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Release is required to produce free standing MEMS structures. These structures are patterned on top of a sacrificial material and must be released. To release a part from the underlying material we have many techniques.
- 1 Release processes
- 2 Figures of Merit
- 3 Critical point drying (CPD)
- 4 References
- 5 Further reading
The processes described here are utilized for Surface micro-machining. They can also be applied to SOI micro-machining, where the buried oxide is the sacrificial layer. For Bulk micro-machining, the entire wafer is dissolved except the features for the device structure.
Photoresist sacrificial layer
This technique is commonly used for metal structures. It has the advantage that it can be easily photo patterned, and it will release in solvents. It is also low temperature so it is compatible with underlying materials and circuits that require low temperature processing. The most commonly utilized solvents (Acetone, Remover PG, Isopropanol, Methanol) will not attack most metals. If only non-polar solvents are utilized, this release process is less susceptible to stiction. The difference between a sacrificial layer process and liftoff is the material on top of the photoresist is anchored so it is not removed. It is not compatible with high temperature processes, gap uniformity is limited to the uniformity of the photoresist (typically 5-10% on a flat surface), and very difficult if there is significant topology on the surface. For a typical process:
- Photoresist is applied to the substrate and patterned
- Metal or other low temperature (<100C) is deposited onto the patterned photoresist
- The top layer of the device is patterned again for either etching of the material or electroplating a thicker layer followed by plating mask and seed layer removal
- All of the photoresist is removed.
This leave a free standing device where the metal was on top of the resist, and anchored where there was no photoresist.
Silicon sacrificial layer
This method most often is used for creating free standing oxide/nitride structures and devices such as microfluidic devices with cavities. Other materials can be deposited onto the Si before release. The silicon is easily removed using dry techniques such as XeF2 to leave the other materials. The advantages of using silicon as a sacrificial layer is:
- XeF2 has a very high selectivity to most other materials and is a dry process so there is no stiction
- LPCVD polysilicon can be deposited very conformally covering large high aspect ratio steps
- High temperature deposition of polysilicon (typically >550C)
- Thermal cycle can cause stress issues
- Not compatible with most already deposited metal layers
A typical silicon sacrificial process:
- Polysilicon is deposited on a glass wafer or Silicon dioxide coated wafer
- Poly is patterned and etched
- A combination of oxide/nitride/low stress nitride and a variety of metals are deposited
- The layer of oxide/nitride/low stress nitride is etched
- At this point, metal can be deposited and patterned
- Put into XeF2 to release the structure
Oxide sacrificial layer
This release method is very common. Free standing silicon or polysilicon structures are the basis for most surface micro-machined and SOI micro-machined devices. The problem with this technique is the release is often in Aqueous HF, and must be rinsed in DI water. Many of these devices have a large surface area and a small gap which can lead to stiction. Stiction occurs when the device will attach itself to the substrate or other feature through Van der waals forces. To eliminate this the water is replaced by methanol through soaking and diffusion, the methanol is replaced with liquid CO2 through critical point drying (CPD). Advantages:
- HF is extremely selective to etch oxide
- HF can attack many metals
- Deposit and pattern oxide layer
- Deposit and pattern poly silicon layer
- Repeat steps 1-2 for multiple layers
- Deposit and pattern metal (usually gold because it in not attacked by HF)
- Release by soaking in HF
After devices are released, they will be rinsed and soaked in Methanol. Then the CPD process is performed.
The aqueous HF can be eliminated using vapor HF, and does not require the CPD.
Figures of Merit
Stiction is a term that is used to describe the effect that happens when you have a large surface area device separated by a small gap. What can happen when doing a wet release in an aqueous solution. The force that is generated while the water is drying will pull the two plates together, and van der waals forces will keep them stuck together. The two primary ways of eliminating this phenomena is:
- Critical point drying: After the device is released and while still wet, the water is replaced by a solvent (methanol), then the solvent is replaced by liquid CO2 at the critical point.
- Dry release: This is achieved through vapor HF for oxide, or XeF2 for Si sacrificial layers
Ideally, the removal of the sacrificial layer will not effect the remaining layer, 100% selectivity. However, while many etches are very highly selective most are not 100% selective so you must time the etch to make sure you are not eroding too much of the layer that makes up the structure.
Coverage of sacrificial layer
To create free standing structures, they must be anchored to the substrate. To achieve this, you must get a good coverage of the sacrificial layer. Without good coverage, it is possible to create weak points in the device structure where the material is thin.
The stress of the free standing structure is very important when determining the structure. There are two types of stress: compressive and tensile. Compressive stress in a free standing structure will cause the device to try to expand when it is released. When it expands it is often seen that the structure will buckle out of plan to relieve the stress. Tensile stress will try to shrink when the structure is released. While devices with more than one clamping location will remain flat, it will effect the force needed to deflect the device out of plane. An example of this is a drum, as it is pulled tighter (higher tensile stress) the frequency of the drum increases (higher vibration frequency).
Temperature is important when determining the type of sacrificial layer that will be used. Higher temperature methods of deposition will tend to have better step coverage/conformality, lower defects, better optical properties, and may have lower stress.
Critical point drying (CPD)
Equipment for CPD: Tousimis 915B Critical Point Dryer
Critical point drying make use of Liquid CO2 under high pressure and lower temperatures to remove any water from around the MEMS device to reduce or eliminate stiction.
This is done in 2 steps.
- First the part is soaked in Methanol to remove the water.
- Depending on how much distance between the edge of the part to the bulk region you will need to give time to allow the water do diffuse out from under the free standing structure. The time it will take is proportional to the square of the distance. This can take anywhere from an hour to 48 hours.
- Typically it helps to change the methanol part way through the diffusion process, though make sure that the part does not dry.
- Once the part has soaked, Methanol is filled in the CPD and the part or wafer is placed into the CPD and the process run.
- The purpose of Critical point drying is to eliminate surface tension/stiction associated with the drying of a liquid by avoiding the phase transition boundary from liquid to gas. The process goes around the high-temperature/pressure side of the gas/liquid transition point by pushing the liquid past the critical point and converting the liquid to a supercritical fluid and then converting that fluid to gas.
- The CPD is a 4-step process (see diagram below, green arrow indicates sample still in methanol.)
- Cool and fill: sample, still in methanol, is cooled and then exposed to a high pressure liquid CO2 (6°C, 850PSI in the case of the LNF CPD tools)
- Purge: methanol is then purged and displaced with liquid CO2
- Heat: chamber is then heated and kept under high pressure until the CO2 is pushed past the critical point (>31°C, >1072PSI)
- Bleed and vent: Pressure is released and you are left with a dried product with no surface tension.
- Overview of Critical Point Drying: Wikipedia article on Supercritical Drying
- Overview of Stiction: Wiki article on Stiction