Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding

Summary

A method for permanently bonding two silicon wafers so as to realize a uniform enclosure is described. This includes wafer preparation, cleaning, RT bonding, and annealing processes. The resulting bonded wafers (cells) have uniformity of enclosure ~1%^1,2. The resulting geometry allows for measurements of confined liquids and gasses.

Abstract

Measurements of the heat capacity and superfluid fraction of confined ⁴He have been performed near the lambda transition using lithographically patterned and bonded silicon wafers. Unlike confinements in porous materials often used for these types of experiments³, bonded wafers provide predesigned uniform spaces for confinement. The geometry of each cell is well known, which removes a large source of ambiguity in the interpretation of data.

Exceptionally flat, 5 cm diameter, 375 µm thick Si wafers with about 1 µm variation over the entire wafer can be obtained commercially (from Semiconductor Processing Company, for example). Thermal oxide is grown on the wafers to define the confinement dimension in the z-direction. A pattern is then etched in the oxide using lithographic techniques so as to create a desired enclosure upon bonding. A hole is drilled in one of the wafers (the top) to allow for the introduction of the liquid to be measured. The wafers are cleaned² in RCA solutions and then put in a microclean chamber where they are rinsed with deionized water⁴. The wafers are bonded at RT and then annealed at ~1,100 °C. This forms a strong and permanent bond. This process can be used to make uniform enclosures for measuring thermal and hydrodynamic properties of confined liquids from the nanometer to the micrometer scale.

Introduction

When clean silicon wafers are brought into intimate contact at RT, they are attracted to each other via van der Waals forces and form weak local bonds. This bonding can be made much stronger by annealing at higher temperatures^5,6. Bonding can be done successfully with surfaces of either SiO₂ to Si or SiO₂ to SiO₂. Bonding of Si wafers are most commonly used for silicon on insulator devices, silicon-based sensors and actuators, and optical devices⁷. The work described here takes wafer direct bonding in a different direction by using it to achieve well-defined uniformly-spaced enclosures over the entire wafer area

Protocol

1. Before Bonding, Wafer Preparation

This step except for 1.8 is done in the Cornell Nanoscale Facility cleanroom.

1. Grow the oxides in a standard thermal oxidation furnace using a wet oxide process for thick oxides and, to achieve better thickness control, a dry oxide process for very thin oxides. Check the thickness for uniformity over the full wafer with ellipsometry.
2. Create a mask for the geometry you wish to etch.
3. Spin photoresist on the wafers being etched.
4. Expose, develop and bake a test wafer and examine with an appropriate microscope.
5. If the test wafer is exposed as desired, etch the t....

Results

Properly bonded wafers will have no unbonded regions. Attempting to split the wafers after annealing will cause the cell to break into pieces due to the strength of the bond. Infrared images of properly bonded wafer are shown in Figures 5 and 6. Often annealing improves the uniformity of the cell, especially if local unbonded regions are due to lack of flatness in the wafers. In Figure 5 the light spots and border are bonded areas. The center bright spot is the hole for .......

Discussion

The development of suitable silicon lithography in combination with direct wafer bonding has allowed us to make vacuum tight enclosures with highly uniform small dimensions over all the full area of a 5 cm diameter silicon wafer. These enclosures have allowed us to study the behavior of liquid ⁴He in the neighborhood of its phase transitions from a normal liquid to a superfluid. These studies have verified predictions of finite-size scaling, as well as pointed out failures which remain to be explored. The work.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
SmartCut	North American Tool	FL 130	Not much is needed per cell. Smaller sizes are available.
Silicon Wafers	Semiconductor Processing Co		There are many suppliers. Pay attention to thickness and thickness variation when ordering.
Deionized Water			General Availability
Peroxide			General Availability
Hydrochloric Acid			General Availability
Ammonium Hydroxide			General Availability
Nitrogen Gas			General Availability
Helium Gas			General Availability
Diamond Paste	Beuler Metadi II	e.g. 406533032
Diamond Drills	Starlite	e.g. 115010
Pyrex Dishes			General Availability
Filter Paper	Whatman	1001-110
Acetone			General Availability
Methanol			General Availability
Quartz tubes for flushing furnace			General Availability
Rubber vacuum hose			General Availability