Analog Micro-Clock Driven by Scratch-Drive Actuators Paul Friedberg University of California at Berkeley, Electrical Engineering Department Introduction Scratch drive actuators (SDAs) have been used in a wide variety of MEMS applications. Generally speaking, SDAs are convenient because the entire SDA structure is displaced during actuation, which is useful for motion over long distances. SDAs are capable of producing 100 µN of force [1] and have a typical step size on the order of 10 nm, so that a driving signal modulating at a frequency of 100 kHz corresponds to a velocity on the order of 1 mm/s. Finally, the step size and force output of an SDA can, to a degree, be tuned simply through the selected geometry. These attractive features of the SDA make it one possible mechanism for actuating an analog micro-clock. SDAs have been researched extensively but certainly not exhaustively, with several papers published in the field of dimension optimization by Akiyama, Fujita, and others [1][2][3]. Additionally, SDAs have been used extensively as actuating mechanisms for rotating movement [4] as well as linear positioning [5]. Design and Fabrication The Sandia SUMMiT process will be used for fabrication of the analog micro-clock. Referring to the processes flow shown below, polysilicon structural layers Poly1 and Poly2 will be used to define the clock hands, and a pin joint around which the hands rotate will be created using the Poly0 and Poly3 layers. A more detailed process flow follows. A pin contact will be created using the Poly0 layer, and then in the SacOx1 layer we will begin to define the shape of the hands of the clock. Specifically, the Dimple1_Cut mask and etch (1.5 µm out of the 2 µm film) will be used to define a “mold” for the bushing of the SDA which will drive the hour hand. The SacOx1_Cut etch will be used to begin the definition of the eventual mold for the bushing of the minute-hand SDA. In addition, a Dimple1_Cut region will be etched surrounding the Poly0 pin contact to promote an electrically conductive path between the hour hand (and consequently, the minute hand which will rest on the hour hand) and the pin upon release Fig. 1. Sandia SUMMiT process layer stack. etch. The subsequent Poly1 deposition and Poly1_Cut steps will define the hour hand and its attached SDA. Next, the 0.5 µm TEOS SacOx2 deposition will serve to separate the Poly1 and Poly2 hands of the clock as well as to finish defining the mold for the SDA of the minute hand (now the bushing of this SDA will not contact the nitride insulating layer, ensuring correct release during sacrificial oxide etch). The Poly2 deposition and Poly2_Cut steps define the minute hand and accompanying SDA. Worth mentioning here is the crucial consideration that the minute hand should be aligned directly on top of the hour hand (being careful to straddle the bushing dimple of the hour hand with the minute hand) to ensure clearance such that the minute hand can pass over the hour hand as will be required. Finally, the SacOx3_Cut, Poly4 deposition, and Poly4_cut steps will be used to create a pin restraining the clock hands but allowing them to freely rotate. Since both the hour and minute hand SDAs sit 0.5 µm above the insulating nitride plane, we know that upon release etch of the sacrificial oxides, both SDAs will contact the wafer surface and each component of the entire structure will be electrical contact, allowing actuation of both hands using a single input signal. Powering the device is relatively straightforward; a contact to the substrate can be cut through the insulating layer outside the radius of the clock, and since the Poly3 layer is deposited above the planarized SacOx3 layer, a contact to the structure can be fabricated by simply extending a “wire” which is suspended above the clock from the pin joint to a point outside the radius of the clock,where the wire can be connected to a second Poly0 pad and metallized. Fig. 2. Cadence layout of an analog clock. For illustrative purposes, the SDAs were drawn much larger (~5x) than scale to show the structure more clearly. Following release etch, the minute hand will drop onto the hour hand, fitting snugly but still being able to pass over the hour hand entirely. Fig. 3. Cross-section of device following release etch. The minute hand is in one mechanical piece, but must avoid running over the bushing of the hour hand. SDA Motion Mechanism The SDA consists of two simple components, which can be fabricated in a single process step. Shown below, these components are a rectangular plate (dimensions LP*WP) and a bushing (height HB) which elevates the plate above the substrate. Fig. 4. Standard SDA, from [3]. As a voltage is applied between the plate and the substrate, the plate is drawn into contact with the insulated substrate and the bushing is jutted out slightly. When the voltage is reversed, the plate is restored to its original position and the new location of the bushing acts as a pivot point around which the SDA “scratches” forward. Fig. 5. Scratch drive actuation, from [3]. Akiyama and Shono [2] showed that the simple geometric relation ∆x = HB2 / 2(LP – LP’) (1) gives the step size of the SDA, where HB is the height of the bushing and LP’ is the cross-sectional length of the SDA that is drawn into contact with the insulator film. Unfortunately, further research has shown that the step of the SDA won’t follow this equation strictly [1,2]. As LP’ increases, the static friction between the section of the plate contacting the insulating plane increases correspondingly, and the SDA might “grab” slightly, so that the step does not increase as much as expected, or might evendecrease. Therefore, this relation should be used only as a rough guideline from which to choose some nominal values of the SDA geometry. Of course, LP’ depends on both LP and HB, as well as the applied voltage and thickness of the SDA plate—therefore, with the right geometrical configuration we should be able to tailor the dimensions of the SDA to generate the desired ratio between step sizes for the hands of the clock (specifically minute:hour = 12:1, since the minute hand must make 12 revolutions for each single revolution of the hour hand). Then, the frequency of the applied signal simply needs to be adjusted so that the hands rotate at proper rates to correctly tell time.
View Full Document
Unlocking...