A Coulomb Force Reconfigurable System

Thaddeus GabaraMetaMEMS Corp. in the EDC at NJIT

211 Warren St., Newark, NJ 07103908-405-0094 thad@metamems.com

Abstract

2. Introduction

The force is the charge Q times the electric field Eas indicated in Equ. 2.

The proposed approach is to generate Coulombforces between substrates to controllably move onesubstrate in relation to another substrate [3]. Anisolated Coulomb plate is placed near the surface of asubstrate. Once the plate is charged, and because it issurrounded by oxide, the charge should remain on theplate indefinitely. Thus, either a repulsion or attractionforce can remain in effect forever. This is veryadvantageous since the electric field will always existnear the plate. The forces that can be generatedbetween this plate and another charged plate locatednear the surface of a second opposing substrate can bedetermined. Assume that the plates are metallic andhave an area A and are separated from each other bythe distance of d. Each of these plates can be formed ondifferent substrates. If both plates are charged, a forceF will be exerted between the plates.

As an example to demonstrate the feasibility of thisapproach, assume that there are two silicon substratesare background to a thickness of 65 J.lm and eachhaving a dimension of 1 cm on a side. The force F ofgravity for each of these substrates is about IE-4 N.Just for demonstration purposes, assume that the entiretop surface of these substrates is covered with aCoulomb plate; furthermore, one substrate is flippedand placed a distance d (5 J.lm) above the othersubstrate. The capacitance C between the two platescan be estimated to be around 175 pF (neglecting theoxide layer for now) as indicated in Equ. 1.

Coulomb forces can be created by placing non­volatile packets of charge on at least two opposinginsulated Coulomb plates. Arrays of Coulomb platescan be formed near the surface of several substrates(chips). The Coulomb forces can be used to levitate andmove any top substrates juxtaposed and placed over abottom substrate by dynamically charging/dischargingselected plates in the array of Coulomb plates of eachopposing substrate. The charge on these plates can bealtered using a control unit that dynamically forms aminimum energy surface that allows the elevatedsubstrates to slide into new positions. A larger topsubstrate can be artificially formed by forcing adjacentelevated substrates together in a patterned sequence toform reconfigurable systems that can be physicallyadjusted to operate at a particular frequency range orto perform manipulative functions. For instance,Coulomb Force Levitated Systems can allow differentantenna structures to be generated on-the-jly.

1. Background

Coulomb forces have been demonstrated when ateam measured the deflection of an exposedconducting beam from a reference beam after theexposed beam was charged by an EEPROM typetransistor. The exposed beam was deflected nearly2J.lm compared to the reference beam. Allmeasurements were performed on a single substrate[1 ].

A reconfigurable system using a ThermobimorphCilia array that moves secondary substrates over thesurface of a first substrate by using frictional forceshas been demonstrated by others [2]. However, if theplanar surface of the system becomes non-horizontal,the secondary substrates may become displaced bygravitational forces. An application of a frictionlessforce between the array and the secondary substrateswould be beneficial to overcome this difficulty.

A Coulomb Force Reconfigurable System

c:AC = _0_ Farads

d

F = QE = CV% Newtons

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(2)

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Using the charge equation (Q = CV) and voltageover displacement (V/d) to replace Q and E,respectively and solving for V leaves Equ. 3.

After - larger gap afternegative charge reduction

Before- gapbetween substrates

Reduced charge on Coulomb plate

(a) (b)Figure 3. (a) Side view of two juxtaposed substrates

each with arrays of charged Coulomb plates illustrating a netstable repulsion force; (b) the repulsive fore is decreased

thereby increasing separation.

Coulomb PlateFigure 2. (top) Side view of Coulomb plate charged by

one of two EEPROM transistors; (bottom) corresponding topview.

Coulomb plate. Note that the entire combined structureis surrounded by an insulator (disregarding the probepoint); thus, the plate and gate of the transistors areisolated by oxide. Fig. 2 (bottom) illustrates the topview of Fig. 2 (top).

Although one pair of plates can generate the forcenecessary to overcome the gravitational force, theability to partition the plate into multiple pairs ofCoulomb plates over the surface of the substrateallows: 1) with a large number of pairs of plates,various combinations of potential variations applied toand among them allow a myriad of manipulationsavailable to the levitated substrate; and 2) the stabilityof controlling the levitated substrate becomes easiersince the forces are spread over a larger area.

Fig. 3a illustrates the cross section image of twoopposing substrates. The top substrate has multipleCoulomb plates while the lower substrate has differentCoulomb plates. These plates help to explain how the

(3)

·on~ ce tenS \ -- --SUI ,8 -- --

-- -- -- -- ~aa\'S"aOd~r ;•••......... . .

electrostatic

100

CD~ 10.10oU.

...-..coi 10.5CDZ"'-'"

{dFV=~CVolts

Placing the values for d, F and C in Equ. 3 andsolving for V gives 1.7V.

Thus, by applying a voltage difference of 1.7Vbetween the plates, a force of 1E-4 N can be generated,causing the upper substrate to levitate 5 f.lm above thelower substrate. The planarity of the substrate'ssurfaces can vary +/-1 f.lm over a large area. Theamount of levitation (or required separation) necessaryto insure a smooth lateral transfer may need to beevaluated for each levitated substrate.

Fig. I reproduces a figure from a publication by R.Fearing [4]. Fig.l depicts the gravitational, electrical,van der Waals and water surface tension attractiveforce between a sphere and a plane. Note that the Vander Waal's force is overtaken by the electrostatic forceat around 10-3 m, providing an approximate lowerbound on the levitating substrate dimension in a dryatmosphere.

10.15.....---------------..10-6 10.5 10-4 10.3

Object Radius (meter)Figure 1. The forces between a sphere and a plane [4].

As shown in Fig. 2 (top), the Coulomb plate canhave an optional opening to directly charge the plate bya probe or by either of the two EEPROM transistorsdevices as indicated in the magnified areas. A singleEEPROM transistor can be used to charge anddischarge the plate. The first device on the left can beused to charge the plate to a negative potential whilethe remaining device can inject (positive charges)holes.

Thus, several non-volatile devices can beconnected to the Coulomb plate simultaneously whereeach non-volatile device can perform a specificfunctional procedure of charging/discharging the

A Coulomb Force Reconfigurable System MetaMEMS Corp.

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substrates can be repelled from one another yet remainin a levitation stable state. For example, the two outsideplates from the top substrate are repelled from theunderlying two outside plates within the lowersubstrate, respectively, since they have a like charge(negative). However, the three inner plates (positive) ofthe top substrate attract the three underlying innerplates (negative) of the lower substrate since theircharges are opposite. The corresponding charges on thethree underlying inner plates have an increasednegative charge as indicated by the double "minus"signs. These charge distributions on the plates causesthe two substrates to be separated by a first distance.

In Fig. 3b, illustrates the situation where thenegative charge on the three underlying inner plates aredecreased from the initial value. Thus, the attractiveforce decreases and causes the distance between thetwo substrates to increase which increases theseparation between the two substrates.

3. Controlled Levitation of Substrates

A 2-D view with depth perspective is depicted inFig. 4. An upper substrate contains only one coulombplate in each comer to simplify the description. Theupper substrate is made transparent to show the forcesformed between the upper and lower substrates.Assume that the four plates of the upper substrate arecharged positive. The lower substrate has an array ofplates where the light colored plates are negative andthe darker colored plates are positive. A minimumenergy Coulomb surface is made by applying potentialsin certain sequences to the Coulomb plates.

Figure 4. The inducement of moving the upper substratein the positive y-direction due to the creation of a minimum

energy Coulomb surface.

Suppose the upper substrate is to move in thepositive y-direction, then the energy surface

A Coulomb Force Reconfigurable System

surrounding each of the plates on the upper substrateneeds to have a minimum energy barrier in the positivey-direction and a larger energy barrier in the threedirections of negative y-direction, positive x-directionand negative x-direction. Assume that a first plate(marked with a #) on the lower substrate directly underthe comer plates of the upper substrate is also chargedpositively. Then the upper substrate is repelled from thesurface of the lower substrate. Whatever is done in thevicinity of one of the plates of the upper substrate onthe lower substrate is repeated at the other three comerplates as well; thus, the formation of the minimumenergy surface can be made for one of the upper plateswhich can then be repeated at the three remainingcomer plates. The lower and side plates adjacent to thefirst plate are charged positively (as indicated) forminga semi-circular shape that acts as a barrier to preventthe upper plate from moving in the negative y-directionas well as either of the positive or negative x­directions. Thus, this energy barrier is not at aminimum in these directions. However, in the positivey-direction the barrier is less if the charge of the lowerthree lighter colored plates (in the upper top row of thelower substrate) and the one remaining plate adjacent tothe first plate are set to have a negative charge. Now, aforce exists to move the upper substrate in the positivey-direction. These forces are shaped to allow the uppersubstrate to be levitated and slide in the positive y­direction.

3.1. Reconfigurable RF System

Fig. 5 shows a sub-miniature infrastructure with areconfigurable system which uses Coulomb force tolevitate and position the upper substrates on the topsurface of the lower substrate. The lower substrate canbe a DSP chip or die while the upper substrates can betop substrates.

Once the substrates are fabricated, placed in asystem, and levitated, the levitated substrate iscompletely isolated from the remaining portions of thesystem eliminating leakage current.

To operate in a new frequency range, it would berequired to replace the Rec-A, Tran-A and Inductor-A(see RF Footprint Channel A in Fig. 5a) which wereoptimum for the prior frequency band with the Rec-B,Tran-B and Inductor-B (see RF Footprint Channel Bin Fig. 5e) that are optimum for the new frequencyband. Replacing the current components with newcomponents offers several advantages at the systemlevel. A few of the advantages are elimination of RFswitches which cause loss, reduction of powerdissipation (since only a minimum number of devicesneed to be powered), optimum frequency bandoptimization (since components that target the desired

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frequency band are used) and improved quality factorof the inductor.

(d) (e)

Figure 5. Sequence representing a reconfigurablesystem; (a) initial placement of substrates on a lower

substrate; (b-d) Coulomb forces used to move top substratesinto position; and (e) final position.

The sequences of steps are outlined in Fig. 5bthrough Fig. 5d. Fig 5a shows Ind-A and Rec-B beingmoved apart, Fig. 5b shows Tran-B and Ind-B beingmoved left, Fig. 5c shows Tran-A being moved downand Rec-A moved right, Fig. 5d shows Tran-B beingmoved right and Rec-B moved up, Fig 5e shows thefinal position. Note that these three new "-B" typesubstrates are placed in the same locations as theprevious three old "-A" type substrates. The positionsof the electrical interface are common to all replaceablesubstrates. Communication is reestablished after the topsubstrates make electrical contact after thereconfiguration has completed.

The control unit has a computation component thatdetermines the current position of the upper substratesand calculates the sequence of steps required toreconfigure the upper substrates to achieve the desiredconfiguration.

Sensors located between substrates are used by thecontrol unit to determine the identity, location, verticalposition, velocity, and direction of movement of theupper substrates. One form of the sensors can usecapacitive coupling between a levitated upper substrateand the DSP substrate to transfer this information (seeFig. 5). A sensing unit on the DSP substrate canmeasure the sensor response. This value can be used toderive several physical parameters of the system.Besides the sensor, the parallel plates on the DSP andupper substrates can be used to form capacito.rs. Thesecapacitors can also be used to send/receIve data,signals, clocks, etc.

3.2. Micro-Surface Studies and LOC

A Coulomb Force Reconfigurable System

Fig. 6 (top) illustrates another sub-miniatureinfrastructure where two drops for surface studies areplaced in contact where each drop has a critical anglegreater than 90°. Current flow through the surfacecontact is indicated in Fig. 6 (bottom) Other LaCconfigurations can be formulated where the substratescan contain micro-pumps and regent reservoirs toconduct biological studies.

Contact

l

Measure TransverseFriction Force

lin lout

Measure Ionic ContactCurrent Flow

Figure 6. (top) Two drops with> 90° critical anglerubbed together under a Coulomb force; (bottom) current

flow through a contact between two drops.

3.3. Reconfigurable Antennas

Coulomb forces are used to create various metallicshapes on substrates. These shapes are formed bycoupling a plurality of levitating substrates togetherwhere each substrate contains its own metallic pattern.The substrates are assembled together over a primarysubstrate and the substrates can be positioned parallelto a planar surface of the bottom substrate formingvarious patterns.

Such a capability is a desirable feature for antennaconstruction. Furthermore, the antenna can bereassembled to adjust the physical dimensions of theantenna on-the-fly while being in the consumerproduct. This can provide a better ~at~h th~ an~e~na toa different frequency band or re-adJust Its dIrectIVIty.

The reconfigurability of the system allows theantennas to adjust themselves as the conditions appliedto the system vary. This allows radiation patterns,impedance bandwidths, operating frequency and ~ower

transfer conditions to be controlled and adjustedwhenever required.

For example, a patch antenna connected to an RFfront end is shown in the block diagram in Fig. 7a. Thephysical realization is given in Fig. 7b. Each of the M'sin Fig. 7b is formed with a metallic pattern. The patch

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antenna is comprised the 4X4 metallic substrate array.This array can be moved up and down to space thearray from the ground plane to help match theimpedance bandwidth. The size of the array can beconfigured to be equal to a portion of the RF carrierwavelength.

(a) (b)Figure 7. (a) Representation ofantenna and front end;

and (b) reconfigurable antenna.

Extra metallic substrates are in the two groupingslocated at the bottom half of the substrate. Thus, thePatch can be changed to either a 3X3 or 5X5 patch.This substrate could be used to create MIM0 antennasby insuring that the front end can accept severalinput/output antenna ports. The M's can be used toconstruct: dipole, patch, Yagi, monopole, bow-tie,meanderline and MIMO antennas. Further more,depending on the size of the bottom substrate and thetotal number of metallic top substrates (M), separateantennas designed for outgoing frequencies can bereconfigured at the same time other separate antennaare being reconfigured for different incomingfrequencies. Thus, this reconfigurable antenna systemcan be very diverse and offers great flexibility to thesystem designer by offering on-the-fly antennaadjustment capabilities.

4. Ohmic Contacts

Electrical ohmic contacts would be desirable sinceDC signals can be applied to the top substrate. Fig. 8aillustrates an upper and lower substrate contacting dueto the Coulomb force causing the metallic pads to makecontact as indicated in Fig. 8b.

Upper Substratet_ MetallicCoulomb ~ PostForcet_

Lower Substrate

Figure 8. (a) An upper and lower substrate under aCoulomb force; (b) the attractive force applies pressure tothe posts; and (c) a close-up view of the posts in contact.

A Coulomb Force Reconfigurable System

These posts are metallic and can be formed eitherby etching back the oxide to expose the highest layermetal, or additional processing steps can be performedto deposit the posts over the metallic regions.

A processing step of wet or dry etching can beused to remove the oxide to expose the metallic traces.If the foundation portion has a non-oxide material, anadditional etching step may be required. On the otherhand, a deposition technique such as sputtering can beused to add material to the exposed metal surface tocreate a post.

A detailed material and chemical analysis arerequired to determine the best processing steps toguarantee reliable contacts. The make and breakcontact between the metal posts will demand repeatableperformance.

5. Conclusion

Coulomb forces offer the potential to levitate andmove top substrates over a bottom substrate. Twoprimary challenges need to be demonstrated: I)levitate substrates using Coulomb forces; and 2)develop materials and process for a repeatableopen/closed Ohmic contact. The charging mechanismcan charge the Coulomb plates independently byusing EEPROM type transistors, for instance. Acontrol unit operates from data received from sensorsembedded in the substrates to determine the chargingsequence that would be necessary to control themovement of these elevated substrates. Controlledlevitation has a potential for the development of manydifferent reconfigurable systems as described in thispaper. These systems have potential applications incommunications, antennas, optical fiber alignment,biological sample preparation and military devices.

6. References

[1] Novel electrostatic repulsion forces in MEMSapplications by nonvolatile charge injection Zengtao Liu;Myongseob Kim; Shen, N.; Kan, E.C.; Micro ElectroMechanical Systems, 2002. The Fifteenth IEEEInternational Conference on 20-24 Jan. 2002 Page(s): 598 ­601

[2] Computational methods for design and control ofMEMS micromanipulator arrays Bohringer, K.F.; Donald,B.R.; MacDonald, N.C.; Kovacs, G.T.A.; Suh, l.W.;Computational Science & Engineering, IEEE Volume 4,Issue 1, Jan.-March 1997 Page(s):17 - 29

[3] Patent submission to the USPTO, Thaddeus Gabara.

[4] Survey of sticking effects for micro parts handling, R.Fearing, Intelligent Robots and Systems 95, Proc. 1995IEEE/RSJ Inter, Conf. Aug. 5-9 1995, pp. 212-217.

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