, he has been mvolved m research on integrated flow sensors, temperature sensors, vacuum sensors, moblhty of MOS devices, CMOS ICs, mlcroactuators and so on Dr Huang 1s now an associate professor of Southeast Umverslty and his mam research mterests are ICs, sohd-state sensors, actuators and interface electronics

Acknowledgements

Pan-Song Mao graduated from Naqmg Ins& tute of Technology m 1965, majormg m radio electromcs Since graduation, Mr Mao has been teaching and researching on the design and process of CMOS ICs, mcludmg high-speed Al-gate CMOS ICs, Darhngton complimentary logic ICs, gate arrays, etc at Southeast Umverslty Durmg 19861987, he worked as a vlsltmg scholar at the National Laboratories of Physics and Engmeermg m New Zealand Mr Mao 1s now an assoctate professor and head of the Semiconductor Research Laboratory of the Umverslty

This project was supported by the National Natural Science Foundation of Chma (No 69006402) and the Doctor Foundation of the State Education Commlsslon of Chma (No 9028611)

References 1 M Mehregany, S F Bart, L S Tavrow, J H Lang, S D Sentuna and M F Schlecht, A study of three mtcrofabncated vanable-capacltance motors, Sensors unddctuators, X-A23 (1990) 173-179 2 L -S Fan, Y-C T;u and R S Muller, K-processed electrostatic mxromotors, 1988 IEEE Inf Electron Devrces Meet , San Francuco, CA, USA, Dee 11-14, 1988, pp 666-669 3 S F Bart and J H Lang, An analysis of electroquaslstatlc mductlon muomotors, Sensors and Actuators, 20 (1989) 97- 106 4 W Tnmmer and R J&ens, Harmomc electrostatic motors, Sensors and Actuators, 20 (1989) 17-24 Electrostatic motors, Phrhps Tech Rev, 30 (1969) 5 D Balk, 178-194 6 S F Bart, T A Lober, R T Howe, J H Lang and M F Schlecht, Design conslderatlon for maxomachmed electnc actuators, Sensors and Actuators, 14 (1988) 269-292 7 J -B Huang, Q -Y Tong and P -S Mao, Gas-lubncated mlcro-bearmgs for mlcroactuators, Proc 6th Int Conf Soled-State Sensors and Actuators (Transducers ‘91), San Francrrco, USA, June 24-28, 1991, pp 894-897

Biographies hn-Btao Huang received B SC , M SC and Ph D

Qzn-YL Tong graduated from Tsmghua Umversity, Belling, China, m 1962, and was awarded a gold medal for an outstanding graduate Smce 1963, he has been teachmg and researching on the design and fabncatlon of rmcroelectromc devices and systems m Southeast Umverslty He has done research on Ge-Sl alloy single crystals, fully-compatible low/high voltage CMOS ICs, mtegrated sensors, device models, etc He was an Honorary Research Fellow at the Umverslty of Edinburgh, UK, from 1981 to 1982 and a Senior Research Fellow m New Zealand m 1986 Currently he 1s the director and professor of the MIcroelectronics Centre of Southeast Umverslty, China