PROCESS OF FORMING A THIN GLASS FILM ON A SEMICONDUCTOR SUBSTRATE

Abstract: A binary glass for use in integrated circuits has a softening or flow point far below temperatures at which glasses normally used in connection with integrated circuits flow. Preferably the glass comprises a mixture of germanium dioxide and silicon dioxide wherein the germanium dioxide is no greater than approximately 50 % by weight of the mixture. Phosphorus is added to the glass film for passivation of the underlying devices.

Description: A LOW TEMPERATURE BINARY GLASS FOR USE AS A NULTILEVEL DIELECTRIC LAYER BACKGROUND OF- THE INVENTION Field of the Invention This invention relates to a glass suitable for deposition by chemical vapor deposition ("CVD") techniques which has a softening (or "flow") point far below tempera- tures at which glasses currently used in the semiconductor industry flow. Description of the Prior Art The vapor deposition of phosphorus-doped silicon oxides onto a substrate is well-known. Such glasses are typically used to passivate the surfaces of integrated circuits made using silicon. Commonly denoted as "PVx" or "phosphorus doped vapox", glasses formed in this manner have very high melting points in the range of 10000C to 1100 C. To reflow these glasses to remove sharp edges and provide curved, gradual-sloping surfaces on which conductive leads can easily be formed with gradual, rather than abrupt, changes in surface height, thereby to reduce the likelihood of such leads cracking or breaking, the integrated circuit must be heated to temperatures within the range of 10000 to 1100 C. Such high temperatures change the diffusion profiles of the various doped regions within the integrated circuit which give to the circuit its electrical characteristics. This is undesirable. Thus the fabrication of a semiconductor integrated circuit, particularly one using more than one level of conductive leads (known as a "multilevel" structure) becomes highly complicated with the final characteristics of the device differing somewhat from the characteristics of the device prior to the formation of the various levels of conductive leads. The formation of glasses suitable for use as passivating and/or insulating layers in integrated circuits which melt at temperatures significantly below the 10000C to 11000C temperatures at which glasses currently used in integrated circuits melt is particularly important now that laser annealing and glass reflow techniques are becoming commonly used in semiconductor processing. In the past, various glass mixtures have been used or proposed to achieve lower melting temperatures. Thus sedimented glasses have been proposed for use in the manufacture of integrated circuits with the constituents of the glasses being selected such that these glasses melt at a substantially lower temperature than the melting temperatures of commonly used phosphorus-doped vapor deposited oxides of silicon. Unfortunately, the techniques used to control the thicknesses of the deposited glasses and the technologies for depositing the glasses differ substantially from the techniques and technologies currently used in the semiconductor industry in the vapor deposition of oxides of silicon (both undoped and doped with phosphorus). SUMMARY OF THE INVENTION This invention substantially overcomes the above problems by providing a thin glass film suitable for use as insulation in semiconductor devices. The glass of this invention utilizes the same equipment as utilized to form the phosphorus doped vapox of the prior art but produces a glass which has a melting point far below temperatures which cause dopant diffusion problems. In accordance with this invention, a binary glass is deposited in a CVD (i.e., chemical vapor deposition) reactor from a germane/silane (GeH4/SiH4) co-deposition source by reacting these two gases at temperatures ranging from approximately 350 C to 5000C with oxygen in a nitrogen carrier gas. The resulting glass (a germanium dioxidesilicon dioxide mixture) has a useful range of flow points typically between approximately 7000C to 9000C thereby making it particularly attractive for use as a dielectric material in multilevel integrated circuit structures. The addition of phosphorus in the form of phosphorus pentoxide (P205) by blending phosphine (PB3) into the deposition gas stream at a selected percentage by weight results in an excellent passivated' glass film which, by controlling the phosphorus level to be relatively low, substantially eliminates corrosion, a common problem in prior art integrated circuits using phosphorus-doped passivation of glass together with aluminum interconnect structures. DETAILED DESCRIPTION OF THE INVENTION In accordance with this invention, a mixture of germanium dioxide and silicon dioxide is deposited on an. appropriate substrate, such as a silicon wafer used in the manufacture of integrated circuits, in a conventional chemical vapor deposition reactor of a type commonly known and used in the semiconductor industry, from a co-deposition source of germane (GeH4) and silane (Sill4). In a preferred embodiment, the deposition is carried out by reacting these two gases at temperatures ranging from 350"C to 5000C with oxygen in a nitrogen carrier gas. A typical preferred ratio for the deposition of this glass is approximately: Constituents Flow Rates GeH4 3.67 cc/min Sill4 7.33 cc/min 02 1.00 cc/min N2 2.00 L/min where "cc" means cubic centimeters and "L" means liters. The above constituents are reacted at 450 C. Of importance, GeH4 is more reactive than. Sill Thus, the result of this reaction is a glass film which will, undoped, in a nominal 50-50 mixture by weight, melt at about 8500C when placed in a furnace with nitrogen at this temperature and held at this temperature for 30 minutes. In a variation of this process, phosphorus pentoxide (P205) is introduced into the gas stream by blending phosphine (PH3) into the gas stream in an amount ranging from three to nine percent (3-9%) by weight. The resulting phosphorus doped glass forms an excellent film and melts between 7000 to 9000C. A characteristic of germanium dioxide (GeO2) is that this material exists in both soluble and relatively insoluble forms in water. When germanium dioxide is grown alone at normal temperatures, the resulting glass is soluble in water. Germane (Ges4) is a commercially available substance supplied by a number of vendors. The germane used in this process is of the quality generally available in the marketplace. As the percentage of germanium dioxide in the resulting germanium dioxide/silicon dioxide glass mixture increases to above somewhere in the 45-55% range, the resulting binary mixture starts to become relatively more soluble in water than phosphorus-doped silicon dioxide. By holding the percentage by weight of germanium dioxide in the mixture less than between 45-55% by weight, the resulting binary glass film is not significantly more water soluble than phosphorus-doped silicon dioxide (that is, its solubility is on the same order as PVx). Such a binary glass film is substantially insoluble in water. When the percentage by weight of germanium dioxide in the binary glass becomes greater than about 50%, however, the oxide becomes relatively more water soluble. The addition of phosphorus in the form of P205 in the range of 3% to 9% by weight to the resulting glass lowers the softening temperature of the glass to about 7000c. Without phosphorus, the glass begins to soften at approximately 8000C. An important feature of the binary glass of this invention is that it can be deposited using standard equipment currently used in the semiconductor industry for the deposition of phosphorus doped oxides of silicon. Thus the basic deposition technology and equipment is already available for use in practicing this invention. By increasing the phosphorus content in the binary glass mixture to 3-9% and heating the glass under 20 atmospheres pressure, the glass has been experimentally flowed at 6500C. This allows glass to be reflowed at a temperature slightly beneath that at which aluminum melts (aluminum melts at about 6600C). Although an aluminumsilicon eutectic melts at 5770C and an aluminum copper eutectic melts at 5480C, the glass of this invention closely approaches these melting points thereby, in some instances, making it possible to reflow glass following the deposition of aluminum interconnects without melting the aluminum interconnects. The glass of this invention is particularly suitable for use with localized glass reflow techniques involving the use of lasers wherein local regions of the glass can be reflowed without significantly increasing the temperature of adjacent portions of the semiconductor circuit. Thus, even though the melting temperature of the glass is slightly higher than that of aluminum, with careful control of the areas heated the melting temperature of this glass is sufficiently low that glass can be locally heated and reflowed without melting adjacent regions of aluminum. Although phase diagrams for germanium dioxide/silicon dioxide mixtures are known for bulk systems, these phase diagrams generally do not apply to the thin films commonly employed in the semiconductor industry. As used herein, the term "thin film" is meant to refer to a film of silicon dioxide/germanium dioxide having a thickness generally less than about 5 microns and typically less than about 2 microns. The thicknesses of films formed using the techniques of this invention for insulation layers in integrated circuits can drop as low several hundred angstroms under proper conditions but more typically range in the thousands of angstroms. Other embodiments of this invention will be obvious to those skilled in the art in view of the above description. -The above description is intended only to be exemplary and not limiting. I claim:

Claims: 1. A binary glass film formed of germanium dioxide and silicon dioxide. 2. The structure of Claim 1 wherein the germanium dioxide is no greater than approximately 50% by weight of the mixture. 3. The glass of Claim 1 wherein said glass film includes phosphorus. 4. The structure of Claim 3 wherein said phosphorus comprises 3-9% by weight of the glass film. 5. Structure as in Claim 4 wherein said phosphorus is in the form of P2O5. 6. A glass film comprising a mixture of an oxide of germanium and an oxide of silicon possessing a melting temperature beneath approximately 9000C. 7. The structure of Claim 6 wherein said glass film possesses a melting temperature between approximately 6500C to 9000C. 8. The structure of Claim 7 wherein the melting temperature of said glass film is between approximately 7000C and 9000C. 9. The structure of Claims 1 or 6 wherein the ratio of silicon to germanium in said glass film is such that the resulting glass film is substantially insoluble in water. 10. The glass film of Claims 1 or 6 wherein the ratio of silicon to germanium by weight in said glass film is such that the resulting glass film is substantially water insoluble and can be etched using standard oxide etching techniques. 11. The method of forming a glass film comprising depositing a binary glass comprising a mixture of oxides of germanium and silicon from the vapor deposition of reaction constituents comprising compounds of germanium and silicon. 12. The method of Claim 11 wherein the method of depositing said binary glass comprises reacting the compounds of germanium and silicon in a chemical vapor deposition reactor at selected temperatures. 13. The method of Claim 12 wherein the reaction constituents comprise germane and silane. 14. The method of Claim 13 wherein said germane and silane gases are reacted at temperatures ranging from .3500C to 5000C with oxygen in a nitrogen carrier gas. 15. The method of Claim 14 wherein the germane has a flow rate of approximately 3.67 cc/min, the silane has a flow rate of approximately 7.33 cc/min, and the germane and silane are reacted with oxygen in a nitrogen carrier gas wherein the oxygen has a flow rate of. approximately 100 cc/min and the nitrogen has a flow rate of approximately 2 liters/min. 16. The method'of Claim 15 wherein the reaction occurs at approximately 4500C. 17. The product made by the process of Claims 11, 12, 13, 14, 15 or 16..

Inventors: LEHRER, WILLIAM I.
Applicants: FAIRCHILD CAMERA & INSTRUMENT CORPORATION
Application year: 1982
Main classification: C03C3
IPC codes: C03C4, C04B41, C03C3, C23C16, C03B19