Introduction

Traditional metal blocks manufactured using conventional machining techniques are used in most successful receiver systems in the submillimeter wavelength range. The components of such receiver systems include oscillators, multipliers and mixers, which mostly incorporate waveguides and their associated transitions. The waveguide is a well characterized, low-loss transmission medium, which affords easy and excellent coupling of the freespace modes to electronic circuit elements. As the desired wavelength of operation becomes smaller, so do the dimensions of waveguide and quasi-optical elements that comprise a receiver system. On the one hand, the small sizes are advantageous in the fabrication of high-resolution focal-plane imaging receiver systems. Such arrays have applications like radio astronomy, commercial and industrial applications like the extension of millimeter technology to collision avoidance radars, aircraft guidance and landing, contraband detection and personal communication systems, all of which benefit from having a large number of elements in a compact, practical sized system. However, as the sizes of components scale down, the cost of fabrication goes up. Indeed, the fractional cost of machined metal blocks containing waveguide electromagnetic elements in the overall budget of receiver systems, especially those that involve arrays, becomes prohibitively high with increasing frequency.

In the past decade or so, non-traditional techniques that can be collectively called ``micromachining'' are demonstrating practical means for producing a variety of submillimeter and terahertz frontend components [1]. Micromachining as a class is derived from manufacturing tools based on batch thin and thick film fabrication techniques of the electronic industry [2]. Many novel micromachining methods such as silicon wet etching [3], laser micromachining [4, 5], mold replication by the method of mastering, molding and casting [6] have demonstrated examples of components with performance equalling that produced by conventional machining. These new techniques, although offering great potential for the future, have many drawbacks, most of which are primarily due to the relative youth of such technologies.

At least for the submillimeter and low terahertz frequencies, conventional machining of metal, which has a long history of manufacturing engineering, can be used to fabricate receiver components. The method often used is the so-called ``split-block'' technique, where the circuit structures are machined on two (or more) metal blocks and then mated together to form complete components. The split-block technique offers the advantage that the machining is in principle straightforward. In addition, circuit components such as RF chokes, diodes and coupling structures can be easily placed on the split blocks prior to assembly of the complete piece. In the past, the main problems of conventional machining as applied to the fabrication of submillimeter and terahertz receiver systems have been (1) the high cost of machine tools and equipment, (2) the high level of expertise required of the machinist and (3) the fabrication time requirements. In this paper, we present a low cost technique of direct machining of waveguide structures on metal blocks that addresses all three of the aforementioned problems.