Pradeep Subrahmanyan

Reactive atom plasma (RAP) processing of mirrors for astronomy

Modern day telescopes for astronomy have very complex requirements. Both ground and space based telescopes are getting much larger placing significant productivity requirements on the manufacturing processes employed. Conventional manufacturing paradigms involving mechanical abrasion have limitations related primarily to the material removal mechanisms employed. Reactive Atom Plasma (RAPTM) processing is a sub-aperture, non-contact, deterministic figuring technology performed at atmospheric pressures. The process has high material removal rates, and given the non-contact and atmospheric nature lends itself very well to scaling up for large aperture mirrors/segments. The process also benefits from its ability to simultaneously remove sub-surface damage (SSD) while imparting the desired figure to the surface. Developments are under way currently to scale the process up towards larger clear apertures while being able to figure in high spatial frequency features.

Rapid fabrication of lightweight SiC aspheres using reactive atom plasma (RAP ) processing

Polishing has traditionally been a process of mechanical abrasion with each iteration removing the damage from the previous iteration. Modern sub-aperture techniques such as CCOS, MRF polishing etc. have added a considerable amount of determinism to this iterative approach. However, such approaches suffer from one significant flaw, i.e., the algorithms are completely guided by figure error. This approach fails when there is a considerable amount of strain energy stored in the substrate and becomes very evident when the aspect ratio of the mirror increases significantly causing relaxation of strain energy to have deleterious and unpredictable effects on figure between iterations. This is particularly pronounced when the substrate is made of a hard ceramic such as silicon carbide requiring a considerable amount of pressure to obtain any appreciable material removal rate. This paper presents an alternate approach involving a stress-free figuring step and a buffing step intended to recover the surface roughness.

Highly sensitive focus monitoring technique based on illumination and target co-optimization

We present a cost-effective focus monitoring technique based on the illumination and the target co-optimization. An advanced immersion scanner can provide the freeform illumination that enables the use of any kind of custom source shape by using a programmable array of thousands of individually adjustable micro-mirrors. Therefore, one can produce non-telecentricity using the asymmetric illumination in the scanner with the optimized focus target on the cost-effective binary OMOG mask. Then, the scanner focus variations directly translate into easily measurable overlay shifts in the printed pattern with high sensitivity (ΔShift/Δfocus = 60nm/100nm). In addition, the capability of using the freeform illumination allows us to computationally co-optimize the source and the focus target, simultaneously, generating not only vertical or horizontal shifts, but also introducing diagonal pattern shifts. The focus-induced pattern shifts can be accurately measured by standard wafer metrology tools such as CD-SEM and overlay metrology tools.

Applications of control systems and optimization in the design of semiconductor capital equipment

This tutorial presents several examples of high performance control systems in the Semiconductor equipment industry. Examples include technologies used in lithography and mask inspection.