One of the challenges in integrated microsystems is the development of multimaterial systems with new functionalities. Heterogeneous integration techniques offer the capability of combining various materials and devices, for instance, optics, electronics, and microfluidics, onto a preprocessed host substrate. Among the demonstrated techniques, fluidic self-assembly (FSA) is widely used for heterogeneous integration because it provides effective self-alignment and parallel manipulation of micro/millimeter scale devices, which are realized in their preferential growth substrates through specific fabrication procedures. However, one of the challenges to be solved in FSA due to its stochastic nature is that the devices might not reach the desired integration locations. In other words, an efficient external guiding mechanism is essential to steer the devices to their predefined binding sites. To address this issue, different guiding mechanisms such as pulsating flow, ultrasonic vibration, magnetic field and electric field have been studied.
The FSA approach with an efficient external guiding mechanism benefits parallel integration especially for highly dense microsystems. However, most of the demonstrated devices are thick and bulky compared with thin-film devices having a thickness of 0.8-3 μm.
On the other hand, the pick-and-place (PAP) approach is comparatively straightforward and useful for a microsystem that does not require massive and dense integration. It can correct the stochastic limitation of the self-assembly process and provide the potential of dexterous manipulation and complex operations with the help of advanced robotic kinematics and automation strategies. However, with the decrease in device dimensions, the conventional PAP method requires modification. For instance, its requirements include safe handling of the devices and tackling the sticking problem during the device releasing step. Compared to a thin-film device, most of the reported devices integrated utilizing PAP and capillarity are thick in nature. Therefore, the risk of damaging them in the storage and handling steps is low.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
A method for self-aligning a thin-film device on a host substrate is provided. A predetermined location on a host substrate is treated with a hydrophobic lubricant to alter its interfacial energy. A needle is used to transfer a thin-film device, under water, to the location. Upon contact with the lubricant, the device adheres and self-aligns to the location to minimize the interfacial energy.
In a first embodiment, a method for self-aligning a thin-film device on a host substrate is provided. The method comprises steps of: preparing a host substrate by depositing a first hydrophobic lubricant on at least one predetermined location on the host substrate; releasing a thin-film device under water from a carrier substrate, wherein the thin-film device is attached to the carrier substrate by a water-soluble polymer; picking up the thin-film device with a hydrophobic needle having a second hydrophobic lubricant at a tip of the hydrophobic needle; moving, while in a water environment, the hydrophobic needle with the thin-film device to the host substrate and contacting the thin-film device to the first hydrophobic lubricant on the predetermined location of the host substrate, the step of moving occurring with the thin-film device, the host substrate and the hydrophobic needle are under water; permitting the thin-film device to adhere and self-align with the predetermined location due to interfacial energy minimization; and evaporating the hydrophobic lubricant.
In a second embodiment, a method for self-aligning a thin-film device on a host substrate is provided. The method comprising steps of: preparing a host substrate by depositing a hydrophobic lubricant on at least one predetermined location on the host substrate; picking up the thin-film device with a magnetic needle; moving, while in a water environment, the magnetic needle with the thin-film device to the host substrate, and contacting the thin-film device to the hydrophobic lubricant on the predetermined location of the host substrate, the step of moving occurring with the thin-film device, the host substrate and the magnetic needle are under water; permitting the thin-film device to adhere and self-align with the predetermined location due to interfacial energy minimization; and evaporating the hydrophobic lubricant.
This brief description of the invention is intended only to provide a brief overview of subject matter disclosed herein according to one or more illustrative embodiments, and does not serve as a guide to interpreting the claims or to define or limit the scope of the invention, which is defined only by the appended claims. This brief description is provided to introduce an illustrative selection of concepts in a simplified form that are further described below in the detailed description. This brief description is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.
So that the manner in which the features of the invention can be understood, a detailed description of the invention may be had by reference to certain embodiments, some of which are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only certain embodiments of this invention and are therefore not to be considered limiting of its scope, for the scope of the invention encompasses other equally effective embodiments. The drawings are not necessarily to scale, emphasis generally being placed upon illustrating the features of certain embodiments of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views. Thus, for further understanding of the invention, reference can be made to the following detailed description, read in connection with the drawings in which:
This disclosure demonstrates the integration of thin-film photonic devices onto a silicon-based host substrate utilizing surface tension-driven FSA and a modified micro-PAP approach. The thin-film format of optical devices allows a topological flat surface after its integration. This leads to a unique advantage: layer-by-layer integration.
The disclosed integration approach provides exciting opportunities for heterogeneous integration of thin-film devices with high repeatability and improved process yield.
In one embodiment a SiO2/Si host substrate was prepared with selective formation of lubricant on binding sites as shown in
Captured pictures in
Thin-Film PD Fabrication
The GaAs metal-semiconductor-metal (MSM) PDs are separately grown, optimized, and fabricated on their preferred growth substrate. The material structure of the starting wafer is as follows: GaAs PD layer/AlAs etching stop layer/GaAs growth substrate with thicknesses of 1/0.2/350 μm, respectively. The top metal patterning with Cr\Au (20\180 nm) was done with a bilayer lift-off process, which includes SF11 and AZ photoresists lithography and thermal evaporation, as shown in
A droplet of a solution comprising 25% polyacrylic acid (PAA) and water (1:3 v/v) was dispensed onto a cover glass, and the PDs were placed on top of the PAA overnight (see
Host Substrate Preparation and Selective Formation of Lubricant
SiO2 (1 μm)-coated silicon wafer was employed as a host substrate (
Treatment of the Needle and an Array Integration of the Thin-Film PDs
A needle used for PD integration was prepared by depositing Cr\Au (20\180 nm) on its tip area using thermal evaporation and KAPTON® tape. After peeling off the tape, only the tip area was coated with gold (
The needle was used to pick up one PD from the fabricated GaAs PD array on the cover glass and was coarsely aligned with the binding sites using the same PAP method as described elsewhere in this specification. During picking up the thin-film PDs, the needle tip was carefully lowered to avoid damage to the PDs caused by excessive pressure. Once the PD touched the lubricant on the integration pads with coarse alignment, the needle was moved up and the PD became attracted by the larger amount of lubricant on the substrate. Due to the interfacial energy minimization, the PD was self-aligned onto the integration locations. Before picking up another PD from the array, the needle touched the lubricant again to form the attracting medium. A 2×2 array of the integrated thin-film PD was completed by repeating the PAP steps. Postprocessing was carried out to ensure the stable electrical contact between the PD and its substrate. The OPTICLEAR® S2, which served as the attracting medium in both guiding and self-assembly, can be easily evaporated with little effect on the PD and substrate. The array of integrated thin-film GaAs PDs on the silicon substrate is shown in
The alignment tolerance of the integrated PDs was also studied. In
The Au—Au contact resistance between the device pads and integration pads was studied. The test device, substrate, and integrated device are shown in
All of the measurements resulted in a resistance of 4.0 ohms. The actual resistance in each of the two gold contacts between the device and the host substrate was experimentally found out to be 0.1 ohms while taking into account the effects of the reference measurements (arrows 1000). As each of the contacts has an area of 50 μm×150 μm, the specific contact resistance can be calculated to be 7.5×10−6 ohms per cm2, which is acceptable compared with reported values.
In this disclosure a technique to heterogeneously integrate thin-film photonic devices on a silicon-based host substrate was demonstrated. Surface tension-driven FSA and micromanipulator-based modified PAP method were used as device integration and guiding mechanisms, respectively. This enables the efficient handling and precise alignment of thin-film photonic devices. GaAs-based thin-film MSM PDs were fabricated in their preferential growth substrate through their specific fabrication procedures. Detailed description of different steps starting from the host substrate preparation, PD release using water-dissolvable polymer, and modification of the PAP tool have been provided along with the fabrication of thin-film GaAs-based MSM PDs. Electrical measurements were performed on the integrated PDs to confirm the functionality of the disclosed integration technique. Thin-film PDs self-aligned to the predefined binding sites on their host substrate due to free energy minimization. This technique opens new avenues for the development of multifunctional photonic systems consisting of multimaterial sub-blocks realized and optimized in their suitable growth substrates and fabrication protocols.
Magnetic Field Based (MFB) Pick-And-Place (PAP) Method
Magnetized Carbon Steel Needle Tip
A carbon steel (CS) needle tip (TED PELLA, INC.) was employed to pick up, transport, and place III-V thin-film devices in water environment. With a cylindrical permanent magnet (N52, NdFeB from K&J Magnetics, with dimensions ⅛″× 1/16″) adhering to it for several days, the CS needle was magnetized. After taking out the magnet, the magnetized needle was mounted into a micromanipulator.
Thin-Film Device Fabrication
LT-grown GaAs based thin-film inverted MSM photodetectors were fabricated in their suitable growth substrate. The starting wafer for the device fabrication consisted of the following layers: LT-grown GaAs device layer/AlAs sacrificial layer/semi-insulating GaAs growth substrate with thickness of 2.5 μm/0.25 μm/350 μm (
Host Substrate Preparation
The host substrate used for this work is a 1 μm thick silicon dioxide thermally grown silicon wafer (
Integration
The integration initiated from the introduction of the magnetized CS needle to close proximity of thin-film PD immersed in fluidic environment (
Self-Alignment of the Device
Self-alignment is one of the most attractive aspects of fluidic self-assembly. A fine probe tip was employed to create a lateral displacement of the integrated device to demonstrate its capability of self-alignment (
Alignment Accuracy and Electrical Characteristic of the Integrated PD
After the integration was completed, a series of post-processing steps were carried out to make a stable metal-to-metal contact between the PD and binding sites on host substrate. The water for the integration was drained out, and the sample was placed in an oven to evaporate the lubricant completely.
A Keithley 2400 SMU (source measurement unit) and a wafer probe station were used for the measurement of dark current and photo current of the PD to verify the electrical contact of the device and substrate (
Magnetization of CS Needle Tip for the Proposed Integration
For a stable PAP process, it is desirable to produce a proper magnetic force at the CS needle tip by modifying its properties. While weak magnetic force cannot pick up a thin-film device, much stronger magnetic force causes jumping of the device during the pick-up step. The CS needle tip was magnetized by a magnetic cylinder (N52, NdFeB from K&J Magnetics, with dimensions ⅛″× 1/16″) for several days. After separating them, the magnetic flux density at the needle tip was measured by a portable digital Tesla meter (model No. HT20@Shanghai Huntoon Magnetic Technology Co., Ltd). The needle tip was brought into contact with the effective sensor head to obtain its magnetic flux intensity value. The magnitude of the induced magnetic field at the tip can be controlled by changing the orientation of the attached cylindrical permanent magnet. For example, the tip with 6mT can be adjusted to 4mT by applying different polarities of the permanent magnet as shown in
Device Storage in Water and Pick-Up Step
This integration process can be further developed by saving the thin-film device array on a substrate in water environment instead of the Mylar diaphragm. Subsequently, with advance automation strategies, the proposed integration approach can directly pick up devices, transport them to the host substrate, and place the devices onto their binding sites and complete the integration. This can be done with help of a water soluble polymer polyacrylic acid (PAA). A droplet of a solution comprising 25% Polyacrylic acid (PAA) and water (1:3 v/v) was dispensed onto a cover glass, and the devices protected by APIEZON® wax were placed on top of the PAA for overnight (
The picking-up of these devices from host cover glass was also studied by the magnetized needle tip with magnetic flux density 4-5 mT (
Force Balancing Analysis for Device's PAP
Simulations were preformed to provide a quantitative comparison between the forces involved in the integration technique. The forces taking into consideration were the attractive magnetic force between the magnetized needle tip and the thin-film device (Fmagnetic(device & needle tip)), the capillary force between device and lubricant (Fcapillary(device and lubricant)).
Numerical simulations for restoring capillary forces between the device and lubricant were performed with help of SURFACE EVOLVER software. Values for different parameters were taken from the literature. The surface energies considered are 53.3 mJ/m2, 47 mJ/m2, and 1 mJ/m2 for the interface of lubricant-water, SAM-water, and lubricant-SAM, respectively. The area of the binding sites is 45 μm×80 μm and dimension of the device is 200 μm×100 μm×2 μm.
F=0.577·B2·A (1)
where F represents the attractive magnetic force, B represents the induced magnetic flux density and A represents the magnetized needle tip pole area. It can be observed from
From the above force calculations, it can be concluded that during the device releasing step, the capillary force between the device and lubricant (Fcapillary(device and lubricant)) is considerably greater than the attractive magnetic force between the magnetized needle tip and the thin-film device (Fmagnetic (device & needle tip)). Also it can be shown that magnetic attractive force is larger compared to the reported experimental values of adhesive force between different flat surfaces under water ensuring safe handling of ultrathin devices during pickup step.
Restoring Forces for Planar Displacement
The SURFACE EVOLVER was also employed to estimate the planar restoring force and investigate the possible maximum lateral displacement of the thin-film device for successful fluidic self-alignment. As shown in
Hydrodynamic Force Study
To discuss the maximum speed the tip can move in water when a device is loaded at a given magnetic flux density, the hydrodynamic force analysis was performed as simplified in
The fluidic drag force Fd can be calculated by
where CD is the drag coefficient for a flat surface, AD is the characteristic area of the device (200 μm×100 μm), ρω is the density of water (999.73 kg/m3) and ν is the average velocity, Re is Reynolds number and CD is obtained by the empirical expression of Re, L is the characteristic linear dimension (100 μm), and μω is the water viscosity (1.002×10−3 Pa·s at temperature 20° C.). Considering the motion is overdamped, i.e. the viscous time scale largely exceeds the inertial time scale, the maximum average velocity of the magnetized needle in water can be calculated as 6.8-9.7 mm/sec based on the above equations and the estimated holding force.
A technique for the integration of an ultrathin device onto a silicon based host substrate has been reported in this work. Magnetic-field-based modified PAP method was utilized to guide the micro devices close to the integration site and surface tension driven fluidic self-assembly was employed to release and self-align the devices onto their predefined binding areas. Different aspects of the process starting from device fabrication, host substrate preparation, PAP tool modification have been described in details. LT-grown GaAs based MSM PDs fabricated in its optimized growth substrate and fabrication procedures have been used for the self-assembly. Surface wetting properties of a silicon dioxide thermally grown silicon wafer was selectively modulated to create the binding sites on the host substrate. Photocurrent and dark current measurements were performed on the integrated device to confirm the electrical contact between the device and the integration pads. This hybrid integration method sustains the advantages of both fluidic self-assembly and robotic PAP, thus providing an attractive alternative as a low cost technique for the integration of ultra-thin devices.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
This application claims priority to and is a non-provisional of U.S. Patent Applications 62/408,298 (filed Oct. 14, 2016) and 62/529,227 (filed Jul. 6, 2017), the entirety of which are incorporated herein by reference.
This invention was made with government support under grant number EEC-0823793 awarded by the National Science Foundation. The government has certain rights in the invention. The applicants also wish to thank the National Natural Science Foundation of China (grant numbers 61404047 and 61501179), the China Scholarship Council and the Natural Science Foundation of Hunan Providence, China (grant number 2015JJ3034).
Number | Date | Country | |
---|---|---|---|
62408298 | Oct 2016 | US | |
62529227 | Jul 2017 | US |