Overview of glass micro machining processes for MEMS applications

by Dave Sarvela
Manager of Applied Technology, IKONICS Corporation

Glass is often considered a more desirable material for particular MEMS device applications due to its unique properties, such as optical transparency and biological compatibility.

As the desire to use glass in the MEMS industry increases, so does the need to develop better methods to micro-machine this material.  Traditional machining methods, such as CNC, are not very effective at machining brittle materials such as glass.  In addition, as the diversity of MEMS applications expands, the desired features continue to get smaller, denser and more intricate.

MEMS applications where micro-machined glass is used include:

* sensors, such as those incorporating pressure, accelerometer, gyroscope transducers
* bioMEMS devices enabled by lab-on-chip and microfluidics technologies   
* membranes
* spacers for cell phone cameras

Currently, there are a few different methods for micro-machining features in glass, each with its own advantages and disadvantages.  These machining methods include powder blasting, laser, wet etching, ultrasonic and DRIE.  This article reviews these glass micro-machining methods and comparers them to each other.

Part 1 -- Powder Blasting

Powder blasting is also referred to as sandblasting, impact abrasive machining or abrasive jet machining (AJM).  In this process, fine abrasive particles are propelled by compressed air at the workpiece and these particles mechanically remove material by small chipping.  When combined with a specially designed photoresist, powder blasting can be a very effective method for micro-machining brittle material. 

UV light passes though the transparent areas of the phototool, changing the chemistry of the photoresist to create a precise and accurately placed pattern that masks the wafer for subsequent abrasive etching (left).  Very fine particles are propelled with compressed air, impacting the wafer.  Material is removed only in the areas defined by the photoresist (right).

The physics of powder blasting produces a taper angle on through holes of about 12°-15°.  Because of this taper, the entry hole opening will be larger than the exit hole opening and the process is limited to a maximum of a 2:1 (material thickness to hole diameter ratio). This can be mitigated by powder blasting from both sides.

Capabilities

Minimum Feature Size: 50 µm
Pitch: <100 µm (feature dependent)
Aspect Ratio:  2-3:1
Feature Size Tolerance: <25 µm
Positional Tolerance: <25 µm
Through-Hole Taper: 12-15°
Maximum Workpiece Thickness: 6 mm
Workpiece Size Limit: >24”
Depth Uniformity:  <25 µm

Powder blasting can be used to create membranes in glass for MEMS applications.  The process can be controlled to leave as thin as 75-100 um of glass material with a depth uniformity within 15 microns.

This image shows a variety of patterns that are possible with powder blasting, including channels, pockets and through holes.

Advantages

One of the advantages of the powder blasting process is that it can quickly create through-holes in brittle materials without creating burrs on the surface.  When powder blasting is used with a specially designed photoresist, it applies the precision, accuracy and high detail of photolithography to the micro-machining of glass.

This process is anisotropic and has an aspect ratio of 3:1 depending on the features.  It can create a wide variety of blind and thru-features that are both detailed and deep, all on the same wafer.  This process is excellent for removing large areas of material to create complex patterns such as posts, mesas and pads with high pitch density.  It can be used for making fixed depth pockets for applications such as membranes or through-holes for spacers or fluid flow.  The process can be adjusted to a light etching to make features such as a suspended diaphragm on a bonded wafer.  It can also be used to create cantilevers for MEMS applications.

All surfaces are fully protected throughout the manufacturing process to avoid any damage from the abrasive by the photoresist film and, therefore, this process works well with wafers with existing features.  The etched surface finish and material removal rate can be influenced by size and type of abrasive.  Powder blasting has similar etching capabilities on borosilicate, fused silica, quartz, soda lime or even silicon; it can also micro-machine tough materials like sapphire and silicon carbide.  With powder blasting there is neither a HAZ (Heat Affected Zone) nor subsurface micro-cracking.

Disadvantages

Other processes can create higher aspect ratios with straight sidewalls.  The machined surface finish is slightly rough unless is it is wet etched after powder blasting.  Finer detail features and shallow etching are possible with other processes.

Part 2 -- Laser Machining

With the laser machining process, glass material removal is from thermal shock or ablation by directed optical energy.  There are several sources of lasers for machining glass and their selection is based on the type of application.  The laser systems that are available for glass include CO2, Nd:YAG and excimer.  The wavelength of the laser needs to be such that it does not pass through the glass without etching it.  Computer-controlled equipment (most commonly an X-Y table) directs the beam to the desired location.

Capabilities

Minimum Feature Size: 50-100 µm
Pitch: <100 µm (feature and controller dependent)
Aspect Ratio:  10-20:1
Through-Hole Taper; 3-5°
Maximum Workpiece Thickness: 1 mm
Workpiece Size Limit: >24”
Positional Tolerance: motion control equipment dependent

Advantages

Laser systems make it fairly easy to create a pattern from a CAD drawing and there is no mask, tooling or tool wear.  Because there is no tooling or mask, this process can be easily automated.  This process has a low taper angle and can be used on large pieces.

Disadvantages

Laser machining creates subsurface micro-cracks and also creates a HAZ (Heat Affected Zone) which results in a kerf or damaged area at the top surface of the hole.  Since it is a thermal process, laser machining can crack or break thin glass pieces.  It is difficult to create blind holes or remove material with a fixed depth across a large area with a laser, since it creates an uneven etching as it progresses across the part.

The capital investment for this process can be significant since the laser system must be combined with the positioning equipment (motion controller for the laser system).  The tolerances of the features are dependent on the quality of the equipment.  For this process, the laser must etch one hole at a time and so the system spends time repositioning itself.  It can be difficult to register to existing features on a wafer.

Part 3 -- Ultrasonic Machining

This is a non-impact process in which a mechanical tool oscillates above the workpiece at a high frequency, roughly 20,000 cycles per second.  The tool end (horn) is formed in the shape of the desired feature.  The tool end (horn) and workpiece are submerged in abrasive slurry.  The majority of the machining occurs by the tool end and abrasive particles hitting the workpiece.

Capabilities

Minimum Feature Size: 200 µm
Aspect Ratio: 25:1
Feature Size Tolerance: <25 µm
Positional Tolerance: +/- 50 µm
Through-Hole Taper: 3-5°
Maximum Workpiece Thickness: 1 mm
Workpiece Size Limit: ~6”
Depth Uniformity: +/- 50 µm

Advantages

One of the primary advantages of this process is its ability to drill straight sidewalls and produce very fine features.  Another big advantage is the high aspect ratios that can be achieved.  It is also easily repeatable until tool wear.  Also, multiple depths and contoured surfaces (2-1/2 D) can be achieved.

Disadvantages

Depending on the features and material, this can be a slow process with a large capital investment.  The tooling needs to be redressed for every 25-50 pieces to avoid feature degradation.  There is a limit to the pattern size and it can be difficult to machine asymmetrical patterns.  It is more difficult to engrave contoured and interior surfaces.

Part 4 -- Wet Etch

Wet etching involves the creation of a pattern in glass by immersing the wafer in an acid, most commonly hydrofluoric.  An acid-resistant mask material can be used for selective material removal and the part can be etched to multiple levels.  Common mask materials are Cr-Au and/or an acid resistant photoresist.

Capabilities

Minimum Feature Size: 1 µm
Pitch: <100 µm (feature dependent)
Aspect Ratio: 1:1
Feature Tolerance: <25 µm
Positional Tolerance: <25 µm
Maximum Workpiece Thickness: >1 mm
Workpiece Size Limit: >24”

Advantages

Wet etching can create very high detailed features and works well for removing thin films.  For this process, the shape and size of the part is not usually limited.  The material removal rate may be comparatively fast since large areas can be exposed to acid at one time in a batch process mode.  Produced features have a low surface roughness and therefore this process can be used with applications where near optical clarity is desired.

Disadvantages

The wet etch process is isotropic and there is undercutting of the photoresist that is equal in distance to the etch depth.  This gives the process a low aspect ratio.  The acid material is extremely hazardous to humans and the environment.

Part 5 -- Deep Reactive Ion Etching (DRIE)

A dry etch micro-machining method is Deep Reactive Ion Etching, or DRIE.  The process uses directional plasma ions to hit the glass causing erosion.  A metal mask can be used to direct the ions and create the desired features.  The process can be used on glass, but the gas chemistry is geared more toward silicon etching.

Capabilities

Minimum Feature Size: 0.5 µm
Pitch: <100um (feature dependent)
Aspect Ratio: 3:1 (glass)
Feature Tolerance: <25 µm
Positional Tolerance: <25 µm
Maximum Workpiece Thickness: >1 mm

Advantages

The process can be extremely accurate with feature creation and can achieve very small features.  There is a low amount of surface roughness and it is a highly anisotropic process.

Disadvantages

Depending on the number and type of features, the DRIE process can be very slow and because of this, it is not good for removing material across a wide area.  Ion etching works well for etching silicon, but the process is extremely slow for etching glass.  In fact, since it etches so slowly, glass is often used as a mask for etching silicon.

References

[1]    “High Resolution Powder Blast MicroMachining,” H. Wensink, J.W. Berenschot, H. V. Jansen, M. C. Elwenspoek.

[2]  “Comparison of via-fabrication techniques for through-wafer electrical interconnect applications,” A. Polyakov, T. Grab, R. A. Hovenkamp, H. J. Kettelarij, I. Eidner, M. A. de Samber, M. Bartek and J. N. Burgbartz

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Dave Sarvela has worked for the IKONICS Corporation for 19 years and current responsibilities include overseeing the Photo-Machining production and developing new markets/application for the IKONICS Industrial division.  He received his Industrial Engineering degree from the University of Minnesota in 1993 and an MBA from the University of Minnesota in 2002.  Dave can be reached at [email protected].

Copyright 2010 MEMS Investor Journal, Inc.

Comments

[Eric Jacobson, Translume]

This article is an excellent overview of the industry's leading glass micromachining techniques. Thanks! Our company offers a novel approach for glass micromachining that is a combination of laser machining and wet etch. We make a pattern in glass with the laser and then etch it out. This allows us to have the machined surfaces that are better than laser ablation and yet get deep aspect ratios.

[Harmen Lelivelt, Micronit Microfluidics]

Great article. A nice overview of available technologies of which many are used by Micronit Microfluidics on a daily basis for its customers in microfluidics and MEMS.