Germane purification

This application claims priority from U.S. Provisional Ser. No. 61/115,132, filed Nov. 17, 2008, the disclosure of which is incorporated herein in its entirety.

The provision of high purity gas streams is critically important in a wide variety of industrial and research applications. The rapid expansion of vapor-phase processing techniques, e.g. chemical vapor deposition, in the semiconductor industry has been associated with the deployment and use of manufacturing equipment that is totally reliant on the delivery of ultra-high purity process gases at the point of use in the semiconductor manufacturing facility.

Considering the impurities which are present in gas streams involved in semiconductor manufacturing, it is to be noted that the growth of high quality thin film electronic and optoelectronic cells by chemical vapor deposition or other deposition techniques is inhibited by a variety of low-level process impurities. These impurities can cause defects that reduce yields by increasing the number of rejects, which can be very expensive. These impurities may be particulate or chemical contaminants.

Chemical impurities may originate in the production of the source gas itself, as well as in its subsequent packaging, shipment, storage, and handling. Although manufacturers typically provide analyses of source gas materials delivered to the semiconductor manufacturing facility, the purity of the gases may change because of cross contamination or from contamination arising from improperly prepared containers, e.g. gas cylinders, in which the gases are packaged. Impurity contamination may also result from improper gas cylinder changes, leaks into downstream processing equipment, or unexpected contamination arising from such downstream equipment.

In numerous industrial and commercial processes, it may be desirable to provide high purity germane. One area in which high purity germane is typically required is in the fabrication of semiconductor devices such as transistors, diodes, integrated circuits, detectors, solar cells, and the like. In many of these applications, high purity germane is oftentimes used as a gas for deposition of silicon-germanium alloys or for doping of substrates. More recently, the commercial use of germanium tetrahydride by the semiconductor and solar cell manufacturers has steadily increased because of new technology that incorporates germanium into active silicon structures. This new technology requires that germane be produced at higher purity levels with less variability in impurity concentration.

Germane may contain a phosphine contaminate due to unexpected variation in raw materials, unexpected manufacturing process conditions or unexpected container related conditions. Separation of two molecules such as germane and phosphine of similar volatility and relatively similar molecular weight thereby reducing the unwanted material down to low, less than 50 ppb, levels has not been reported in the literature.

FIG. 1 is a chromatogram of a sample of germane in hydrogen containing no phosphine,

FIG. 2 is a chromatogram of a sample of germane in hydrogen containing 4300 ppb phosphine and

FIG. 3 is a chromatogram of a sample of germane in hydrogen spiked with 130 ppb phosphine.

A process for the purification of germane gas contaminated with phosphine has been developed. The method is based on the discovery that phosphine, a compound with similar volatility and molecular size as germane may be selectively removed from a germane gas containing phosphine by a properly conditioned molecular sieve having an effective pore diameter of about 4 Angstroms or greater. Phosphine and germane have effective diameters of between 3 to 4 Angstroms. Phosphine is retained by the molecular sieve in preference to germane. Preferably, the molecular sieve has an effective pore diameter of about 5 Angstroms or greater. More preferably, the molecular sieve has an effective pore diameter of about 5 Angstroms.

Method:

Drying the Adsorbent:

The molecular sieve beds used in the present invention were dried by heating to 200° C. to 300° C. under a purge of dry helium for 4 to 12 hrs, then cooled to about 23° C. under a purge of dry helium. Dry gases such as nitrogen and hydrogen and the like may also be utilized for both the heating and cooling purges.

Conditioning:

The molecular sieve bed must be conditioned by being saturated with germane. Germane is strongly adsorbed onto freshly dried molecular sieves. This adsorption process releases heat and is notable for the near complete disappearance of the germane gas from the input gas stream. Typically an input conditioning gas stream comprising up to about 40% germane preferably from about 20 to 40% germane in hydrogen is passed through a sieve bed or maintained in contact with a sieve bed. Conditioning gas flow is maintained until the germane component is no longer appreciably retained by the sieve bed. Care must be taken to avoid bed overheating and potential decomposition/deflagration of germane. The sieve bed temperature is monitored to avoid overheating. A preferred sieve bed temperature is about 60° C. or less. The sieve bed temperature must be maintained below the decomposition/deflagration temperature of germane.

Molecular Sieve:

Typical examples of molecular sieves suitable in the present invention include types 4A, 5A, 10X and 13X. Such molecular sieves are available from a number of suppliers. These molecular sieves have an effective pore diameter of from about 4 Å or greater. Molecular sieves are synthetically produced zeolites characterized by pores and crystalline cavities of extremely uniform dimensions. Type 4A (4 Å) Molecular Sieve is the sodium form of the Zeolite. Type 4A will adsorb those molecules having a critical diameter of less than 4 Å (0.4 nm). Type 5A (5 Å) Molecular Sieve is the calcium form of the Zeolite. Type 5A will adsorb those molecules having a critical diameter of less than 5 Å (0.5 nm). Type 10X is a modified form of the sodium zeolite with an effective pore diameter of about 8 Å Type 13X is a modified form of the sodium zeolite with an effective pore diameter of about 10 Å (1 nm).

The preferred molecular sieve is type 5A which has the composition 0.80 CaO:0.20 Na 2 O:Al 2 O 3 :2.0±0.1 SiO 2 :x H 2 O. Divalent calcium ions in place of sodium cations give apertures of about 5 Å.

The invention will be illustrated in more detail with reference to the following Examples, but it should be understood that the present invention is not limited thereto.

Purifying Germane