Molded electromechanical shift register memoriesGary M. McClelland and Bayu Atmaja Citation: Applied Physics Letters 89, 161918 (2006); doi: 10.1063/1.2364458 View online: http://dx.doi.org/10.1063/1.2364458 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Six-input lookup table circuit with 62% fewer transistors using nonvolatile logic-in-memory architecture withseries/parallel-connected magnetic tunnel junctions J. Appl. Phys. 111, 07E318 (2012); 10.1063/1.3672411 Asymmetric magnetic NOT gate and shift registers for high density data storage Appl. Phys. Lett. 96, 262510 (2010); 10.1063/1.3457998 Bidirectional magnetic nanowire shift register Appl. Phys. Lett. 95, 232502 (2009); 10.1063/1.3271683 Amorphous silicon shift registers for display drivers J. Vac. Sci. Technol. A 22, 981 (2004); 10.1116/1.1722376 Layered tunnel barriers for nonvolatile memory devices Appl. Phys. Lett. 73, 2137 (1998); 10.1063/1.122402

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Molded electromechanical shift register memoriesGary M. McClellanda� and Bayu Atmajab�

IBM Research Division, Almaden Research Center, 650 Harry Road San Jose, California 95120

�Received 16 March 2006; accepted 11 September 2006; published online 20 October 2006�

Synchronous, nonvolatile, shift register memories storing �1000 data bits can be constructed of aseries of bistable mechanical levers, interacting electrostatically and actuated by a single electrode.An entire memory can be formed by a single molding step. A working laboratory-scale model hasbeen constructed, and two submicron designs have been modeled. In one design using conductinglevers, momentum briefly carries information. In another design using charged levers, informationis passed between primary and secondary portions of a memory cell. Input and output, errorpropagation, fabrication tolerance, and memory integration are discussed. © 2006 AmericanInstitute of Physics. �DOI: 10.1063/1.2364458�

Many proposals for mechanical memories1–5 offer littlepractical advantage over electronic memories because theymust be electronically written and read as well as moved.And electrical contacts can be unreliable. However, nanotubememory architecture could simplify these issues.6 Heinrichet al. have constructed devices of interacting CO molecules,7

but these cannot store a sequence of data. Here we describenonvolatile shift register memories that can be formed by asingle molding step. The memory elements interact electro-statically, so that only two conductors are required to storekilobits of information. Fabrication would require only wetprocessing and no precise alignment. Many shift registerscould be fabricated together in large sheets with edge con-tact, enabling large memories.

A simple shift register can be formed from a series ofbistable conducting “levers,” all electrically and mechani-cally connected, and all actuated by a single electrode �Fig.1�. First consider just two levers pointed toward each other�Fig. 1�a��. An electric field pulse �Fig. 1�b�� brings charge tothe levers’ ends, repelling both levers to their other stablepositions �Fig. 1�c��. In a shift register �Fig. 1�d�� data areentered by moving the leftmost input lever to the forward �F�or backward �B� position. As in Fig. 1�b�, a pulse pushesapart any two levers leaning toward each other. The net ef-fect is to move data forward �to the right�.

This scheme fails if two adjacent levers are both forward�Fig. 1�f��, in which case no force pushes the last of theselevers �circled in Fig. 1�g�� backwards. To avoid this problema 0 can be encoded by one B, and a 1 encoded by an Ffollowed by a B �Fig. 1�h��. Disallowing adjacent F’s givesthe shift registers its directionality.

Given that the proper motion of a lever depends on thelever after �to the right of� it, how can the register end? Thelast lever behaves properly if it is followed by an additionalrigid element �“the terminator”� shaped like a B lever �Fig.1�i��. Supposing an error creates two sequential F’s, we findthat the register evolves to remove the sequential F’s withonly the loss of 2–3 bits, which can be restored by an errorcorrection code. To initialize to all B’s in an N-lever registerfabricated with randomly oriented levers, the register can be

shifted a few more than N times, while terminating with a Band always writing a B.

We made a working model of this memory with pivot-ing, conducting paper levers and pulses of several kilovolts8

�Fig. 1�i��. To read, a light beam sensed the position of the

a�Author to whom correspondence should be addressed; electronic mail:mcclell@almaden.ibm.com

b�Present address: Department of Chemical Engineering, Stanford Univer-sity, Stanford, CA 94305.

FIG. 1. Shift register based on field-induced repulsion. �a� Two conductinglevers each in one of two stable positions, B and F. �b� A field brings chargeto the ends. �c� Repulsion has flipped the levers. �d� A shift register. �e� Aftera pulse. �f� Two sequential forward lever positions. �g� Improperly advancedinformation. �h� An encoding scheme. �i� A laboratory-scale memory. �j� Amemory formed from a conducting, swollen polymer. �k� Side view �beforeswelling� of �j�, viewed from the right. �l� Optimized design.

APPLIED PHYSICS LETTERS 89, 161918 �2006�

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final lever. To write data into the first lever before shifting,we could have flipped it by a separate electrode, but weinstead “broadcasted” the input and shift pulses using asingle electrode. The first input lever was modified by addingweights above and below the pivots to increase the momentof inertia. The weights were balanced to nearly cancel thegravitational torque. The input lever was flipped by a 2.7 kV,300 ms pulse, and the data were shifted by an 8.5 kV, 40 mspulse. A sub-micron input lever could be differentiated by itsgeometry.

A microscopic memory can be molded as rectangulartrenches �Fig. 1�j�� into a conducting soft elastomer with lowviscoelastic damping �e.g., poly�dimethylsiloxane��. To makethe walls bistable, the substrate would be held fixed whileswelling the elastomer9 �e.g., by a solvent which is thencross-linked� to buckle the walls �indicated by the arrows inFig. 1�j��. In our “molecular dynamics” simulations,8 we findthat for an elastic modulus of 900 kPa, swelling of 26% byvolume, 40-nm-thick walls, and other dimensions indicatedin Fig. 1�k�, a 2.7 V, 40-ns-long pulse flips the levers in60 ns.

How well must the geometry be controlled? Irrespectiveof its neighbors, each lever is pulled upward by a voltagepulse V. If a lever is too weak, this force flips the lever, evenif it and another lever are not pointed toward each other. Theupper limit of V is set by this effect, while the lower limit isset by the need to flip properly a pair of levers pointed to-ward each other. If the levers have too large a range of stiff-ness, there will be no V for which the memory shifts prop-erly. We have determined the stiffness tolerance, i.e., thefractional range of stiffness variation among levers whichallow operation with a single V �Table I�. We optimized thetolerance to ±21% for the geometry of Figs. 1�j� and 1�k�. Bylowering the ends of each lever �Fig. 1�l�� to concentrate thefield, the tolerance was increased to ±29%.

If V and all dimensions are scaled by a factor �, the timescales as �, while the elastic barrier scales as �3. At 350 K,and at an attempt frequency of 10 MHz, a barrier of 2�10−19 J holds the thermal flipping rate to 10−4 /y. For thegeometry of Fig. 1�j�, this barrier corresponds to a Young’smodulus of 900 kPa, and can be surmounted by V=2.7 V. Toscale smaller, the modulus must be increased by �−3 to main-tain the flipping barrier while increasing V by �−1/2 to main-tain the electrostatic energy.

Consider generally the state of a memory cell called C2situated behind �i.e., to the right of� C1 and in front of C3.While shifting, C2 must assume the state of C1, while re-membering its state to pass it on to C3. In the design of Fig.1, this conflicting requirement is resolved by the behavior of

momentum in the lever L2 in cell C2. This momentum actsas a sort of temporary buffer. During a pulse, L1 gives itsstate in the form of momentum to L2. Although L2 has ac-quired momentum, its position changes only gradually, sothat its original position can be encoded in the momentum ofL3. Then L2 moves to its new position, determined by itsmomentum.

In a liquid the momentum is quickly damped, and thedesign of Fig. 1 fails. An alternative is to include in each bittwo levers, called a primary �P�, which is the quiescent stor-age element, and a secondary �S�.10 In the first half of a shiftcycle, information is transferred from the P to the S lever ofa cell. In the second half, it is transferred from the S to thenext P lever.

Without additional lithography steps, P and S can beoppositely charged8 and both up and down fields are used�Fig. 2�. In a downward field, the positive lever is pulleddown into one of its two bistable positions, in Fig. 2�b�, theposition leaning toward the negative lever. The negative le-ver is not bistable but is pulled upward and is slightly at-tracted to its neighbor. When the field is reversed, the posi-tive lever is pulled up, while biasing the negative lever to bepulled toward it into the left bistable position �Fig. 2�c��.Information has been transmitted between the levers.

Figure 2�d� diagrams a moldable shift register using thisprinciple. We modeled flexible levers by rigid pivotedbeams, damped to approximate a liquid of viscosity of0.5 cP. A 16-lever memory was modeled using periodicboundary conditions.8 At the end of the P and S levers arecharges of +2.35�10−16 and −2.35�10−16 C, respectively.A rigid post with an end charge of −1.67�10−17 C biases the

TABLE I. Manufacturing tolerance of elastomer memories. The table en-tries are the � percent range of lever stiffness variation allowable for properoperation of the geometries of Figs. 1�k� and 1�l�.

Electrode spacing �nm�

Flat top Optimized

35 55 75 45 55 90

Nearest leverspacing �nm�

32 21 17 12 28 29 2352 18 17 15 24 23 2272 12 14 13 20 21 21

FIG. 2. Shift register using a two-lever cell. �a� Two insulating levers in zerofield. �b� A positively charged lever is in the rightmost of two bistablepositions, pulling the other lever slightly toward it. �c� Reversing the field,the positive lever has caused the negative lever to be pulled into its leftmoststable position. �d� Perspective view of shift register, with positive primarylevers, negative secondary levers, and fixed negative posts. �e� Field for ashift cycle. �f�–�j� P at the times indicated in �e� drawn as plan views.

161918-2 G. M. McClelland and B. Atmaja Appl. Phys. Lett. 89, 161918 �2006�

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P’s outward and limits the outward motion of the P’s to 28°from the vertical.

A 4 bit section of a shift register is pictured in Fig. 2,with the P and S components of the Nth cell labeled by �and �. The data shifts between successive cells by the at-tractive interaction of P1+ with S1−, S1− with P2+, etc.Figures 2�f�–2�j� are frames of the lever motion in the exter-nal field plotted in Fig. 2�e�. In Fig. 2�f�, the memory is in itsquiescent state at a field of E=−3.2 V/�m �downward�. Allthe P’s point outward from the center line �called the 0 po-sition�, except for P2 which is tilted inward in the 1 position.To start shifting, E is first lowered to −1.8 V/�m for 1 �s,then reversed in direction to 5.2 V/�m, pulling the P’s to-ward the vertical �Fig. 2�g��; S2 is attracted by P2 and theexternal field to tilt inward. Next E is switched to zero, al-lowing P3 to be attracted by S2 �Fig. 2�h��. Switching E to−5 V/�m hastens the return to the quiescent state �Fig. 2�i��.After E has been returned to −3.2 V/�m, the 1 state of P2has been transferred to P3 �Fig. 2�j��. A P starting at 0 �out-ward� �e.g., P1� never causes its S to be pulled toward thecenter.

The specific geometry here suffers from the interactionof neighboring cells, a problem which can probably be cor-rected by further optimizing the geometry. For example, twosequential inward primary positions give a spurious interac-tion with inappropriate data shifting.

To sense the last lever, an optical reader could be used ifthe registers were to be molded into the surface of a wave-guide. The final moving lever would be shaped to modulatetransmission of light between the register and the waveguideelectrode above. These input and output schemes could beincorporated into a cross bar architecture, in which a 2 kbitspiral shift register �Fig. 3�a�� is positioned at each crossingof a rectangular array of molded waveguides �Figs.3�b�–3�d��. Using the monopolar design, data can be shiftedin all registers along a word row by applying a voltage pulseto the row while selecting the bit lines to be read. To read therow, the light propagated between the row and each columncan be sensed.

For a low cost/bit, many memory layers need to be as-sociated with a driver chip. A cross bar memory needs onlyto be addressed along a single diagonal edge �Fig. 3�d��. Byshingling the layers �Fig. 3�e��, a memory layer 1 cm widecan be attached directly to the driver chip by large contacts,which need not be precisely designed. Optical detectionwould enable remote sensing on the complementary metal-oxide semiconductor chip.

Typically, in microelectronics, a variety of functions areenabled by forming several types of elements from differentmaterials and varying their attachments. In the two moldedmemories we have described, different functions arise fromvaried geometries and charges of a single material. Usingonly local interactions, data is accessed using only two con-

ductors. Different types of elements �here, the input and stor-age levers� are “tuned” to respond to different signals be-tween the conductors, eliminating the need for preciseoverlay. The sense elements can be formed to couplewaveguides molded simultaneously with the memory.

The authors are grateful to Loan Vo for her stimulatingexperiments and Sally Swanson for help with chemistry. Theauthors enjoyed helpful discussions with Charlie Rettner andJim Hedrick.

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8See EPAPS Document No. E-APPLAB-89-207643 for further descriptionof the laboratory-scale model, details of the simulations, and video anima-tions of the results of the simulations. This document can be reachedthrough a direct link in the online article’s HTML reference section or viathe EPAPS homepage �http://www.iap.org/pubservs/epaps.html�.

9J. N. Lee, C. Park, and G. M. Whitesides, Anal. Chem. 75, 6544 �2003�.10In the electrical engineering literature, this primary-secondary design has

been called “master-slave.”

FIG. 3. Integration of an array of shift registers. ��a�–�c�� Shift registersmolded in a waveguide and operated between an array of crossedwaveguides. �d� Connections required along only one edge. �e� Shinglingthe shift registers of F.

161918-3 G. M. McClelland and B. Atmaja Appl. Phys. Lett. 89, 161918 �2006�

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