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.
Sub-surface damage is a serious issue in the manufacturing of precision optical elements. For very lightweight mirrors,
changes in surface stresses through various process steps that sequentially relieve stored up strain energy lead to poor
convergence to eventually desired figures. For high fluence laser applications damage sites can prove to be deleterious
to the functioning of the optic. For precision refractive optics, birefringence resulting from damage and stress can be an
issue as well.
Conventional methods of optical finishing rely mostly on mechanical abrasion, requiring an iterative process of subsurface
damage mitigation from earlier process steps while minimizing damage from the current process step. This
manufacturing paradigm leads to very long lead times and costs in producing high precision optics.
Reactive Atom Plasma (RAP) based figuring is introduced as a technique to simultaneously remove damage from prior
steps while imparting no further damage and figuring the surface of the optic. RAP based figuring demonstrates a new
approach to the figuring of precision optics using a non-contact sub-aperture atmospheric plasma footprint to shape the
surface. RAP figuring has been illustrated to remove Twyman stresses caused by conventional optical processing
technologies. Twyman stresses on coupons of various glass materials and ceramics have been characterized and RAP
removals of the damage layer have led to removal of the strains and thence the associated stress. The process is
deterministic, enabling the figuring of high-precision surfaces with little to no sub-surface damage.
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.
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.
Emerging applications in the microelectronics industry impose special manufacturing requirements that are not well addressed by conventional manufacturing techniques. On the other hand, advances in laser technology, optics and beam steering combined with a better understanding of laser-material interaction make laser micromachining a viable, attractive, cost-effective and in some cases enabling technology to support these applications. This paper reviews some of the emerging applications in the microelectronics industry that are well served by laser micromachining and discusses the advancements in lasers, optics and beam steering that enable cost effective laser micromachining. It also discusses some open issues that are the subject of current and future research.
Conference Committee Involvement (1)
Laser Applications in Microelectronic and Optoelectronic Manufacturing IX