Stiction free removal of organic sacrificial layers in MEMS manufacturing
YIN XU, VLAD TARASOV, WEI CHEN, and KOUKOU SUU, ULVAC Technologies, Inc., Methuen, MA
To resolve "stiction" yield failure issue in the manufacturing of MEMS devices, special release processes have been developed based on a combination of structural and sacrificial materials.
The fabrication of Micro-Electro-Mechanical Systems (MEMS) derived from the integrated circuits (IC) industry, but has developed in its own ways and directions never anticipated by its IC counterpart. Now a highly specialized discipline in its own right, MEMS manufacturing utilizes not only all of the modern IC process techniques, but also novel fabrication methods and non-microelectronic materials to create complete microsystems with sophisticated structures and empty space inside the structure. MEMS devices have a wide variety of applications performing basic signal transduction operations as sensors and actuators. The unique nature of MEMS devices introduces new challenges and failure mechanisms in the manufacturing process, which is different from those in IC device fabrication. 
The single foremost cause of yield failure in the manufacturing of MEMS devices is "stiction": the unintentional adhesion of structural components to one another or to the substrate. Because of the complicated topography of the MEMS devices, their surface area to volume ratio is very high, typically 100:1 through 10,000:1. At the same time, they are manufactured just a few microns above their supporting substrate. The combination of these characteristics makes MEMS devices very susceptible to surface forces, which can deflect the suspended members towards each other or the substrate. If the deflecting force is sufficiently strong, the MEMS structures can contact with and permanently adhere to the underlying substrate, causing stiction failure.
Release processes and their specific recipes must be developed and optimized based on the combination of structural and sacrificial materials involved. In this work, an all dry plasma ashing process is utilized to remove the photoresist organic sacrificial layer in the polysilicon MEMS release process. The exposed and embedded photoresist sacrificial layer is removed by a low temperature oxygen based microwave remote plasma process with a small amount of fluorine in a proprietary way. This process results in fully released MEMS structures without stiction failures. Existing fabs are able to use this MEMS release process with higher throughput and higher yield than their traditional processes.
All work was performed on the ULVAC EnviroTM dry resist and polymer removal system. This system has been widely used in the semiconductor industry for various solvent free processes, it incorporates both a microwave downstream plasma source and a non-damage RIE plasma source for resist stripping and residue cleaning (Fig. 1). The effect of temperature, fluorine content and plasma source is investigated. After the release process, the MEMS structure is inspected by optical microscope and SEM. Yield analysis is performed after the wafer has completed its entire process flow.
FIGURE 1. Schematic illustration of ULVAC's EnviroTM system with MW and RF power source.
The release step in the MEMS fabrication process selectively etches the sacrificial layer and releases the microstructures, creating the freestanding micromechanical structures such as cantilever beams. Several important criteria need to be considered here: (i) Release of MEMS structure without stiction failure, (ii) High ashrate removal of the sacrificial layer (in this case, photoresist), (iii) Complete residue removal, (iv) High selectivity of sacrificial layer to MEMS structure (in this case, MEMS structure is made of polysilicon).
Key process parameters in the release process are: (i) Process temperature, (ii) Process gases and gas ratios, (iii) Process pressure, (iv) Plasma sources. Three different processes were developed for the MEMS release process based on these key parameters.
FIGURE 2. Focal planes are different.
Process A is a high temperature microwave process. After Process A, the MEMS structures are not properly released. The focal planes are different for different structures, and the gaps between structures are not equal, as shown in Fig. 2 and Fig. 3.
FIGURE 3. Gaps are not equal.
Process B is a low temperature process with microwave and RF bias assistance. After Process B, the MEMS structures are not properly released either, similar problems are observed, i.e., different focal planes and unequal gaps.
FIGURE 4. Focal planes are the same.
Process C is a low temperature process with microwave only, with a small amount of fluorine. After Process C, the MEMS structures are properly released. Different MEMS structures are in the same focal plane now, and the gaps between structures are equal, as shown in Fig. 4 and Fig. 5.
FIGURE 5. Gaps are equal.
Process C has been applied to different types of MEMS release. Another example is the Digital Micro ShutterTM MEMS release. Schematic illustration of stuck and released MEMS structures are shown in Fig. 6.
FIGURE 6. Examples of stuck and released MEMS structures.
Pictures of the actual Digital Micro ShutterTM MEMS structures after release are shown in Figs. 7a and 7b, courtesy of Pixtronix. The sacrificial polymer layer was successfully removed without stiction failure to release the MEMS structure.
FIGURE 7A, 7B. Digital Micro Shutter MEMS structures after release.
Process C is a low temperature, oxygen based microwave process with a small amount of fluorine. The selectivity of sacrificial layer to polysilicon MEMS structure is about 900:1. The the selectivity of photoresist to polysilicon is shown in Fig. 8.
FIGURE 8. Selectivity of photoresist to polysilicon.
1. Process A: High temperature process.
To activate atomic oxygen's reaction with photoresist, high temperature is needed (>200 ??C). However, at elevated temperature, photoresist layer starts to reflow, forming a meniscus between the structure and the substrate, yielding an attractive capillary force. During the release process, surface tension forces the suspending structure to collapse and contact the substrate, followed by interfacial adhesion sufficiently large to overcome the elastic restoring force.
A dimensionless number, NEC, shown in Equation 1, defined as the elastocapillary number by the Mastrangelo group, , can be used to illustrate an elastic microstructure under a capillary pull:
In Equation 1, ?? is the spring constant, ??l is the liquid surface tension, ??c is the contact angle. The numerator of Equation 1 can be regarded as the elastic force, whereas the denominator is the capillary force. When the elastic force is greater than the capillary force, NEC > 1, the structure is free; otherwise, when NEC < 1, the structure is pinned.
For the high temperature Process A, once photoresist starts to reflow, it develops a capillary force, which eventually overcome the elastic force and causes stiction failure.
2. Process B: Low temperature process with MW and RF bias assistance
This process has been used in the manufacturing in the semiconductor devices, no plasma damage is seen. In addition, no electrical charging damage is observed from CHARM?? wafer monitor test either.
However, MEMS has different failure mechanism from IC manufacturing due to its unique nature. Transient electrostatic interaction/Coulomb force induced by RF plasma causes the suspending structure to collapse.
3. Process C: Low temperature process with MW only
Process C is conducted at room temperature without RF bias, in order to avoid both photoresist reflow problem and any transient electrostatic interaction. A small amount of fluorine gas is added to the gas mixture in order to effectively remove the photoresist sacrificial layer at room temperature conditions. In addition, fluorine treatment also reduces surface energy and helps to prevent stiction. After process C, the MEMS structures are fully released without stiction failure.
A low temperature, oxygen based microwave remote plasma process with a small amount of fluorine is utilized to completely remove the organic sacrificial layer and fully release the MEMS structures without stiction failure. The selectivity of sacrificial layer to MEMS structure is about 900:1.
1. R. Ghodssi, P. Lin, Editors, "MEMS Materials and Processes Handbook".
2. C. H. Mastrangelo, "Suppression of Stiction in MEMS", invited paper, 1999 Spring MRS Meeting, Boston, MA.
3. Y. P. Zhao, "Stiction and Anti-Stiction in MEMS and NEMS", Acta Mechanica Sinica, Vol. 19, No. 1, Feb. 2003.
YIN XU is Supervisor, Process Engineering, VLAD TARASOV is senior process engineer, and WEI CHEN is Senior Director Of Etching And Ashing Technology, ULVAC Technologies, Inc., Methuen, MA. KOUKOU SUU is General Manager, Institute of Semiconductor & Electronics Technologies; ULVAC, Inc., Susono, Japan.
Solid State Technology, Volume 55, Issue 4, May 2012