Universal selective transfer printing via micro-vacuum force

Abstract

Transfer printing of inorganic thin-film semiconductors has attracted considerable attention to realize high-performance soft electronics on unusual substrates. However, conventional transfer technologies including elastomeric transfer printing, laser-assisted transfer, and electrostatic transfer still have challenging issues such as stamp reusability, additional adhesives, and device damage. Here, a micro-vacuum assisted selective transfer is reported to assemble micro-sized inorganic semiconductors onto unconventional substrates. 20 μm-sized micro-hole arrays are formed via laser-induced etching technology on a glass substrate. The vacuum controllable module, consisting of a laser-drilled glass and hard-polydimethylsiloxane micro-channels, enables selective modulation of micro-vacuum suction force on microchip arrays. Ultrahigh adhesion switchability of 3.364 × 10⁶, accomplished by pressure control during the micro-vacuum transfer procedure, facilitates the pick-up and release of thin-film semiconductors without additional adhesives and chip damage. Heterogeneous integration of III-V materials and silicon is demonstrated by assembling microchips with diverse shapes and sizes from different mother wafers on the same plane. Multiple selective transfers are implemented by independent pressure control of two separate vacuum channels with a high transfer yield of 98.06%. Finally, flexible micro light-emitting diodes and transistors with uniform electrical/optical properties are fabricated via micro-vacuum assisted selective transfer.

Fig. 1. Concept of micro-vacuum assisted selective transfer printing (μVAST).

a An exploded scheme of the VCM with interdigitated μ-channels and μ-holes arranged with regular distances. b Pressure change inside the μ-channel during the μVAST process. c Schematic of μVAST procedure for heterogeneous integration by selectively picking-up microchips from diverse wafers. Colored SEM image of transferred Si arrays with various shapes including pentagon, circle, and star shapes. d SEM image of freestanding μLED arrays on GaAs substrate. e Colored SEM image of transferred μLEDs on the final substrate. f Comparison of adhesion switchability of μVAST with other transfer printing methods.

Fig. 2. Laser-induced etching (LIE) of μ-hole arrays on a glass substrate.

a Schematic illustration of LIE technology on a glass substrate. b 3D XRM image of a laser-irradiated glass. Top and side view XRM image of the laser-irradiated glass. The right upper inset shows an SEM image of laser shots on top of a glass and laser-induced defects inside a glass substrate, respectively. c Comparison of porosity between LAZ and unaffected zone. The data point indicates the mean value and the error bars indicate the standard deviation. d Comparison of chemical wet etch rate between LAZ and unaffected zone. e SEM image of 20 μm-sized μ-hole arrays on a glass substrate. The right upper inset shows a magnified SEM image of a through-glass μ-hole.

Fig. 3. Mechanism analysis of μVAST.

a Optical image of the VCM. The right upper inset shows an OM image of μ-channels and μ-holes. b SEM image of μ-holes surrounded by ring-shaped μ-pillars. The inset displays the magnified SEM image of a μ-pillar. c Cross-sectional SEM image of VCM with μ-holes aligned at the center of each μ-channel. d Nano-indentation results of μ-bridges to experimentally investigate the fracture forces. The inset displays the fracture behavior during the nano-indentation. e Calculated stress distribution at the μ-bridge during the pick-up process. f Comparison of suction force and fracture force of μ-bridge depending on the bridge width. The inset shows the fracture behavior during the pick-up process. g OM images in each step of μVAST. (i) Alignment, (ii) Contact and depressurization, (iii) Lift-up, (iv) Alignment with target substrate, (v) Contact and vent, (vi) Release. h The pressure difference and suction force depending on the diameter of μ-hole. i The pressure difference and suction force depending on the number of μ-hole. j Average transfer yield and suction force during 100 μVAST cycles. The inset shows the SEM image of a μ-pillar after 100 μVAST cycles.

Fig. 4. Universal transfer printing of thin-film semiconductors via μVAST.

a Optical image of transferred μLEDs on a flexible PI substrate. b Optical and SEM images of transferred μLEDs on various substrates including human skin (left), non-sticky rubber sheet (right upper), and paper (right lower). c SEM images of transferred μLEDs on a hornet wing, leaf, and fabric. d Colored SEM images of multiple transfer processes with various chip sizes. (i) Transfer printing of 80 × 80 μm² and 120 × 120 μm² Si chips with a certain distance for 3rd transfer. (ii) 3rd transfer (Si chips with 100 × 100 μm² size) between the previously transferred Si chips. (iii) 4th transfer of 140 × 140 μm²-sized Si chips on the right side of previously transferred chips. e Schematics and results of selective transfer printing enabled by independent pressure control of each μ-channel. The inset shows SEM images of a donor substrate after selective transfer. f SEM images of heterogeneous integration of various microchips. The inset shows the EDS mapping results of AlGaInP μLED, bare Si, and polymer-patterned Si chips on the same plane. The right image exhibits a magnified colored SEM image of thin-film semiconductors with diverse sizes, materials, and thicknesses, transfer-printed on the same substrate.

Fig. 5. Flexible devices fabricated by μVAST.

a Exploded illustration of vertical thin-film μLEDs on PI substrate. b The photoluminescence (PL) maps of μLED arrays before (left) and after (right upper) μVAST. The right lower graph shows the PL spectrum of μLEDs before and after μVAST. c A photograph of a flexible μLED attached to a cylinder (bending radius: 1 cm). The right upper shows an OM image of 10 × 10 arrays of 80 × 80 μm²-sized μLED. The right lower exhibits a cross-sectional SEM image of transfer-printed μLED packaged via ACF. d I–V characteristics of transferred μLEDs and μLEDs on the as-grown wafer. e Luminescence distribution of 10 × 10 μLED arrays transfer-printed on a PI substrate. f Forward voltage and normalized optical power change of flexible μLEDs depending on bending radius. g Exploded schematic of flexible Si transistors on PI substrate. h OM image of a single cell Si transistor, transfer-printed via μVAST. i The transfer characteristics (I_D–V_G) of the flexible transistor.