1、90 FY06 Engineering Research and Technology Report Gray-Scale Lithography for Sloped-Surface 3-D MEMS Structures Christopher M. Spadaccini (925) 423-3185 spadaccini2llnl.gov T echBase M icrofabrication techniques that originated from the IC community typically yield 2-D extruded geometries or struct
2、ures with limited angles due to crystallographic orientation. Gray-scale lithography in MEMS is capable of generating a gradient height pro le in photoresist, and subsequently in silicon, after deep reactive ion etching (DRIE) or other dry etching techniques. Our work has sought to establish this 3-
3、D micro- fabrication capability at LLNL. Gray- scale lithographic techniques previously reported in the literature were used to baseline and calibrate arbitrary sloped- surface, 3-D microstructures at LLNL. Typically, photolithographic pro- cesses involve a photomask with only an opaque “dark eld” a
4、nd a transparent “clear eld,” resulting in 2-D features with relatively straight sidewalls in the photoresist. The gray-scale technique is performed by using a photomask with multiple, discreet “gray-levels” or with pixilated features to locally modulate the intensity of UV light used in the standar
5、d photoresist exposure process. This results in locally varied photoresist exposure and correspond- ingly varied depth/thickness upon wet chemical development. After DRIE, the 3-D depth pro le is transferred to the silicon substrate and altered, based on the etch selectivity to silicon ver- sus phot
6、oresist. By varying the optical density and spacing of the gray levels on the photomask, an arbitrary angle in the silicon microstructure can be achieved. This capability will enable a whole new class of microstructures not previously considered manufacturable at LLNL. Project Goals The result of th
7、is work will be the capability to fabricate arbitrary angle, sloped-surface, 3-D microstructures. Deliverables include: 1. a well-de ned process for fabricat- ing 3-D MEMS structures (sloped surfaces) of arbitrary angle based on a combination of parameters such as photomask optical density, spacing
8、of Figure 1. Calibration squares of varying optical density and micro-lens arrays on the HEBS glass gray-scale photomask.91 Lawrence Livermore National Laboratory Micro/Nano-Devices and Structures gray levels, photoresist thickness, and photoresist development time; and 2. documentation of the proce
9、ss and parametric study for general use so that the LLNL microfabrication com- munity at large can easily fabricate a microstructure with an arbitrary sloped surface. Relevance to LLNL Mission The ability to fabricate sloped surfaces at an arbitrary angle in silicon microsystems allows for a host of
10、 new geometries not previously considered, and will shift the overarching microfab- rication paradigm away from 2-D struc- tures. The availability of this technique will advance the core microfabrication competencies at LLNL, which provides vital support to both internal and exter- nal customers. FY
11、2006 Accomplishments and Results High-energy beam-sensitive (HEBS) glass from Canyon Materials, Inc. was se- lected as the material for the photomask. HEBS glass is doped with a photoinhibi- tor that changes opacity when exposed to an electron beam. The change in optical density of the glass varies
12、with the inten- sity of the beam, thus controlling the gray levels. The theoretical minimum feature size for a gray level is the thickness of the beam, and 1000 discreet gray levels of varying optical density are possible. The photomask used in this work contained a variety of features intended to c
13、alibrate the gray-scale process in LLNLs clean- room, and show some of the capabili- ties of the technique. Features included micro-lens arrays, grating structures, tapered structures of varying height, and calibration squares of varying height to correlate photoresist thickness to mask optical dens
14、ity. Figure 1 shows some of the features on the photomask. Two types of photoresist were calibrated with the gray-scale mask. These included AZ4620 thick resist and AZ1518 thin resist. A range of thicknesses (1 to 10 m) of each of these photoresist types were spun onto silicon wafers, exposed to UV
15、light, and developed. The photoresist profiles were then characterized using a SEM and an optical interferometer. Figure 2 shows interferometer mea- surements of photoresist profiles for the micro-lens features. By care- ful measurement of the remaining photoresist height, a calibration curve of res
16、ist thickness versus photomask optical density for a given set of pro- cess conditions was generated. These curves can then be used by engineers to design gray-scale photomasks for custom applications. Figure 2. Optical interferometer measurement of micro-lens photoresist pro les. Figure 3. SEM of v
17、arying height calibration squares after DRIE in silicon. Figure 4. SEM of micro-lens arrays after DRIE in silicon.Finally, DRIE of the photore- sist profiles was performed and 3-D features in the silicon substrates were obtained. Figures 3 and 4 show SEMs of the micro-lens arrays and the varying hei
18、ght calibration squares in silicon. The overall etch depths for these features were 120 m and the AZ4620 thick photoresist was used.As a result of this work LLNL now has the capability to produce arbitrary, 3-D, sloped-surface microstructures for a variety of applications. 0.4 Distance (mm) +2085.45 1 0 0.2 0 0.6 0.8 Height (m) 1 2 3 701.46 701.46 0 +2085.45 nm mm mm 0 1.48 1.11 nm
