Research Papers

Microfabricated Instrumented Composite Stamps for Transfer Printing

[+] Author and Article Information
Numair Ahmed

Department of Mechanical Science and Engineering,
University of Illinois at Urbana-Champaign,
Mechanical Engineering Building,
1206 W. Green Street,
Urbana, IL 61801
e-mail: ahmed19@illinois.edu

John A. Rogers

Fellow ASME
Department of Material Science and Engineering,
University of Illinois at Urbana-Champaign,
Materials Science and Engineering Building,
1304 W. Green Street,
Urbana, IL 61801
e-mail: jrogers@illinois.edu

Placid M. Ferreira

Fellow ASME
Department of Mechanical Science and Engineering,
University of Illinois at Urbana-Champaign,
Mechanical Engineering Building,
1206 W. Green Street,
Urbana, IL 61801
e-mail: pferreir@illinois.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received April 25, 2014; final manuscript received March 2, 2015; published online March 27, 2015. Assoc. Editor: Liwei Lin.

J. Micro Nano-Manuf 3(2), 021007 (Jun 01, 2015) (12 pages) Paper No: JMNM-14-1034; doi: 10.1115/1.4030001 History: Received April 25, 2014; Revised March 02, 2015; Online March 27, 2015

Transfer printing is an emerging process that enables micro- and nano-scale heterogeneous materials integration for applications such as flexible displays, biocompatible sensors, stretchable electronics, and others. It transfers prefabricated micro- and nano-scale functional structures, referred to as “ink,” from growth or fabrication donor substrates to functional receiver substrates using a soft polymeric “stamp,” typically made from polydimethylsiloxane (PDMS) with patterned posts for selectively engaging the ink. In high throughput implementations of the process, where several structures or inks are transferred in a single cycle, the ability to detect contact and monitor localized forces at each post during critical events in the printing process allows for the development of a robust and reliable manufacturing process. It also provides a unique vantage point from which to study fundamental issues and phenomena associated with adhesion and delamination of thin films from a variety of substrate materials. In this paper, we present a new composite stamp design consisting of SU-8 cantilevers instrumented with strain gauges, embedded in a thin film of PDMS patterned with posts, and supported by a backing layer. The fabrication of such a stamp, its testing and calibration are discussed. The use of the instrumented stamp in measuring adhesion forces between silicon and PDMS is demonstrated. New modes of programming the print cycle that monitor forces to control the stamp–substrate interaction are also demonstrated. Finally, a classifier-based approach to detecting failed pick-up or release of the ink is developed and demonstrated to work within a transfer printing cycle.

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Fig. 1

Representation of transfer printing using a patterned stamp with four posts, showing the individual process steps: (a) translation and alignment to donor wafer. (b) Selective engagement with ink. (c) Ink retrieval from donor wafer. (d) Contact with acceptor substrate. (e) Device release onto acceptor substrate.

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Fig. 2

Schematic representation of some failure modes encountered during transfer printing process. (a) Devices not properly undercut. (b) Devices that have been accidently removed during fabrication. (c) Angular misalignment of stamp. (d) PDMS stamp collapse. (e) Acceptor substrate defect (waviness/bowing).

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Fig. 3

(a) A commercial off the shelf strain gauge attached to a glass slide forming a cantilever. (b) The cantilevered stamp mounted on the transfer printer. This system was used to perform pilot studies to test the feasibility of using instrumentation to detect different process events during the printing process taken from Ref. [21].

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Fig. 4

Solid model of the composite stamp. (a) Exploded view of the individual layers. (i) PDMS web with posts. (ii) SU-8 layer that forms the cantilevers. (iii) Metal layer with the sensing strain gauge along with the compensation resistors and contact pads. (iv) SU-8 handle layer with windows to allow free movement of the cantilever and optical feedback. (b) Cut out of a stamp showing how the various layers are integrated into the final stack and what the cantilever looks like.

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Fig. 5

(a) Plot of applied force and calculated deflection from the FEA studies. (b) Image of the model used for the FEA. (c)–(f) The four frames from the FEA study done to determine the required dimensions of the composite stamp to achieve the target stiffness. Frames show the deflection of a single stamp when a distributed force of magnitude 1 mN, 2 mN, 3 mN, and 4 mN is applied to the post surface, respectively.

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Fig. 6

Schematic representation of the composite stamp fabrication process showing the constituent layers

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Fig. 7

Image of an array of nine instrumented composite stamps prior to release from the SU-8 mold

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Fig. 8

(a) Close-up image of the composite stamp attached to the transfer printing. The image shows the ACF film used to connect contacts with the interface circuitry and the array of PDMS posts. (b) Image shows the composite stamp attached to the transfer printed and the test setup used for stamp characterization.

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Fig. 9

Schematic representation of interfacing scheme of the instrumented stamps with the microtransfer printer. The stamp is connected to a Wheatstone bridge; the bridge output is supplied to instrumentation amplifiers for signal amplification. The amplified signal is sampled using a data acquisition system. The acquired digital signal is then used in the transfer printer software.

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Fig. 10

Schematic of the experimental setup used for stamp characterization

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Fig. 11

(a) Graph of measured forces as a function of deflection of (the cantilevers in) the composite stamp when each of the four posts is pressed against a substrate. (b) The voltage output from the bridge circuit (for each of the cantilevers) plotted against the applied forces at each of the corresponding posts (insets in both graphs are meant to show the typical standard deviation in the measurements).

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Fig. 12

(a) Graph of voltage signal generated during adhesion testing experiment. The graph shows the voltage trace for repeated adhesion force measurement experiments. (b) Close-up of a single contact and delamination event.

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Fig. 13

Graph of adhesion force between PDMS and silicon and its dependence on delamination velocity for a bulk PDMS stamp and the composite stamp. The error bars represent the RMS error for the measured variable.

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Fig. 14

Schematic representations of a conventional transfer printing cycle (left) and one with an instrumented stamp (right). With an instrumented stamp, the stamp's engagement with the donor and receiver substrate can be controlled by monitoring the output of the strain gauges at each post.

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Fig. 15

Voltage trace recorded for a post during engagement with the donor substrate in a printing cycle. The figure shows how the voltage output from the stamp changes with time during various events during engagement. Motion of the machine Z-stage stops once the output voltage reaches the threshold of 0.75 V.

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Fig. 16

Image of the test donor wafer. (a) Micrograph of training area of donor wafer showing individual ink used for training. (b) Micrograph of teat area of donor wafer showing engineered defect location used for testing error detection.

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Fig. 17

Image of stamp and ink during automated transfer printing process. Only three of the four cantilevers are visible due to the limited field of view of the optics. (a) Image of stamp above the donor substrate. (b) Silicon chips (ink) printed on the acceptor. (c) Finished 2 × 2 array of ink for two of the four posts on the stamp.

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Fig. 18

Plot of the two probability distributions of voltages representing peak delamination forces for successful and failed pick-up of ink in a transfer printing. The distributions are calculated based on the mean and standard deviation calculated from the data captured on training samples.

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Fig. 19

Plot of values calculated by the discriminant function based on the measured voltages during ink pick-up. (a) Plot of calculated values for the entire data set. (b) Close-up plot showing values of discriminant function for failure events in relation to the threshold.

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Fig. 20

Plot of values calculated by the discriminant function during ink printing. The solid line represents the threshold value for discrimination.



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