Contractor: Stanford University
Agreement Number: F30602-96-2-0308
Project Number: HJ1500-3221-0599
DARPA Order Number: N-7-0504/AO E117/12
Contract Period: September 13, 1996 - September 12, 1999
Date: January 31, 1999
Sponsor: Department of the Air Force
Technical POC: Robert W. Dutton
Report Prepared by: Robert W. Dutton
Task 2
Task 3
Develop and demonstrate fully-integrated, FEM-based prototype solver capabilities to model behavior and fabrication process-dependency of MEMS devices. This will include capabilities to consider multi-physics and materials dependencies as well as other process induced factors--geometry effects due to deposition/etching. The overall tool integration strategy will be to develop and test key components that overcome limits currently seen to be "bottlenecks" in commercial systems. Application-specific lumped modeling will be developed and used to guide the overall direction of the CAD efforts, including improved parameter extraction schemes that cross-link device performance with layout parameters that can in turn support design (and hopefully optimization) of practical MEMS devices (i.e., RF switch).
Approach:
Progress:
Pull-in voltage and buckling height (center deflection) measurements along with detailed 2D simulations were used to characterize electrostatically-actuated test structures fabricated in MUMPs. Effects of stepup anchors, dimples, and steps over underlying POLY0 layers were analyzed and included in the detailed simulation model. The extracted parameters, with no further fitting parameters, predict the behavior of a more complex three electrode device accurately. This confirms the validity of the parameter extraction technique, and presents the canonical benchmark as a viable verification test case to which coupled electromechanical simulations under development can be compared. [1]
The thickness and stress of polysilicon structures in MUMPs were shown to depend on whether the structures were connected to gold pads or not. Polysilicon layers connected to gold pads were thinner and had rougher surfaces than those not connected to gold pads, and exhibited larger stress gradients. Electrochemical potentials set up during the HF release etch are believed to be the cause. Such nonuniformities among nominally similar structures were previously unknown.
Significant progress has been made in the domain decomposition technique (in particular with respect to multiple depositions) and it has proven useful for creating realistic 3-D MEMS geometries (e.g. the canonical RF Switch test structure). The domain decomposition technique detailed in [4] decomposes the wafer into regions requiring 1-D, 2-D, and 3-D process simulation and carries out the appropriate level of simulation to automatically create the desired geometry. In the current implementation, the pseudo-3D technique (detailed in [2] & [3]) is used to perform the process simulation in the 3-D regions. While this is adequate for the RF switch, in general devices (e.g. the comb drive) a more robust 3-D technique is often desired. The investigation of commercial 3-D process simulators has been delayed due to delays in the release and availability of current tools (e.g. Taurus by TMA). Since conformal deposition is of great interest to the MEMS community due to the popularity of the MUMPS process, we are exploring the use of 3-D levelset to perform 3-D conformal deposition. The domain decomposition technique is the enabling technology for the use of 3-D levelset since it significantly reduces the size of the 3-D regions to simulate.
Preliminary tetrahedral meshes of the canonical structure have been created. The use of automatic tetrahedral mesh generation required by the complex geometry created from process simulation is discussed in [5]. While promising, they indicate the need to be able to modify the geometry prior to meshing to represent curved surfaces in a "meshing friendly" manner.
Software Interoperability:
Closure has been achieved on the demonstration of VRML as a portable, platform independent way of representing MEMS geometry. As the canonical RF switch test structure demonstrates, VRML is a capable and robust way of representing geometry of interest to the MEMS community.
Effort aimed at incorporating the constitutive information obtained for microbeams (Task #2) into a finite element framework have made considerable progress. This has been accomplished in a one-dimensional setting. A numerical integration algorithm has been devised for the viscoplastic material model. This allows the solution of a boundary value problem for various cases of loading, material parameters and temperature conditions.
The next step in the progression of this work is the extension of the constitutive model to multiple dimensions. This will be followed by the design of numerical algorithms to integrate the material model in a multidimensional setting. Thus the solution of realistic 3D problems with completely general boundary conditions will be possible, which, in turn, will enable the prediction of fatigue lifetime of the actual device.
Goals targeted for the next quarter:
Develop extraction schemes for materials properties using simplified MEMS test structures. In combination with the CAD tools efforts, the physical models (including microstructure) will be evaluated and modified as needed. The scope of material characterization efforts include both standard metal layers (possibly graded as well) and reliability issues such as plastic yield and fatigue. The extraction and validation steps for physical models is a key linking between the three sub-tasks.
Approach:
Progress:
1. Fatigue Study
Cyclic and static relaxation tests at various strains have been performed for Al microbeams of two micron thickness. The results of this study allow the construction of stress-strain curves for both cyclic relaxation and static relaxation. Both of these curves are located well below the tensile stress-strain curve, which is usually used to design mechanically loaded structures. These new curves let one infer how the material behaves due to cyclic loading as opposed to just one application of stress.
A post-test sample extraction procedure has been developed which allows the removal of tested, but not fractured microbeams without application of additional loads. Through this procedure it is possible to "freeze" the dislocation arrangement which has evolved throughout testing. Additionally, we have developed TEM sample preparation techniques. These allow us investigate and illustrate the dislocation states within the tested samples. An effort is now under way to link changes in mechanical properties, measured during testing, to microstructural changes which are observed through TEM analysis.
Aluminum microbeam samples of one, two, four, and eight micron thickness have been fabricated. Due to changes in the backside etching process the overall die yield increased from 26% to 65%.
2. In-situ Stress Measurement
Ex-situ stress measurements were conducted on samples of existing IR Detectors that have been modified for our measurement purpose.
A 3" sputtering system was used to deposit 1,100A thick Tantalum thin films onto the deflection membranes of the sensors. Before testing the sensors, we deposited blanket Ta films onto silicon wafers under the same conditions and measured the film stress by wafer curvature. The stress was between 0.5 and 1GPa, tensile. We measured the deflection voltage of the sensor at a constant stable tunneling condition, both before and after Ta deposition. According to our initial theoretical modeling, we expected to see a deflection voltage increase for a tensile Ta film.
The original IR Detector comprises three levels, i.e. the top Absorber-Wafer, the middle Membrane-Wafer, and the bottom Tip-Wafer. In our initial trials, the absorber membrane of the original IR Detector was removed to open a top window (2mmX2mm), while the Absorber-Wafer was left intact. Ta film was deposited through this window onto the deflection membrane. However, because the deflection membrane lies 1mm below this top window, significant shadowing occurred during sputtering deposition, which led to thickness non-uniformity of the deposited Ta thin film. When we measured the change in deflection voltage at constant stable tunneling condition, we actually measured a voltage decrease for the tensile film, opposite to what was expected. To determine the cause of this anomaly, we examined the film surface topography using interferometry, and observed that the thickness was indeed not uniform. The film appeared as an island sitting on top of the deflection membrane. We simulated this situation using finite element modeling and were able to reproduce the experimental anomaly. Based on this, in order to achieve a uniform coverage of the thin film we had to further remove the entire Absorber- Wafer. The final modified device comprises only the Membrane-Wafer and the Tip- Wafer. We repeated the experiment and measured the deflection voltage before and after Ta deposition. According to our initial theoretical prediction, the observed voltage increase of 11.2V corresponds to a stress increase of about 0.5Gpa, which is consistent with the stress measured by wafer curvature.
Fabrication on the first generation dedicated stress measurement sensors has resumed.
Difficulties/Problems:
Design and implementation of MEMS structures, including materials parameter extraction, will be used to test the CAD and physical models based on MEMS devices of interest to DARPA. The experimental measurements made with these structures will facilitate the critical evaluation of models, physical parameters and overall simulation accuracy of CAD for MEMS devices. The test vehicles from this work also support canonical benchmarking of both new materials for MEMS and accuracy of Composite CAD in MEMS applications, specifically the RF switch.
Approach:
Progress:
This past quarter was spent addressing fabrication issues necessary to generate a reliable materials system. Before we can consider an end-user application, we must have confidence that our process flow will be successful.
We performed several process experiments involving the deposition and patterning of the aluminum membrane layer which finally yielded well patterned structures. Unfortunately, this process flow also caused the suspended layer to delaminate from the sacrificial layer. A processing adjustment has been made to improve the adhesion between the suspended layer and the sacrificial material with a dramatic reduction in the delamination. These structures now need to be released.
This process flow has been pursued because it is a purely sputtered film materials system. Our alternative deposition system, the Innotech Evaporator gives us the capability of depositing unique metal layers. However, this evaporator will also bombard any on-board circuitry with high energy radiation. It has been historically proven that most of this damage can be annealed. Since this is an area of exploration, it is necessary to develop both sputtered and evaporated processes.
We have also begun the design of end-user devices. This design will incorporate the cananonical test structures being pursued by the modeling portion of this project. Having recently completed a Silicon-on-Sapphire (SOS) process run, we have a proven capability to generate high voltages on chip. It is now our desire to integrate the control circuitry and the MEMS RF components. To guarantee success, the structures must be design with the voltage generation limits of the SOS process.
Difficulties/Problems:
Technology Transfer:
Our interactions with AFRL, Hanscomb AFB continue. We have regular communications with them regarding fabrication concerns.
II. Financial Information: AF F30602-96-2-0308
| Total Amount Funded | $1,627,185 | |
| Salaries and Wages | $541,528 | |
| Staff Benefits | 90,215 | |
| Travel | 26,988 | |
| Expendable Mat./Services | 166,927 | |
| Repairs and Maintenance | 19,693 | |
| -------- | ||
| Total Direct Costs | $845,351 | |
| Capital Equipment | 128,593 | |
| Student Aid (Tuition) | 84,619 | |
| Indirect Costs | 461,267 | |
| Total Expenses to date | $1,519,830 | |
| ---------- | ||
| Funds to date, 12/31/98 | $ 107,355 | |