| This thesis proposes a systematic design and control approach to allow the cheaper, simpler, and more reliable wet etching technique to be used to fabricate future fine geometry, high capacity recording heads. An Optimal Open Width design principle is proposed that is capable of generating near-vertical wall profiles, which is highly desirable for small geometries, but currently only achievable via the expensive dry etching process. Furthermore, close-loop optimal control of the wet etching process, accompanied with augmented Kalman filtering, can result in higher yields, less equipment down time, reduced overall handling and human error, as well as tighter process control and greater flexibility.;This research started with the experimental modeling of the wet etching of sputtered aluminum oxide (alumina, ;Although considered amorphous, the sputtered aluminum oxide film displayed significant anisotropic behavior when etched in warm phosphoric acid. A novel double-layer model was proposed to explain the observed anisotropy. It was hypothesized that there was a fast-etching thin layer at the interface of photoresist and alumina. Side wall profiles simulated by this double-layer model agreed satisfactorily with the scanning electron microscopy (SEM) pictures taken of actual parts. Analysis using electron spectroscopy for chemical analysis (ESCA) supported the double-layer hypothesis.;The shallow wall angle caused by the anisotropic etch is detrimental to the process. An unique metal masking technique was invented by depositing a thin chromium layer between the photoresist layer and the alumina film. Experiments showed that isotropic etching was achieved, and the undercut was decreased by fifty percent.;The optimal open width (OOW) design principle was proposed to allow, for the first time, the simultaneous control of the critical dimension (CD) and the wall angle. The OOW design principle utilizes isotropy obtained by employing a metal mask, and is capable of driving both the CD and the wall angle to their setpoints. This design principle will also help alleviate the photoresist lift-off problem by minimizing undesired undercutting.;A closed-loop optimal control system was then designed to implement this OOW design principle. A time-optimal control problem with a dual-term terminal cost, including both the critical dimension and the side wall slope, was formulated. Pontryagin's Maximum Principle was applied to solve this problem. A singular arc solution did not exist and the control action was bang-bang. The optimal switching time was calculated analytically. (Abstract shortened by UMI.). |
