Micro-masonry for 3D Additive Micromanufacturing

Summary

This paper introduces a 3D additive micromanufacturing strategy (termed ‘micro-masonry’) for the flexible fabrication of microelectromechanical system (MEMS) structures and devices. This approach involves transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.

Abstract

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called ‘inks’) from a substrate where they are generated to a different substrate by utilizing elastomeric stamps. Transfer printing enables the integration of heterogeneous materials to fabricate unexampled structures or functional systems that are found in recent advanced devices such as flexible and stretchable solar cells and LED arrays. While transfer printing exhibits unique features in material assembly capability, the use of adhesive layers or the surface modification such as deposition of self-assembled monolayer (SAM) on substrates for enhancing printing processes hinders its wide adaptation in microassembly of microelectromechanical system (MEMS) structures and devices. To overcome this shortcoming, we developed an advanced mode of transfer printing which deterministically assembles individual microscale objects solely through controlling surface contact area without any surface alteration. The absence of an adhesive layer or other modification and the subsequent material bonding processes ensure not only mechanical bonding, but also thermal and electrical connection between assembled materials, which further opens various applications in adaptation in building unusual MEMS devices.

Introduction

Microelectromechanical systems (MEMS), such as the miniaturization of large scale ordinary 3D machines, are indispensable for advancing modern technologies by providing performance enhancements and manufacturing cost reduction^1,2. However, the current rate of technological advancement in MEMS cannot be maintained without continuous innovations in manufacturing technologies^3-6. Common monolithic microfabrication primarily relies on layer-by-layer processes developed for the manufacture of integrated circuits (IC). This method has been quite successful at enabling mass production of high performance MEMS devices. However, owing to its complex layer-by-layer and electrochemically subtractive nature, manufacturing of diversely-shaped 3D MEMS structures and devices, while easy in the macroworld, is very challenging to achieve using this monolithic microfabrication. To enable more flexible 3D microfabrication with less process complexity, we developed a 3D additive micromanufacturing strategy (termed ‘micro/nano-masonry’) which involves a transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.

Transfer printing is a method to transfer solid microscale materials (i.e., ‘solid inks’) from a substrate where they are generated or grown to a different substrate by using controlled dry adhesion of elastomeric stamps. The typical procedure of micro-masonry starts with transfer printing. Prefabricated solid inks are transfer printed using a microtip stamp that is an advanced form of elastomeric stamps and the printed structures are subsequently annealed using rapid thermal annealing (RTA) to enhance ink-ink and ink-substrate adhesion. This manufacturing approach enables the construction of unusual microscale structures and devices that cannot be accommodated using other existing methods⁷.

Micro-masonry provides several attractive features not present in other methods: (a) the ability to integrate functional and structural solid inks of dissimilar materials to assemble MEMS sensors and actuators all integrated within the 3D structure; (b) the interfaces of assembled solid inks can function as electrical and thermal contacts^9,10; (c) the assembly spatial resolution can be high (~1 μm) by utilizing highly-scalable and well-understood lithographic processes for generating solid inks and highly-precise mechanical stages for transfer printing⁷; and (d) functional and structural solid inks can be integrated on both rigid and flexible substrates in planar or curvilinear geometries.

Protocol

1. Design Masks for Fabrication of Donor Substrate

1. Design a mask with desired geometry. To fabricate 100 μm x 100 μm square silicon individual units, draw an array of 100 μm x 100 μm squares.
2. Design a second mask with an identical geometry, with each side extending out an additional 15 μm. For the array of 100 μm x 100 μm squares, draw an array of 130 μm x 130 μm squares that can cover the squares in step 1.1.
3. Design the anchor geometry. Draw four 20 μm x 40 μm rectangles, each centered along one edge of a square. Place the structures so that the first 15 μm covers the original 100 μm x 100 μm square in step 1.1 and the remaining 25 μm extends outward (as shown in Figure 2).
   NOTE: Any shape and dimensions can be used as long as the anchor contacts both the patterned material and the substrate. One end of this anchor covers the original geometry in step 1.1 and the other end should extend out the geometry in step 1.2.

2. Prepare Retrievable Donor Substrate

1. Prepare a p-type doped silicon on insulator (SOI) wafer with 3 μm device layer thickness, with sheet resistance of 1-20 Ω•cm and box oxide layer thickness of 1 μm. NOTE: For various applications these parameters can be altered.
2. Spin coat photoresist (AZ5214, 3,000 rpm for 30 sec, 1.5 μm thick) and attach the mask designed in step 1.1.
3. Using a reactive ion etching (RIE) instrument, pattern the device layer of the SOI wafer and remove photoresist mask. After this step, the RIE etched region has exposed the box oxide layer (Figure 2A).
4. Spin coat photoresist (AZ5214, 3,000 rpm for 30 sec, 1.5 μm thick) and pattern with mask designed in step 1.2.
5. Heat the wafer at 125 °C for 90 sec on a hot plate.
6. Immerse the wafer into 49% HF for 50 sec to etch the exposed box oxide layer from step 2.3. After completely drying, remove the masking photoresist (Figure 2B).
7. Spin coat (AZ5214, 3,000 rpm for 30 sec, 1.5 μm thick) and pattern the anchoring design from step 1.3.
8. Heat the wafer at 125 °C for 90 sec on a hot plate.
9. Immerse into 49% HF for 50 min. This step etches the box oxide layer remaining underneath the remaining patterned device layer silicon, resulting in suspended silicon individual units on the photoresist (Figure 2C).

3. Design Masks for a Microtip Stamp

1. Design a mask with a single 100 μm x 100 μm square.
2. Design a mask with multiple 12 μm x 12 μm squares inside a 100 μm x 100 μm area.

4. Make the Mold for a Microtip Stamp

1. Clean a silicon wafer with crystalline orientation of <1-0-0>, deposit 100 nm of silicon nitride using Plasma Enhanced Chemical Vapor Deposition (PECVD) equipment.
2. Spin coat photoresist (AZ5214, 3,000 rpm for 30 sec, 1.5 μm thick) and pattern with mask designed in step 3.2.
3. Pattern the silicon nitride layer using 10:1 Buffered Oxide Etchant (BOE).
4. Dissolve 80 g of potassium hydroxide (KOH) in 170 ml of deionized water and 40 ml of isopropyl alcohol (IPA) mixture a beaker.
5. Heat the KOH, IPA, and water mixture at 80 °C on a hot plate.
6. Vertically place the prepared wafer in the beaker with KOH mixture to etch the exposed silicon in crystalline structure (etching rate is around 1 μm/min).
7. After the exposed silicon is fully etched, remove the wafer from KOH mixture, etch away the silicon nitride using HF, and perform RCA 1 and RCA 2 cleaning (Figure 3A).
8. Spin coat with SU-8 100 and pattern with the prepared mask from step 3.1 with following recipe: 3,000 rpm for 1 min, soft bake at 65 °C for 10 min and 95 °C for 30 min, expose with 550 mJ/cm², and post bake at 65 °C for 1 min and 95 °C for 10 min (Figure 3B).
9. After the SU-8 100 is fully cured, apply a monolayer of (tridecafluoro-1,1,2,3-tetrahydro octyl)-1-trichlorosilane by dropping 3-5 drops of (tridecafluoro-1,1,2,3-tetrahydro octyl)-1-trichlorosilane into a vacuum jar and placing the wafer in the jar and applying the vacuum.

5. Duplicate a Microtip Stamp Using a Mold

1. Mix polydimethylsiloxane (PDMS) base and curing agent with the ratio of 5:1.
2. Degas the mixture by placing it in a vacuum jar.
3. Pour a small portion of the degassed PDMS mixture on the mold and let the PDMS reflow to achieve a flat top surface (Figure 3C).
4. Place the mold with PDMS in the oven at 70 °C for 2 hr to fully cure the PDMS.
5. Remove the mold from the oven and peel the PDMS off (Figure 3D).

6. Retrieve Ink from the Donor Substrate and Print on the Target Area

1. Place the donor substrate onto motorized rotational and x,y-translation stages equipped with a microscope.
2. Attach the microtip stamp to an independent vertical translational stage.
3. Under the microscope, align the microtip stamp with the Si ink on the donor substrate using translational and rotational stages. Furthermore, do the tilting alignment between the microtip surface and the Si ink by adjusting a tilting stage. Afterwards, bring the microtip stamp down to make contact.
4. Slowly bring the microtip stamp down further after initial contact, so that small tips are fully collapsed and the whole surface is in contact with the Si ink on donor substrate.
5. Quickly raise the z stage, breaking the anchors due to the large contact area between the microtip stamp and the Si ink, to retrieve the Si ink from the donor substrate and attach it to the microtip stamp.
   NOTE: When the microtip stamp is free of any stress, the compressed microtip restores to its original pyramidal shape, making minimal contact with the retrieved Si ink.
6. Place the receiver substrate onto an x,y-translation stage and align the retrieved Si ink under the microtip stamp at the desired location.
7. Descend the z stage until the retrieved Si ink barely makes contact with the receiver substrate.
8. After making contact, slowly raise the z stage to release the Si ink, printing it on the desired location.

7. Bonding Process

1. Program a rapid thermal annealing furnace to cycle from RT up to 950 °C in 90 sec, remain at 950 °C for 10 min and cool down to RT (by removing any heat supply in the furnace).
2. Place the printed receiver substrate in the furnace in an ambient air environment and anneal at 950 °C for 10 min for Si-Si bonding or at 360 °C for 30 min for Si-Au bonding.

Results

Micro-masonry enables heterogeneous material integration to generate MEMS structures that are very challenging or impossible to achieve by monolithic microfabrication processes. In order to demonstrate its capability, a structure (called a ‘micro teapot’) is fabricated solely through micro-masonry. Figure 4A is an optical microscope image of fabricated Si inks on a donor substrate. The designed inks are discs with different dimensions made of single crystalline silicon, which are the building blocks of t...

Discussion

Micro-masonry, presented in Figure 4, involves silicon fusion bonding in a material bonding step. Silicon fusion bonding is achieved by placing the sample in a rapid thermal annealing furnace (RTA furnace) and heating the sample at 950 °C for 10 min. This annealing condition is both adoptable between Si – Si and Si – SiO₂ bonding^10,11. Alternatively, the Au bonded with a Si strip as found in Figure 5C adopts eutectic bonding, and therefore, the...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Az 5214	Clariant		1.5 mm thick Photoresist
Su8-100	Microchem		100 mm Photoresist used in mold
Sylgard 184	Dow Corning		PDMS mixed to fabricate stamp
Hydrofluoric acid	Honeywell		Acid to etch silicon oxide layer
Silicon on insulator	Ultrasil		Donor substrate was fabricated
Trichlorosilane	Sigma-Aldrich		Chemical used to help pealing of PDMS from mold