Transfer Printable Single-Crystalline Coupons for III-V on Si Integration

Abstract

The next-generation internet (6G) requires highly functional devices that e.g. realize frequencies in the THz range for higher data rates and lower latencies. Those requirements exceed the physical limits of established CMOS technologies based on silicon (Si). Hence, there is demand for other semiconductor materials with superior electronic and optical properties that complement Si. One of the key candidates is the III-V compound semiconductor, indium phosphide (InP). Due to its high electron mobility and direct band gap, InP-based devices allow access to frequencies >100 GHz and operate at the optical fibre compatible wavelength of 1.55 μm.1 With the perspective of leveraging the advantages of Si-based CMOS technology and III-V semiconductors, hetero-integration of III-V materials on Si is of great interest. However, existing integration approaches entail certain disadvantages: (i) High dislocation densities due to the lattice mismatch of InP and Si for integration via hetero-epitaxial growth;2 (ii) limited integration density and the requirement of accurate alignment for flip-chip integration; and (iii) high process-related losses of Si and III-V materials as well as thermal stress and low thermal conductivity of adhesive layers degrading device performance for wafer/die bonding technologies.3 Another promising approach for III-V-on-Si integration is micro-transfer-printing (μTP) that involves pick-up and transfer of µm-small chips from a source substrate to a target substrate with high alignment accuracy by using an elastomeric stamp. Advantages of μTP are high integration densities and efficient material use. The technique was already implemented for III-V-on-Si photonic integrated circuits by transfer of epitaxial III-V layers.4 However, using sacrificial III-V interlayers for release and adhesives for bonding still leads to transfer issues and low operation temperature for the devices, respectively.We pursue a new approach to hetero-integration of III-V on Si that aims at the transfer of single-crystalline InP coupons onto Si via μTP. This will be achieved by obtaining crystalline coupons with a thickness of d ≤ 10 µm and two polished surfaces that attain low roughness, needed i.a. for µTP. If the high structural quality of the single-crystalline InP source material can be maintained, this process will provide high quality templates for subsequent epitaxial growth. Towards this goal, we developed a sophisticated micro-preparation process in cooperation with the Leibniz Institute for High Performance Microelectronics IHP.5 Starting from 4-inch single crystals with homogeneous, low dislocation density of 2×103 cm- 2 grown at IKZ,6 thinned InP dies were obtained by sawing, grinding and employing an optimized two-step chemical mechanical polishing (CMP). In order to produce µm-sized transfer-printable coupons, the InP dies were micro structured by means of photolithography assisted patterning and wet (under-)etching (Fig. 1). The coupons can then be picked up with a stamp and transferred to the target wafer. Main innovation of this process is the resin which serves as low stress fixing layer for CMP as well as sacrificial layer for later release.The optimized CMP process with abrasive-free final polishing yielded InP platelets of the desired thickness below 10 μm with low thickness deviation < 1 µm and excellent surface roughness of S q ≈ 0.3 nm (Fig. 2a,b,d). This value even meets the requirements for adhesive-free bonding (S q ≤ 2 nm) and subsequent epitaxial growth (S q ≤ 0.5 nm). X-ray rocking curve mapping provides accurate spatial maps of lattice deformations in the material that may be a consequence of the mechanical processing. Rocking curve widths mappings of the 004 reflection of a (001) sample before and after thinning are homogeneous and below 25 arcsec in the majority of the sample area. Overall no signs of systematic crystal quality deterioration in the product platelets compared to bulk samples have been detected.In summary, the feasibility of μm-thin InP platelet fabrication was demonstrated. Final platelets meet the prerequisites of low and uniform thickness, high planarity, low roughness and little crystal quality deterioration. Furthermore, first InP platelets could successfully be patterned to 100–400 µm-sized coupons using optical lithography and wet etching (Fig. 2c). This opens a path to take the next steps towards hetero-integration on Si by means of µTP with high potential for adhesive-free bonding.