Process

The many steps required to convert polycrystalline germanium into a gamma-ray spectrometer are depicted in Fig. 1. The description of these steps follows the flow of germanium through the process, some steps being more intricate than others.

The initial starting material, electronic grade polycrystalline germanium "metal" (because of its metallic appearance), is zone refined in a quartz "boat" having a pyrolytic graphite coating. A zone refiner uses the principle that most impurities concentrate in the liquid phase as the material begins to freeze. The rf heating coils of the zone refiner melt a small section of the germanium ingot or bar held in the quartz boat.

Fig. 1. Germanium Detector Manufacturing Process.

Fig. 2. 3-Coil Zone Refiner.

Fig. 3. Zone-Refined Ingot.

Fig. 4. Czochralski Technique Schematic.

Fig. 5. Germanium Crytsal Being Grown.

Fig. 6. Mounted Crystal Being Sliced.

Fig. 7. Grinding the Germanium Crystal.

Fig. 8. Schematic of Both P-type and N-type Coaxial Detectors.

The rf coils are slowly moved along the length of the ingot, causing the liquefied portion beneath the coils to move also. Thus, the ingot is continuously melting at the advancing solid-liquid interface and freezing at the receding interface. The impurities tend to remain in the molten section and hence are "swept" to one end of the ingot by this process. This "sweeping" operation is repeated many times, until the impurities are concentrated at one end of the ingot. This end is then removed, leaving the remaining portion much purer than the original starting material. Figure 2 shows a three-coil zone refiner in operation. The improvement or reduction in impurity concentration actually realized is about a factor of 100 or more at the completion of this process. Figure 3 shows a zone-refined ingot. The tapered end contains the high concentration of impurities and is cut off. The impurity concentration of the remaining portion is then determined by a Hall effect measurement, and the ingot is sliced into pieces suitable for loading into the crystal-growing equipment.

Large single crystals of germanium are grown using the Czochralski technique, which is schematically illustrated by Fig. 4. A precisely cut seed crystal is dipped into the molten germanium and then withdrawn slowly, while maintaining the temperature of the melt just above the freezing point. The rate of crystal withdrawal and temperature of the melt are adjusted to control the growth of the crystal.

Figure 5 shows a crystal during the growth process. High-purity germanium crystals suitable for detector fabrication are almost always grown in a quartz crucible under a hydrogen atmosphere. Near the completion of the growth process, the crystal is tapered gradually at the tail to minimize thermal strain. It is imperative that the crystal be grown to the exhaustion of the melt, because germanium both wets quartz and expands on freezing. The valuable quartz crucible might be fractured if any germanium were left after completion of the crystal growth.

After the crystal is grown and cooled, it is mounted in a Plaster-of-Paris cast for slicing. Figure 6 depicts a mounted crystal during the slicing process. The completed crystal is cut by an ORTEC-designed string saw that causes virtually no damage to the crystal. A slurry of water and silicon carbide is pulled along by a wire, resulting in a sawing action. Sections of the crystal from both top and bottom are checked by Hall effect measurements to determine the impurity concentration and type (n or p). On the basis of the Hall effect results, that part of the crystal which contains detector-grade material is selected.

The rejected material is returned to the zone refining operation.

The section of crystal which has both adequate purity and crystallographic perfection for coaxial detector fabrication is then ground perfectly cylindrical. The edge at one end is beveled to a radius ("bulletized") to improve charge collection and timing performance. Figure 7 illustrates the grinding operation. Afterwards, a hole is machined into the unbeveled end so that the central contact of the device may be made later. The detector subsequently is hand lapped all over to remove damage caused by the machining processes.

A lithium diffusion to form the n⁺ contact is then performed over the entire outer surface except the flat, unbeveled end for p-type coaxial detectors and on the "walls" of the central hole for n-type coaxial detectors. This lithium-diffused layer is about 600-µm thick. After the lithium diffusion operation, the detector is lapped once more, chemically polished, and a surface protective coating applied. The coating is amorphous germanium hydride deposited by a sputtering process, similar to that described by Hansen, et al., (Ref. 1). Next, the p⁺ contact is formed by the ion implantation of boron ions. This last step completes the fabrication process for the coaxial detector element itself. Figure 8 shows schematically the structure of both p-type and n-type coaxial detectors.

Because many of the steps in the manufacturing process (Fig. 1) have less than a 100% yield, a detector element may spend an extended time in a "loop" before being shipped.