Substrate Materials Micromachining and Surface Considerations

Abstract

Subject material presented describes substrate materials and their processing as key elements in microsystems technology manufacturing (MST) for Biochips (DNA arrays), Electronic MEMS (Micro-Electro-Mechanical) Devices and electro-optics. Material choices are primarily glass, quartz and silicon, each of which may require mechanical features and exact surface finishes to enable design function and manufacturing. This paper will address some basic approaches to determining what substrates require for specific designs and manufacturing processes.

INTRODUCTION

As engineers and scientists design prototype devices, they are challenged with the task of deciding what material best meets their requirements. Considerations of the type of mechanical, physical, and chemical machining will provide the critical features, determine the optimum substrate material choice.

This list of materials is generally used for applications requiring the following physical attributes:

Common Glass — (Soda Lime base typically) laboratory slide quality for surface chemistry. Low cost. Can be machined and polished to fairly good finishes. Has many undesireable elements present such as: sodium, chromates, metals, and some phospates to name a few.

Borosilicate Glass — (Pyrex) most commonly used material substrate for anodic bonding and mechanical features. Has thermal expansion coefficient that nearly matches that of silicon. Readily machined surfaces, known chemical and physical properties. Produced to property content not material content. A surface finish of 4–7 angstroms can be processed. Readily available and easy to work with. Review of chemical and physical properties. (Figures 1-5)

Figure 1. Figure 2. Figure 3. Figure 4. Figure 5.

Fused Silica — known physical, chemical and optical qualities, used in applications requiring discriminating high frequency light wave transmission and other optic related mechanisms. Material is readily available and varies tremendously in quality and price dependent on application requirements. A surface finish of 2–4 angstroms can be obtained.

Other Material — Ceramics, plastics, and films are being looked at for applications where cost is primary force driving the device design. Recent technology developments similar to CD Recording imprinting processes, and thermal micro-compression with plastics are being evaluated for future products.

This paper will deal with the requirement of hard surface substrates with and without features, with a focus on gass and fused silica.

Once substrate material is decided on, the following decisions can be determined. The table below shows materials, fabrication and assembly processes for microsystems.

Material	Assembly Process	Fabrication Process
Silicon	Gluing	Machining
Glass	Welding	Photolithography
Ceramic	Soldering	Laser Ablation
Plastic	Press Fit	Molding
Composites	Bonding	Plating
Metals		Sputtering
Carbon		Plasma Etching
Organics		Chemical Etching
		Reactive Ion Etch

A typical start for a device consideration begins with working on a single piece of glass or silicon. If the glass is to be anodically bonded to silicon or fused with another piece of glass the following must be considered. The glass must have a relative thermal expansion coefficient that closely matches up to that of the silicon. (Figures 2-5)

Dimensions — standard dimensions are expressed in the desired format of end users. Standard wafer formats used in MEMS devices and semi-conductor manufacturing are round wafers in a 75mm, 100mm and 150mm dimensions. These glass materials can be supplied with orientation features such as flats and edge-work bevels to allow for easy identification of the process side or if work is on both sides a side indicator can be provided. Squares, rectangles and shapes can also be produced dependent on specified requirements.

Machined Features — hard surfaces can be machined using mechanical and chemical processes. Holes, vias, channels, junctions and slots are typical feature requirements. Ultrasonic drilling is a commonly used machine method for a variety of shapes and configurations. Ultrasonics work on the principle of ultrasonic frequency energy interaction with a slurry between the tool face and the substrate to machine the desired feature(s). Alignment of features, dimensions and depths can be controlled to provide exact details and their location to specification (Figures 7-10).

Figure 6. Figure 7. Figure 8. Figure 9. Figure 10.

Machined Surfaces — Surface grinding and polishing will provide additional surface and feature attributes dependent on bonding methods. For example, wafer formats requiring silicon contact to the edge of the hole or feature requires crisp edges as opposed to a feature desiring a “roll off” to a well or hole. This can be machined in the post processing polishing after ultrasonic drilling.

Silicon to glass wafer bonding requires exact surface smoothness to prevent voids in the bonding interface. If the glass or silicon has high surface roughness or exceeds a critical value, the wafer will not bond properly. A variety of bonding methods such as: bonding at room temperature to bonding in a high temperature (400°C) in a vacuum and pressure will determine some basic parameters'. Typical substrate surface roughness values stated as Ra range from 15–20 angstroms (average) to 8–10 angstroms bondable to 4–8 angstroms bondable and etch able. These surfaces can be specified and determined by utilizing optical interferometers and profilometer surface detection devices. Atomic Force Measurement (AFM) can also be utilized if one has access. Flatness of the substrate will also effect the bondability of the material. Flatness is often expressed as Rms and refers to the overall peak to valley (highest to lowest) measurements that will reduce bondability. Glass substrates can be purchased directly from companies that will specify the surface roughness and flatness parameters to meet a range of requirements (Figures 6, 11, 12).

Figure 11. Figure 12.

Cleaning — cleaning of glass wafers is of critical importance due to the following considerations:

• Charged particles adhere to glass.

• Ions from glass and cleaning solutions will migrate to new surfaces.

• Particles will contaminate surfaces.

• Material handling issues will cause surface scratches.

• Ion exchange or transference to base material.

It is recommended that end users clean incoming materials. It is best to work with a supplier to define cleaning and packaging requirements to prevent permanent damage to wafer surfaces.

Packaging — Substrates can be provided in a variety of shipping and storage containers. Low cost cosmetic round containers with clean room paper leafing to clean room packaging such as Empak or Fluroware containers can be specified dependent on the end user's requirements and budget. Custom packaging is available similar to that used by photomask processors. Substrate size and shipment method will determine the best method to package. Every attempt should be made to use standard wafer sizes so that standard wafer containers can be obtained.

Etching — a variety of chemical and dry etches can be used to machine surface features to exact dimensions and tolerances. Hydrofluoric acid and dry etch procedures are quite effective to produce desired feature dimensions (Figures 8-11).

Post Processing — In some instances it may be more appropriate to use a combination of processes to obtain desired device design. It is quite common to bond silicon to glass or glass to glass with features machined at different intervals of the manufacturing process. Drilling holes through silicon on glass is routinely performed to achieve wire contacts or fluid connections between the layers of substrates. Typical feature sizes and tolerances can be maintained between 25–50 microns in width, any length, and 50–250 microns in depth. See Figures 4 and 5 of various configurations.

CONCLUSION

The relationship between substrate materials, machined features, surface finishes and device design is important to function and cost. Most designers work with materials they have experience with, however they may not be familiar with current machined feature capabilities that would enable them the opportunity to produce a more advanced device at a reasonable cost.

As microsystems technology grows, there is more information available through a number of organizations relating to materials, processes and suppliers providing services to technically assist those developing their unique devices.