mems

Mat. Res. Soc. Symp. Proc. Vol. 741 © 2003 Materials Research Society J5.2.1 Fabrication Of Flat RF MEMS Switch Membrane By Minimizing Of Stress GradientsIn The Au Membrane StructureJong-Seok Kim, Hoon Song, Jin Woo Cho1, Eun Sung Lee, Sun Hee Park, Mun Chul Lee,Dong Hwa Shim, In Sang Song, Jung Woo Kim2, Seok Jin Kang and Ki Moo SongME Center, MEMS Laboratory, Samsung Advanced Institute Of Technology,416, Mae-tan 3 dong, Pal-dal-gu, Su-won city, Korea 442-742E-mail : kjs10000@samsung.co.kr1CSE Center, 2MD LaboratoryABSTRACTRF MEMS(Micro-Electro-Mechanical-System) switch technology is one of powerful solution for future RF systems. This technology provides low insertion loss, High linearity and broad bandwidth. Wide driving membrane used MEMS switch can reduce driving voltage but it is easy to bend because of the stress gradient. In order to solve this problem we fabricated Au cantilever in various sputtering condition and various substrate materials. As a result of this experiment, we fabricated cantilever which was bent within 1 um, with 2 um thickness and 340 um length. We applied this condition to RF MEMS switch and we fabricated switch membrane within 1 um bend, under 10MPa stress gradient.INTRODUCTIONThe most remarkable tendency in the recent cellular phone technology is a multi-functional integration with smaller size and lighter weight. In this purpose, not a just miniaturization but device integration by the new method is essentially needed. One of methods is a fabrication of RF device by MEMS technology. MEMS device may be fabricated various ways using processes have been extended to achieve three dimensional structures by the techniques of bulk micro-machining, in which large amounts of the substrates are removed, or surface micro- machining in which layers are deposited on the surface followed by the removal of a sacrificial layer to release a moving structure.[1] RF MEMS switch fabricated by MEMS technology has many advantages such as low insertion loss, high isolation, low current consumption, excellent IP3 characteristic and high power capability. However, it requires a higher driving voltage than conventional switches. Recently, many researchers have extensively studied RF switches operated in low voltage. In the development of low voltage driving RF switch, one of the big problems is that driving voltage is increased by the bend of Au membrane. This kind of bend is caused by internal stress degree of slop. The range of the bend is several micro-meter. Figure 1( a ) is a photograph of RF MEMS switch and figure 1( b ) image is a picture taken by optical 3D profiler. By using this equipment, we could measure X and Y direction profiles of switch membrane which bended 2.4um in X direction, 2.1um in Y direction. In order to find out control methods and critical factors that affect stress gradient, we fabricated Au cantilever with different width, different length and by changing sputter deposition pressure, temperature, RF powers, sacrificial layer materials and substrate materials.( a ) ( b )( c ) ( d )Figure 1. Bended RF MEMS switch ( Optical 3D profiler image )FABRICATIONFabrication method of cantilever is shown in figure 2. On the 500um thickness quartz wafer (a), we deposited Al (or Cu) as a sacrificial layer (b). After patterned sacrificial layer (c), Ti (or Cr) and Au metal was deposited as a buffer and structure layer by sputter (d). Au structure was patterned by wet etching solution or ion milling (e) and released the structure by removing sacrificial layer (f). Size of cantilever is shown in table Ⅰand Fabricating condition is shown in table Ⅱ(a) (b)(c)(d)(e) (f)Figure 2. Cantilever fabricating processTable Ⅰ. Cantilever sizeW: Cantilever widthL : Cantilever length g : Cantilever pitch to pitchTable Ⅱ. Fabricating conditionCondition RangeSputter pressure 10, 3 mTorr Sputter temperature 25, 100°C Sputter power 160, 90 watt Structure buffer layer Ti, Cr Structure patterningmethod Ion milling, Wet etching Sacrificial layerCu, AlMEASUREMENTAfter fabrication, we measured the length and the deformation of cantilevers with optical 3D profiler (Figure 3-a). Stress gradient of Au cantilever was calculated by simulation (Figure 3-b). When we measured cantilevers, sticky samples were excluded. We compared cantilever deformation with simulated data and found out stress gradient of them indirectly.(a) Measured cantilever deformation (b) Simulated deformation vs. stress gradientFigure 3. Stress gradient measuring methodCONCLUSIONWe used DOE (Design Of Experiment) method and analyzed these data by using Minitab® soft-ware. As a result, we found out that buffer layer, Au patterning method and sputter working pressure were main factors that affected stress gradient of Au structure. In order to minimize the stress gradient of Au membrane we deposited Al as a sacrificial layer, 300Å thickness Ti as a buffer layer and 2um thickness Au as a cantilever structure. Au cantilever was deposited by sputter with 4.5mTorr as a working pressure, 110Watt as a RF power. Au metal was patterned with wet etching solution. With this optimal condition, we fabricated a cantilever that was bended within 1um, with 2um thickness and 340um length. We applied this condition to RF MEMS switch and we fabricated Au membrane within 1 um bend, under 10MPa stress gradient ( Figure 4 ).( a )( b ) (c)Figure 4. RF MEMS switch within 1um bendREFERENCES[1] N. Maluf, “ An Introduction to micro-mechanical systems engineering”. Norwood, MA:Artech House, 2000, CH.1.Y profile。