The Chemically Assisted Ion Beam Etching System at UMBC

Chemically Assisted Ion Beam Etching (CAIBE) is a technique used to etch patterns into a substrate material in a very controllable, high fidelity fashion. CAIBE provides the ability to etch vertical or angled sidewalls with mirror-like smoothness. This has been exploited in the field of photonics to make integrated lasers, mirrors and diffraction gratings which generate, route and diffract light on the surface of semiconductor chips. At UMBC, we have designed and constructed a CAIBE system with unique characteristics to advance the art, science and applications of this process.

The CAIBE process combines the action of a broad area, collimated ion beam and a reactive gas to remove material from a substrate in areas which are not protected by a patterned masking material. The etching process occurs under conditions in which the substrate does not spontaneously etch when exposed to the reactive gas, but does etch when the ion beam is also present - leading to the alternative name "ion beam assisted etching" (IBAE). The ion beam by itself will etch the surface by physical sputtering. This process, also known as "ion milling", is caused by atoms being knocked off the surface by the impact of the incident ions. The addition of the reactive gas in CAIBE greatly increases the substrate material removal rate for a given ion beam flux.

The characteristics of CAIBE are due to its mixing of "physical" and "chemical" attributes. The ions in the collimated beam travel in nearly parallel paths, so the etching proceeds in a "line-of-sight" fashion in the unmasked areas of the substrate. Control of the etching angle can be achieved by tilting the sample with respect to the beam direction. The etching rate and profile can be made insensitive to crystal orientation and alloy composition. The reactive gas adds the benefit of reducing the number of incident ions required to achieve a desired etching depth. This reduces both the amount of ion-induced crystal damage, and undesirable trenching and redeposition effects associated with physical sputtering. Use of the reactive gas also allows one to choose a masking material which does not react with the gas. Deep etches can be made with relatively thin masking layers and little degradation of the mask pattern.

The CAIBE system at UMBC was designed to incorporate advanced concepts to achieve excellent quality etching of III-V semiconductors such as alloys of gallium arsenide and indium phosphide. To perform etching of these materials, we chose argon for the beam ion and chlorine as the reactive gas. One of the major goals in the design was to provide an etching environment free of interfering contaminants. In particular, the presence of oxygen-containing molecules is known to interfere with the etching of semiconductor alloys, especially those including aluminum. The system was designed using ultrahigh-vacuum technology throughout, with stainless steel, metal seals and compatible materials. Samples are introduced to the chamber through a high vacuum load lock, so all internal surfaces of the etching chamber and sample mount remain at ultrahigh vacuum constantly. The unbaked chamber achieves a pressure of low 10-9 torr routinely, utilizing a 1500 liter/second corrosion resistant turbomolecular pump. Samples are transferred into the etching chamber from the load lock, which is first pumped down to 5x10-7 torr in 20 minutes using a combination of turbomolecular and cryopumps. The ion source is a 5 cm Kaufman ion source manufactured by Ion Tech, Inc.. A key feature of this source is its use of a hollow cathode, rather than a filament cathode. The filament cathode used in most ion sources has a fairly short life, especially in the presence of corrosive gases, requiring venting of the chamber frequently for filament replacement. The use of the hollow cathode has allowed us to operate with our chamber under vacuum continuously for times up to greater than one year. We have also recently added a hollow cathode neutralizer, to which the same consideration applies. Due to the inherent cleanliness of the system, we have avoided the use of a cryogenic trap, used in CAIBE systems to remove water vapor contamination. Such traps add to the time and cost of system operation due to the regeneration time, liquid nitrogen consumption, and added chlorine consumption (and disposal).

Sophisticated sample manipulation is another special feature of the CAIBE system at UMBC. The samples are mounted on a virtual axis gearbox (a semi-custom manipulator from Thermionics Northwest, Inc.) which provides three computer-controlled motions about the center of the sample surface. The motions provided include tilt of the sample normal with respect to the beam axis, azimuthal rotation of the sample about the sample normal, and variable distance between the sample and the ion source. An additional unusual feature is the chlorine gas feed, which is provided through four gas jets, which tilt and translate with the sample, retaining fixed gas input geometry, regardless of the sample tilt or distance from the ion source. The sample stage is also equipped with a substrate heater with temperature sensor capable of controlled temperatures up to at least 200 C.

Our group at UMBC has used CAIBE to fabricate a number of photonic device structures. In a joint project with Westinghouse, we fabricated an individually adressable two-dimensional surface emitting laser array using CAIBE etched 45 degree mirrors to deflect the light out from the surface. We have fabricated submicron period etched sidewall gratings suitable for a planar spectrometer on a chip by the combination of CAIBE and direct write electron beam lithography. These gratings have also been used to fabricate a multi-wavelength laser array with grating feedback. Ridge etches suitable for semiconductor lasers and waveguides have been etched at UMBC with tight depth control (run-to-run depth variation +/- 3%).

The CAIBE system at UMBC is being actively used for photonics research, and will be used to develop new techniques and applications. The CAIBE system will be a key tool in work being done with the Laboratory for Physical Sciences at College Park, MD on a wavelength division multiplexing (WDM) network system (the Lightning program). For this program, etching will be used to define sidewall gratings for wavelength multiplexing and demultiplexing, and etched facets for integrated diode lasers and amplifiers. In addition to etching GaAs/AlGaAs structures, processes will be developed to etch InP based alloys. The CAIBE system is used to fabricate curved sidewall gratings for monolithically integrated wavelength division multiplexing laser arrays under an ARPA program. Work is also underway to extend CAIBE techniques, and explore new applications.

A number of people helped this system become a reality:

The funding for building, operating and maintaining the CAIBE system has come from the Laboratory for Physical Sciences at College Park, MD, mostly through the Joint Program for Advanced Electronic Materials. Funding for projects utilizing CAIBE have come from LPS, ARPA, Martin Marietta Laboratories, Westinghouse and the State of Maryland MIPS program.

  Special thanks to John Hryniewicz for his contribution to this page !

ee-web-editor@engr.umbc.edu

Last Update: June 26, 1995 (Monday)