Design of Tool for Exfoliation of Monocrystalline Microscale Silicon Films

Abstract

This paper presents the development of a prototype exfoliation tool and process for the fabrication of thinfilm, single crystal silicon, which is a key material for creating high-performance flexible electronics. The process described in this paper is compatible with traditional wafer-based, metal-oxide-semiconductor (CMOS) fabrication techniques which enables high performance devices fabricated using CMOS processes to be easily integrated into flexible electronic products like wearable or Internet of Things (IoT) devices.The exfoliation method presented in this paper uses an electroplated nickel tensile layer and tensioncontrolled handle layer to propagate a crack across a wafer while controlling film thickness and reducing the surface roughness of the exfoliated devices as compared with previously reported exfoliation methods.Using this exfoliation tool, thin-film silicon samples are produced with a typical average surface roughness of 75 nm and a thickness that can be set anywhere between 5 μm and 35 μm by changing the exfoliation parameters. Ward 3 implantation, [10,11] but these approaches have disadvantages. For example, CMP requires that the entire backside of the wafer be ground down, which is a slow and expensive process, while ion-beam implantation does not allow for a device to be prefabricated on the bulk substrate before becoming a thin-film. Another promising technique for creating thin-film single crystal silicon that leverages existing CMOS infrastructure is exfoliation (also referred to as "kerfless-wafering" or "spalling"). In this process, the top layer of a silicon wafer is mechanically removed by brittle fracture, including any devices contained in that layer. The fracture is driven by a tensile layer placed on top of the bulk substrate that creates a stress concentration just below the surface. Once the stress intensity reaches the fracture toughness of the material, the crack will propagate. A similar phenomenon occurs as a mode of failure in other applications, where dissimilar coatings coupled with thermal stresses cause the substrate to fracture [12]. The stress intensity can be increased by raising the stress in the tensile layer or by applying an external load.Several methods of exfoliation have been developed, each with a different method of creating the tensile stress and applying the final load. In 1986, Tanielian et al.[13] first mentioned exfoliation as means to produce thin-film single crystal silicon in a patent, but neither commercial results nor discussion in literature can be found. The patent describes a process that only uses thermal stresses in the tensile film to propagate the crack and does not implement any kind of control. A similar method was demonstrated by Dross et al. [14] using printed metal pastes which are thermally treated so that as the wafer cooled, the stress in the tensile film would increase until A c c e p t e d M a n u s c r i p t N o t C o p y e d i t e d Journal of Micro-and Nano-Manufacturing. Ward 4 sufficient energy was present to spontaneously gene...