Nb oxide thin film resistors

Recently Kerr and Pan have proposed development of integrated image separating mixer circuits. To realize the 50 and 100 ohm terminations required by these circuits, a 4.2 K thin film resistor technology is required. It is also desirable to implement each resistor with one or two "squares" of material so that they are electrically short. Existing commonly used Ti-Au based resistor technologies are limited to values well below 50 /spl Omega///spl square/. We are investigating a new Nb-oxide technology where the resistivity is controlled by varying the oxygen content of the sputtered films. Initial experiments using a DC magnetron gun with a Nb target proved difficult to control due to the changing deposition rate and electrical characteristics of the gun with target erosion. Greatly improved control was obtained with the use of a RF diode sputter arrangement and a 5" diameter target. As target voltage is decreased or oxygen partial pressure is increased, the deposited films become less metallic and more resistive. Films with ohms//spl square/ values ranging from five to 1,000 have been obtained with this technique while a partial pressure of oxygen on the order of 10/sup -3/ Pa is needed for 50 /spl Omega///spl square/ material. A valuable attribute of these resistive films is that the measured room and liquid He temperature resistivities are the same.