(Invited) Micro-Transfer-Printing at X-FAB: Demonstration of a New Technology for Wafer-Level Packaging
Authors: Wicht Sebastian , Krieger Uwe , Guenther Daniela , Heise Niclas , Naumann Falk , Fecioru Alin 
Published: December 2020

Introduction

Within this publication micro-transfer-printing (µTP) will be introduced as a promising new technology for 3D wafer-level packaging (WLP). The main advantage of µTP arises from the substantial parallel chip transfer from the source wafer to a target device by a visco-elastic stamp. To execute and develop this versatile technology a pilot line has been installed in the X-FAB MEMS Foundry GmbH clean room facilities within a funded EU project (MICROPRINCE). To demonstrate the general process capabilities recent results on the preparation and integration of silicon-based ICs, optical filters and III/V semiconductors shall be discussed.

Contents

The general principle of µTP is based on the utilization of an elastomer stamp to transfer devices or arrays of devices from their native substrate (source) to a non-native target material [1]. A mandatory requirement for this transfer is that the stamp, typically made of polydimethylsiloxane (PDMS), provides tunable adhesion properties to the printable chiplet [2]. Thereby, high adhesion during the “pick-up” as well as low adhesion forces during the “placing” steps can be achieved.
The general process flow for micro-transfer-printing is illustrated in Figure 1 .Therein, the main advantage of µTP, namely the extensive parallelization of the chip placement (schematics a.-e.), can be recognized. Such a parallelization offers the possibility of a major process time reduction.
Prior to the transfer operation, the source wafer has to be prepared for the printing process. This involves the formation of reliable anchor and tether structures which carry the printable chiplets after the release etch. Another crucial requirement on these structures is that they provide a controlled micro-fracture and release the transfer chiplets to the stamp [3]. An example of a print-ready optical filter device is furthermore illustrated in Figure 1g.
The target on the other hand has to be coated with an adhesion providing polymer (see red layer in Figure 1d.-f.) for which bisbenzocyclobutene (BCB) is applied at X-FAB.
After the printing itself, a post-processing comprising the patterning of applied adhesion media, a wiring via a Cu redistribution layer (RDL) and a passivation is required to allow the functionality of active devices (see Figure 1f.). Figure 1h shows two 3D-integrated demonstrator devices which have been printed on a XL035 technology from X-FAB and which were afterwards connected via a Cu-RDL layer. Within the printing experiments of these chiplets a print yield of about 99.9% with a placement accuracy of down to 3σ ≤ 0.78µm has been achieved.

Conclusions

By this publication the Micro-transfer-printing (µTP) technology and its technical implementation will be introduced. To build up and develop µTP processes a pilot line has been installed in the X-FAB MEMS Foundry GmbH via a funded ECSEL project called “MICROPRINCE”. To execute the µTP technology for 3D integration several process steps have to be executed to prepare a source wafer for the printing operation, transfer the chiplets to a non-native target and connect them afterwards during the prost-processing. The process sequence itself will be explain in more detail within the presentation on examples of optical filters, silicon ASICs as well as III/V semiconductor chiplets.

Acknowledgements

This work is funded by ECSEL JU under grant agreement No 737465 and by the BMBF No ESECS16204. Further, the authors like to thank all X-FAB internal and external MICROPRINCE partners for their support.

References

[1] C. A. Bower, M. A. Meitl, S. Bonafede and D. Gomez, ”Heterogeneous Integration of Microscale Compound Semiconductor Devices By Micro-Transfer-Printing”, IEEE 65th ECTC Proceedings (2015).
[2] M. A. Meitl, Z.-T. Zhu, V. Kumar, K.J. Lee, X. Feng, Y.Y. Huang, I. Adesida, R.G. Nuzzo, and J.A. Rogers, “Transfer Printing by Kinetic Control of Adhesion to an Elastomeric Stamp,” Nature Materials 5, 33-38 (2006).
[3] J. Yoon, S.-M. Lee, D. Kang, M. A. Meitl, C. A. Bower and J. A. Rogers, „Heterogeneously Integrated Optoelectronic Devices Enabled by Micro-Transfer Printing”, Advanced Optical Materials 3, 1313-1335 (2015).

Figure
1
: Schematic illustration of the µTP process (a. - f.), a released optical filter (g.) and two printed and contacted demonstrator devices (h.). Details of the process sequence: a.-c. device pick-up from source wafer, c.-e. transfer and “diluted” placement on the target and f. target post-processing including adhesive patterning and metallization via RDL layer.

Figure 1