Bulk vertical micromachining of single-crystal sapphire using inductively coupled plasma etching for x-ray resonant cavities

Abstract

To provide coherent x-ray sources for probing the dynamic structures of solid or liquid biological substances on the picosecond timescale, a high-aspect-ratio x-ray resonator cavity etched from a single crystal substrate with a nearly vertical sidewall structure is required. Although high-aspect-ratio resonator cavities have been produced in silicon, they suffer from unwanted multiple beam effects. However, this problem can be avoided by using the reduced symmetry of single-crystal sapphire in which x-ray cavities may produce a highly monochromatic transmitted x-ray beam. In this study, we performed nominal 100 µm deep etching and vertical sidewall profiles in single crystal sapphire using inductively coupled plasma (ICP) etching. The large depth is required to intercept a useful fraction of a stopped-down x-ray beam, as well as for beam clearance. An electroplated Ni hard mask was patterned using KMPR 1050 photoresist and contact lithography. The quality and performance of the x-ray cavity depended upon the uniformity of the cavity gap and therefore verticality of the fabricated vertical sidewall. To our knowledge, this is the first report of such deep, vertical etching of single-crystal sapphire. A gas mixture of Cl2/BCl3/Ar was used to etch the sapphire with process variables including BCl3 flow ratio and bias power. By etching for 540 min under optimal conditions, we obtained an x-ray resonant cavity with a depth of 95 µm, width of ~30 µm, gap of ~115 µm and sidewall profile internal angle of 89.5°. The results show that the etching parameters affected the quality of the vertical sidewall, which is essential for good x-ray resonant cavities.

Pillars or Pancakes? Self-Cleaning Surfaces without Coating

“…Finally pancake structures on the micron scale can be mass produced comparatively cheaply in a huge range of materials (i.e., with nanoimprint and etching). This is particularly relevant for sapphire where it is very difficult to make high aspect ratio pillar structures with controllable sidewall profile. − …”

Atomic-Scale Etching Mechanism of Aluminum with Fluorine-Based Plasma

“…High selectivity and precise control of alumina etching by Ar/HBr/CH 3 F/O 2 plasma were discussed by Suzuki and co-workers . A gas mixture of Cl 2 /BCl 3 /Ar was used to etch the sapphire with process variables including BCl 3 flow ratio and bias power by Chen and co-workers . On the other hand, the etching properties of alumina are characterized by Koo and co-workers using N 2 /Cl 2 /BCl 3 and Ar/Cl 2 /BCl 3 gas chemistry.…”

“…11 A gas mixture of Cl 2 /BCl 3 /Ar was used to etch the sapphire with process variables including BCl 3 flow ratio and bias power by Chen and co-workers. 12 On the other hand, the etching properties of alumina are characterized by Koo and coworkers 13 using N 2 /Cl 2 /BCl 3 and Ar/Cl 2 /BCl 3 gas chemistry. Further, the common product of alumina etching with fluorinebased plasma, AlF x was characterized using XRD phase analysis by Pakpum.…”