Laser Micromachining of Silicon: A New Technology for Fabricating THz Imaging Arrays

The vast majority of radio receivers, transmitters, and components that operate at millimeter and submillimeter wavelengths utilize waveguide structures in some form. This is because waveguide is a well characterized transmission medium which can be readily fashioned into a variety of complex radio frequency circuits. The long history of development of waveguide components provides a broad base of knowledge from which to synthesize and evaluate new designs. However, at frequencies above 700 GHz waveguide becomes so small (less than 0.3 by 0.15 mm) that fabrication utilizing conventional machining and electroforming techniques becomes extremely difficult, expensive, and/or impossible. This situation is unfortunate, since, due to their superior optical efficiency, lower transmission line loss, and tuning flexibility, waveguide mixers have been found to outperform quasi-optical mixers at frequencies where both have been constructed. We propose to use a new, revolutionary, laser micromachining technique to fabricate the 2 THz waveguide mixers for the array. In the past, wet etching techniques have been employed to micromachine silicon-based submillimeter waveguide components. The main disadvantage of the wet etching technique is that one is forced to follow the <100> or <110> crystal plane in the silicon. This severely limits the types of structures that can be produced. The process is well suited to making wide-angle pyramidal horns or straight sections of single height waveguide. However, it cannot readily be used to make waveguide structures in which the height of the waveguide is stepped down (e.g. in an impedance transformer) or tapered (either rectangular or circular). These structures are needed to make efficient feedhorns. However, a recently developed laser machining technique overcomes these problems. The laser etching technique operates independently of the crystal plane orientation and thereby permits a wide variety of structures to be made.

Process Description

This approach makes use of fast laser microchemical etching, combined with a digitally addressable laser scanner and computer aided design (CAD/CAM) software. A schematic diagram of the laser etching system that inspired our experiment is shown below:

Schematic diagram of galvanometer based scanning apparatus. A higher power objective (not shown) can be slid in place of the focusing optic to allow more detailed analysis of the surface. For etching high aspect ratio structures, a circular polarizer is placed in the beam path to improve edge uniformity due to the selective reflectivity of the S and P polarization components along the side-walls.

Below is a CAD drawing of the system we are currently building.

CAD view of the laser micro machining system. The whole system rests on a heavy optical table and is enclosed in a vented sheet metal hood to contain a possible chlorine leak. Inside the chlorine enclosure an open steel box can be seen, the beam steering, and video monitoring equipment can be seen on top. Inside the steel box the motion stages and chlorine milling chamber can be seen.

Below is a cut away of the system that allows a better look at the optical design.

The laser light beam can be seen emerging from the Argon-Ion laser on the far right of this drawing. After bouncing off an optimized mirror, the beam goes through a shutter, before bouncing off a second mirror. The beam now heading left then goes through a beam enlarger, and a half wave plave before being elevated to the upper optical bench. On the upper bench the beam hits a beam splitter. The half wave plate is set so that almost all of the light will go through the beam splitter, and go on through a quarter wave plate to the steering mirrors then on to the scanning lens and into the milling chamber. Light reflected comes back the same way but when it reaches the beam splitter the polarization this time is such that most of it will go to the monitoring camera

Schematic representation of laser direct write etching of silicon in a chlorine ambient. Using high NA optics, the reaction zone can be confined to the necessary micrometer resolutions demanded for microelectronics processing.

Of course handling chlorine gas requires special care. The chlorine storage area, milling chamber, etc need to be enclosed in vented cabinets. Chlorine flow needs to be carefully regulated, temperature controlled etc. Below is a draft of our chlorine flow system.

A small chlorine tank provides chlorine gas to the system through stainless steel piping. The flow is regulated using a mass flow controller visible to the right of the milling chamber. The chlorine then goes through a vacuum pump to a cold trap where chlorine and silicon chloride are stored before beig chemically neutralized after every run.

Interfacing the 3D CAD/CAM software to the scanner permits the rapid prototyping of waveguide components. The silicon substrate is contained in a flat vacuum cell under a slowly flowing ambient of chlorine. The thermal microreactions of silicon in chlorine are chosen because of their speed. The chlorine ambient reacts with the silicon at temperatures near the melting point to form volatile silicon chlorides, which are pumped away from the surface (Figure 1b). A ~1 micron³ of silicon is brought to just above its melting point by a focused, CW argon-ion laser operating at 488 nm wavelength. The (circularly polarized) beam is deflected in the x,y plane with a pair of computer-controlled galvos. A field size of 256 x 256 pixel elements can be addressed in random access speeds up to 5 x 10⁴ pixels/s, or in a raster mode up to 2.5 x 10⁶ pixels/s. The chlorine gas pressure, laser power, and scan rate are adjusted to give optimum surface quality. Waveguide surface roughness values measured with atomic force microscopy are typically on the order of 200 nm RMS. This surface quality is already sufficient to provide low-loss waveguide performance to > 10 THz. The RMS surface roughness can be reduced even further, to under 25 nm using standard polishing etch solutions. When necessary, multiple fields are stitched using a 4 inch travel x,y stage driven with stepper motors. Once the micromaching process is completed, gold is sputtered on the micromachined structure to make it conducting. The University of Arizona and MIT Lincoln Laboratory particpated in a Collaborative Research Agreement (CRDA) to investigate the application of laser micromachining to the fabrication of THz waveguide components. As a proof-of-concept demonstration, we made of a portion of an 810~GHz and 2~THz waveguide mixer block. The following figures show the impressive results:

Scanning Electron Micrograph (SEM) of portion of an 810 GHz feedhorn structure. The structure was etched using 4.3 Watts of laser power focused into a 6 micron spot in 200 Torr of chlorine gas. The laser beam was scanned at 5 cm/s and incrementally moved 2 micrometers between line scans. Under these conditions, nominally 1 micron shavings are removed per pass of the laser over the surface. The total etch time is one hour, not including the overhead time for pattern generation and stage motion.

SEM close-up of 810 GHz feedhorn ridges

SEM micrograph of replicated version of 2 THz waveguide structure. The original structure was etched using 3 Watts of laser power focused into a 4 micron spot in 200 Torr of chlorine gas. The laser beam was scanned at 4 cm/s and incrementally moved 2 micrometers between line scans removing 0.65 micron shavings per pass of the laser over the surface.

SEM close-up of 2 THz feedhorn ridges

Cross-scans on the prototype 2 THz feedhorn indicate the feedhorn's beam is nearly Gaussian with no measurable sidelobes to the noise floor of the map (~11 dB). The FWHM of the beam profile is ~20, close to the value of 19.3 derived from the horn's beamwaist (omega = 0.33), where d is the diameter of the feedhorn aperture (0.506 mm). The differences between the theoretical and measured beam profiles are within the resolution limit and amplitude sensitivity of the test set-up.

Assembly Drawing of the 850 GHz Array Mixer Block

Bolometer Array Block showing the Duroid PC Board containing the IF matching ntewrok and DC bias lines for the HEBs.

Overall 3-d isometric view of the mixer block with lenslet array, feedhorn section, junction and backshort blocks and the IF matching network PC board. The size of a dime is shown for comparison alongside the mixer block.