Control of a drop-ejector used as photo-resist dispenser P.—E. Rochea, A. Hanssonb and B. T. KhuriYakubL a L. Ginzton Laboratory, Stanford University Stanford, CA 94305, USA binformation Systems Laboratory, Stanford University Stanford, CA 94305—9510, USA ABSTRACT Photo-resist dispensers traditionally apply a generous amount of resist on the wafer and then spin the wafer to reach a uniform desired thickness. With this technique, over 95% of expensive and hazardous liquid is wasted. The goal of this project is to reduce the waste by using a drop-on-demand ejection technology to apply the photo-resist. In practice, a controller turns out to be necessary to compensate for the variability of the performance of the ejectors, to insure the stability of the ejection, and to speed up the transient regime for drop-on-demand operation. This paper reports on the design and simulation of this controller. Keywords: Smart Structures, Control, Piezoelectric, Drop-Ejector, Modeling, Non-Linear 1. INTRODUCTION A key step in ICs' and nanostructures' fabrication is photo-resist coating. Present technology for this coating is very wasteful and does not allow spatial discrimination of the coated region. In this work is explored an alternative way of performing this task by adapting drop-ejection technology. This technology is currently used for printing.1 The ejector is built around a circular membrane glued at the bottom end of a cylinder. The resulting cavity acts as a photo-resist tank. When the membrane is perforated at its center and vibrated at resonance, drops of photo-resist are ejected. A flat piezoelectric ring, glued on the bottom side of the membrane and driven by a voltage source provides the membrane actuation. The ejector has previously been described in a couple of references.2'3 A micro-machined version is also being developed.4'5 Similar devices have been developed by others.68 Research on piezoelectric flexural transducers has a long history.9 Due to imprecise manufacturing and aging there is a certain variability in the behavior of the micro-machined/macro drop-ejectors. The objective is to use feedback control to compensate for this variability. A crucial point is to obtain an adequate model suitable for model based controller design. The model used is a third order non-linear differential equation,1° where especially the non-linear part is uncertain. It will be seen that is possible to synthesize a controller that makes the closed loop robust with respect to the uncertainty. The controller has been implemented in analog electronics and successfully tested on a drop-ejector. The dynamics and control of piezoelectric laminated circular plates is an active are of ' Piezoelectric actuators are also used for control in other areas such as gust alleviation control of wings,'4 position and force control for micro-robotics,15 and vibration control of flexible space structures,16 flexible manipulators,'7"8 and cantilever 19 Tue remaining part of the paper is organized as follows. In Section 2 the model is derived. In Section 3 the controller is synthesized and evaluated in simulations. Finally, in Section 4 some concluding remarks together with suggestions for future research are given. P.-E. Roche was on leave from CRTBT CNRS - 25 avenue des Martyrs, BP 166 - 38042 Grenoble Cedex 9, France. Send correspondence to Anders Hansson, E-mail: andersh©isl.stanford.edu. Part of the SPIE Conference on Mathematics and Control in Smart Structures • San Diego, California . March 1998 446 SPIE Vol. 3323 . 0277-786X/98/$10.oo