US 3916822 A
A chemical vapor deposition reactor produces deposits which are uniform in composition and thickness by minimizing convective currents in the immediate vicinity of the substrate. Heating of the substrate, positioning the substrate to face in a downward direction at the upper extremity of an essentially convection-free zone, and a radiation shield are utilized.
Description (OCR text may contain errors)
' United States Patent 91 Robinson Nov. 4, 1975 CHEMICAL VAPOR DEPOSITION REACTOR  Inventor: McDonald Robinson, Chester, NJ.
 Assignee: Bell Telephone Laboratories,
Incorporated, Murray Hill, NJ.
3,677,329 7/1972 Kirkpatrick 165/105 3,690,635 9/1972 I-Iarker et a1 l18/49.5 X
FOREIGN PATENTS OR APPLICATIONS 218,262 5/1968 U.S.S.R 118/49.5
OTHER PUBLICATIONS Western Electric Co. Tech. Dig. Apparatus For The Deposition Of Silicon Nitride, Whitner, R. A., No. 11, (July 1968) pp. 5,6.
Primary Examiner-M0rris Kaplan Attorney, Agent, or Firm-G. S. Indig  ABSTRACT A chemical vapor deposition reactor produces deposits which are uniform in composition and thickness by minimizing convective currents in the immediate vicinity of the substrate. Heating of the substrate, positioning the substrate to face in a downward direction at the upper extremity of an essentially convectionfree zone, and a radiation shield are utilized.
6 Claims, 1 Drawing Figure U.S. Patent Nov. 4, 1975 H i K 1 CHEMICAL VAPOR DEPOSITION REACTOR BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is concerned with the desposition of films on substrates; the Substance of the film being produced by a chemical reaction involving a gaseous phase.
2. Description of the Prior Art Chemical vapor deposition (abbreviated in the following as CVD) is a method of plating solid objects in which deposits are produced by chemical reactions near, at, or on the surface of a substrate. The method involves the introduction of one or more gaseous reactants into the vicinity of a substratewhere the substance to be deposited is produced by a change in chemical state such as a breakdown or a combination of reactants. CVD has found a variety of commercial applications; examples are the metallization of mirrors, the pigmentation, reinforcing, protection, and decoration of surfaces, and the manufacture of semiconductor devices and integrated circuitry..CVD can be used to deposit elemental substances as well as chemical compounds such as bromides, carbides, nitrides, oxides and silicides. Deposited films may be amorphous, polycrystalline, or epitaxial and they may be electrically insulating, semiconducting, or conducting. A survey of fundamentals, techniques, and applications of CVD is given in C. F. Powell et al (Ed.), VapbnDeposition, John Wiley and sons, Inc., 1966.
In many applications of films deposited byCVD the uniformity in thickness and composition of the deposit is a major concern; this is the case, for instance, with deposited films as they are used in the semiconductor industry. One apparatus designed to produce filmsof superior uniformity is the CVD device proposed in F. H. Nicoll, The Use of Close Spacing in Chemical- Transport Systems for Growing Epitaxial Layers of Semiconductors, Journal of the Electrochemical Society, Vol. 110, November 1963, pp. 1165-1167, where the source material is placedparall el to and at close range of the substrate. Howeve'r',the design of this device limits its use to solid source materials and does not allow for the introduction of gaseous reactants.
SUMMARY OF THE INVENTION The invention represents a chemical vapor deposition reactor which, by minimizing convective currents in the vicinity of the substrate, produces films of superior uniformity.
Convection is prevented (1) by positioning the substrate at the upper boundary of the zone into which gaseous reactants are introduced, (2) by orienting the substrate to face in a downward direction, (3) by uniformly heating the substrate and the gases from above, and (4) by utilizing a radiation shield between the manifold through which gaseous components are introduced and the zone in the immediate vicinity of the substrate.
BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a cross sectional view of an embodi ment of the invention.
DETAILED DESCRIPTION THE DRAWING The FIGURE shows enclosure lcontaining substrate support2 equipped with heater 3 and shown as supporting substrates 4 which face in a downward direction. Auxiliary peripheral heater 5 helps to maintain a uniform temperature distribution in the vicinity of, the substrates. Perforated radiation shields 6 and 7 protect inlet manifold 8 from thermal radiation emanating from support 2 and substrates 4. Reactant gases 9 pass upward through the perforated-radiation shields 6 and 7 and diffuse upward across the essentially convectionfree vicinity of substrate 4. Residual product gases 10 diffuse across the vicinity of substrate 4 in a downward direction and leave the reactor.
Substrate support 2 is a stainless steel plate equipped with a cavity 21 which-is partly filled with sodium 22, evacuated, and sealed. Cavity 21 containsone or more piecesof wire mesh 23. in contact with the ceiling and the bottom of cavity 21. Under the influence of heat emanating from heater 3, sodium 22 liquefies, rises by capillary action along wire mesh 23 to the ceiling of cavity 21, evaporates, and condenses again at the bottom of cavity 21.
Operational Principles l The invention achieves uniformity of deposition through minimization of convection in the vicinity of the substrate. This minimization is accomplished by maintaining the substrate at an elevated temperature relative to the introduced reactant gases and by positioning it to face in a downward direction thus ensuring essentially planar and horizontal isotherms. Heating of the substrate may be accomplished by a variety of means, the key requirement being the uniformity of temperature at the surface ofthe substrate. One heater design which has been particularly effective in assuring uniform substrate temperature encompasses a so-called heat-pipel integral to the substrate support. In this design, a stainless steel plate supports the substrate and contains an evacuated and sealed cavity which is partly filled with a heat transfer medium such as liquid sodium.-Pieces-of,wire mesh are placed inside the cavity in contact with the' ceiling as:well as the bottom of the cavity to serve as a wick along which, through capillary action, the heat transfer medium reaches the top of the cavity. Alternately, steel wood, fiberglass, or an equiva- .lent 'could be used instead of wire mesh. The heat transfer medium evaporates at the heated ceiling of the cavity and condenses at the cooler lower end of the cavity, efficiently transferring heat in a downward direction.
Unless the width-to-height ratio of the zone between the radiation shield and the substrate is very large, peripheral heat and reactant losses should preferably be guarded against. In an experimental embodiment of the invention in which this ratio was about 6:1, peripheral heat loss was compensated by auxiliary heating from a heater tape wrapped around the reactor whose enclosure had a circular horizontal cross section. Depending on the application, differently shaped enclosures may be advantageous.
In order to prevent premature heating of the entering gases as they pass through the inlet manifold, a radiation shield is placed between the substrate and the inlet manifold. The shield protects the inlet manifold from thermal radiation emanating from the substrate and its support and helps prevent forced convection in the vicinity of the substrate. For this purpose the experimen tal embodiment utilizes a pair of gold plated fused quartz plates placed about 4 millimeters apart. The
plates are perforated and positioned relative to each other so that entering gases follow a tortuous rather then a straightthrough vertical path before reaching the vicinity of the substrate. While the experimental embodiment utilizes a pair of plates as a radiation shield, the use of only one or of more than two such plates is not precluded.
During deposition, reactant gases diffuse upward from the radiation shield. At the surface of the substrate reactants are consumed and products created by the deposition reaction. The residual gaseous products in turn diffuse downward and away from the vicinity of the substrate through the radiation shields. The inlet manifold, located in the convection zone below the radiation shields, supplies fresh gases, preferably in a uniform, periodic array ofjets to the underside of the radiation shields. The incoming gases dilute the product gases which are forced out the bottom of the reactor; typical gas flow and deposition rates are shown in the examples below.
EXAMPLE 1 Silicon dioxide was deposited onto silicon and tungsten metallized silicon at a temperature of 720C according to the chemical reaction Sil-l, 2N O SiO 2H, N,
using helium as a carrier gas. Flow rates were 1.5 cm lmin for Sil-l 187.5 cm lmin for N 0, and 91 cm /min for helium. Under these conditions a deposition rate of 0.8 micrometers per hour was realized.
' Variation in thickness of the deposited layer, measured with a spectrophotometer, was found to be no more than 4 percent over the 1% inch diameter silicon wafers. This compares favorably with variation of up to percent resulting from the use of commercially available apparatus.
EXAMPLE 2 Silicon nitride was deposited onto stainless steel at a temperature of 720C according to the chemical reaction using helium as a carrier gas. Flow rates were 1.5
cm /min for SiH cm lmin for NH and 91 cmlmin for helium. A deposition rate of 1.2 micrometers per hour was achieved.
What is claimed is:
1. Chemical vapor deposition apparatus for depositing a layer of a substance on a surface of a substrate comprising means for supporting said substrate,
means for introducing at least one gas producing said substance into a zone adjacent to said surface,
means for extracting any residual gaseous material,
means for uniformly maintaining said substrate at a temperature higher than that of said introduced gas characterized in that 1. said surface generally defines the upper extremityof said zone,
2. said surface faces in a downward direction,
3. the lower extremity of said zone is-generally defined by a radiation shield permeable to reactant, carrier and product gases, and
4. means are provided for introducing said gas at a position below said radiation shield.
2. Chemical vapor deposition apparatus of claim 1 in which said radiation shield consists of at least one perforated plate in a position essentially parallel to said surface of said substrate and of which at least the plate closest to said surface is equipped with a heat-reflecting coating.
'3. Chemical vapor deposition apparatus of claim 2 in which said radiation shield consists of a pair of plates separated by a distance of 0.140 centimeters.
4. Chemical vapor deposition apparatus of claim 1 equipped with an auxiliary heated positioned at the periphery of the zone between the substrate and the radiat ion shield.
5. Chemical vapor deposition apparatus of claim 1 in which said supporting means is a heated plate.
6. Chemical vapor deposition apparatus of claim 5 in which said plate is equipped with a heat pipe for transfer'ring heat from the upper to the lower extremity of said plate.