Analog tunable electrostatic and piezoelectric diffractive gratings

We have designed and fabricated tunable gratings with period tunable to within fraction of a micrometer. We actuated the gratings by electrostatic and piezoelectric means, and demonstrated period changes of order 1nm. Introduction Optical MEMS devices have included micro-mirrors [1, 2], tunable gratings, tunable Fabry-Perot resonators, etc. For example, Silicon Light Machines has commercialized the Grating Light Valve [3] as a light modulator. The period of the GLV is controlled digitally by moving the grating beam in the vertical direction. Analog control of the beam height permits modulation of the diffraction efficiency. Senturia and collaborators demonstrated the Polychromator [4] which utilizes similar actuation principle. The angular resolution of these systems is limited by the minimum beam width (on the order of 2-3 microns). We here show a tunable grating design which permits analog control over the diffraction angle. This is accomplished by analog actuation, electrostatic or piezoelectric, of the grating beams in the lateral direction. Thus, our device trades deflection range for angular resolution. Applications for high resolution analog tunable gratings include microspectrometers, external cavity tunable lasers, thermal compensators for wavelength multiplexer-demultiplexers, etc. Device design The schematic design of the electrostatic device is shown in Fig. 1(a). Two comb -drives [5] pull on both sides of a periodic structure. The structure is composed of the grating bars in the center window and the flexures which connect each bar. The flexures are essentially springs which determine the stiffness of the whole structure. The whole suspended structure is attached to the silicon substrate through four anchors. The key design parameters in this design are the flexure stiffness, the comb -drive finger pairs, and the grating period. The stiffness of the flexure can be estimated by k=Ewt/L, where the effective spring constant for one period is on the left side E is the Young’s modulus of the material, t is the thickness of the structure, w is the width of the flexure beam, and L is the length of the folded beam. The flexure stiffness is selected based on a trade-off: low tuning voltage (<100V) requires the device to be compliant. Additionally, we require the device to be stiff enough that the resonant frequency be high (10 KHz or higher). The driving force is rendered by the two comb -drives on the sides. Comb -drive draws essentially no current; therefore, it minimizes power consumption. The disadvantage is that comb -drives deliver small force, usually limited to microNewton or less. The force can be estimated as F=-N?tV/2g, where N is the number of finger pairs, ? is the permittivity of air, t is the thickness of the structure, g is the gap distance between two adjacent fingers, and V is the applied voltage. The minimum grating period is set by the resolution of the available lithography tool. Since the flexures on the sides of the grating must be defined, we find the minimum grating pitch is, at best, 4 times the design rule for 75% duty cycle or 6 times the design rule for 50% duty cycle. In the piezoelectric version, the driving force is via the deposited thin-film piezoelectric actuators. The diffractive grating was etched above the membrane such that its period could be tuned progressively to a desired value in response to stretching of the membrane. Fig. 1(b) illustrates the device design, in a specific doubly-hinged membrane configuration. Other configurations, such as a free cantilever and a perforated membrane, were also designed so as to have a larger period change. In contrast to the electrostatic version, the piezoelectric design neccessitates more difficult fabrication, on a total of five masks to fabricate, although it is a very compact and has accurate control at sub-nanometer resolution. In addition, it permits low voltage actuation, less than 5V, although there is somewhat higher power consumption compared to the electrostatic comb -drives. Piezoelectric device modeling suggests a possible deformation to the magnitude required. A multi-morph structure model [6,7] was implemented and the devices were designed such that the deformation, for different characteristics of the piezoelectric material, would be on the order of 1-2 nm per period at an applied voltage of 5V. Fig. 1 Device schematic: (a) Electrostatic comb -drive actuation, (b) Piezoelectric thin-film actuation The diffraction angle of a grating is given by sin?=m?/p, where m is the diffractive order, ? is the wavelength of the incident light, and p is pitch of the grating. Expanding the previous equation for a smaller period change ? p, we find the response angle ? ?? -m?? p/pcos? which reduces to ? ?? ?? p/p for m=1 and ? <<1 rad. For example, if ? p=10 nm we obtain ? ?=10 degrees at ?=532 nm. Fabrication process We start the fabrication process, for the electrostatic version, with an SOI (Silicon On Insulator) wafer which has a 10-micron thick device layer and a 0.5-micron thick buried oxide. First we etch through the device layer with DRIE (Deep Reactive Ion Etching) technology. The advantage of using DRIE is that it allows us to obtain grating beams and flexures that are thick (10 microns) in the vertical direction. This increases the vertical stiffness of the structure and avoids potential stiction problem during the releasing step (see below). The design also includes lateral bumps (see Fig. 2) to ensure that no lateral stiction occurs either. More importantly, the structure is essentially residual-stress free because there is no film deposition. Since the buried oxide behaves essentially like a good etch stop, our design also avoids the loading effect (an etching non-uniformity due to different exposed areas) which shows up almost in any etching process. The DRIE process is followed by an HF etching step to release the moving parts. Since the lateral dimension of the movable parts is much smaller than that of the fixed parts, we have large process latitude during the time-control releasing process. After releasing, we deposit an aluminum film to form the electrodes and the reflective surface on the gratings. In the piezoelectric version, a 0.2-micron oxide is first grown as a diffusion barrier for the piezoelectric material. The platinum bottom electrode is then deposited via evaporation and patterned. The piezoelectric material, Pb(Zr,Ti)O3, is deposited and patterned with a HF/HCL reagent. The top electrode and the gratings are then deposited and patterned similarly like the bottom electrode. The final process step consists of a potassium hydroxide etch from the backside of the wafer to release the membrane structure.