PROCESS FOR THE FORMATION OF PHOSPHOSILICATE GLASS COATING

Abstract: A coating of phosphosilicate glass is deposited on a substrate by a chemical vapor deposition method, using a reaction gas consisting of monosilane, phosphine, and oxygen, in admixture with ammonia gas, thereby to minimise the undesirable formation and adhesion to the substrate surface of particulate by-­ products such as Si0₂, P₂0₅, and H₂Si0₃. At the same time step coverage can be improved.

Description: This invention relates to a chemical vapor deposition method. More particularly, this invention relates to a process for the formation of a coating of phosphosilicate glass on a substrate by a chemical vapor deposition method (hereinafter, referred to as "CVD" or "CVD" method. This phosphosilicate glass (phosphorus-­ doped silicon dioxide, P-glass, or PSG) coatings obtained by CVD are widely used in the production of semiconductor devices such as IC, LSI and VLSI circuits. These PSG coatings act for example as electrical insulating layers between metals within the device, as passivation layers which protect the device from the environment, and as gettering sources. A variety of deposited films or coatings such as polycrystalline silicon (or polysilicon), silicon nitride, silicon dioxide, and phosphorus-doped silicon dioxide (PSG) are widely used in the fabrication of semiconductor devices, and among these coatings, PSG coatings are more frequently used because of their excellent properties. Generally, the PSG coatings are deposited on the substrate by using various CVD methods, and the CVD methods and reactions can be carried out under many different conditions. In the formation of the PSG coating by CVD, a gaseous mixture of monosilane (SiH₄), phosphine (PH₃), and oxygen (O₂) is generally used as a reaction gas. The reaction gas is introduced into a reaction chamber to cause the following chemical reactions, thereby depositing the PSG coating: SiH₄ + O₂ → SiO₂ + 2H₂ 4PH₃ + 5O₂ → 2P₂O₅ + 6H₂ The reactions can be carried out at atmospheric pressure or at a reduced pressure. The main advantage of these reactions is the low deposition temperature, lower than 500°C, which allows the resulting PSG coatings to be deposited over aluminum metallization. This advantage will be appreciated when it is understood that, since a PSG coating as insulating layer or as a passivation or covering layer is deposited on the substrate or wafer after the formation of aluminum circuitry thereon, it is necessary to carry out the above reactions at temperatures lower than the melting point of aluminum, i.e., 660°C, in order to avoid fusion of the aluminum circuitries. Generally, the above CVD reactions are referred to as low-temperature CVD. More particularly, hitherto, coating of PSG as an insulating layer has been effected on the substrate or wafer surface by using a hot-wall, reduced-pressure CVD reactor, such as shown in the accompanying Fig. 1. The CVD reactor of Fig. l has a reaction chamber l in the form of a quartz tube provided with two gas inlet tubes 2 and 3 and a gas exhaust tube 4. A gaseous mixture of SiH₄ and PH₃ is introduced through the inlet tube 2 into the reaction chamber l, and oxygen gas is introduced therein through the inlet tube 3. Wafers 5 on which the PSG coating is deposited by CVD are stood vertically, perpendicular to the gas flow, in a basket 6. The reaction chamber l is heated with a heater 7 surrounding the chamber l. Pressures in the chamber l are maintained in the range of from a reduced pressure of about 1.33Pa (0.01 Torr) to atmospheric pressure by controlling a reduced pressure exhaust system (not shown) connected with the exhaust tube 4. 26.6Pa (0.2 Torr) is maintained in the chamber l. The reaction gases are introduced into the reaction chamber at different flow rates. The flow rates of (SiH₄ + PH₃) mixed gas and O₂ gas depend on various factors, such as the size of the chamber, number of wafers to be disposed in the chamber, and the like. The reaction gases are heated and reacted in the chamber while maintained at temperatures of from 300 to 450°C. An atmospheric-pressure CVD process also has been used for the formation of a PSG coating. The atmospheric-pressure CVD process can be carried out in the reactor shown in Fig. 2 of the drawings. A reaction chamber ll has a susceptor 9 on which a wafer l5 is positioned, and which is heated to temperatures of from 300°C to 450°C by the attached heater l0. A reaction gas consisting of SiH₄, PH₃ and O₂ is introduced, in turn, through a gas feed pipe l2 and a gas outlet 8 provided with orifices, into the reaction chamber ll. After the CVD reaction is completed, the used gas is discharged through gas exhaust pipes l3 and l4. The above-discussed reduced-pressure and atmospheric-pressure CVD processes can provide PSG coatings having many advantages, such as a uniform and highly oriented coating, good electrical properties, and excellent resistance to the environment, but suffer from two serious problems. As is well known in the art, one problem is poor step coverage, and another is particles of the by-products produced during the CVD reactions, because the particles adhere to the wafer surface and thus the wafers are unacceptable for use. In the formation of the PSG coating by CVD, the following reactions are generated: SiH₄ + O₂ → SiO₂ + H₂ or SiH₄ + 20₂ → SiO₂ + 2H₂O 4PH₃ + 5O₂ → 2P₂O₅ + 6H₂ or 2 PH₃ + 40₂→ P₂O₅ + 3H₂O As a result, SiO₂ is grown on the wafer, and P₂O₅ is doped into the growing SiO₂ layer to form the PSG coating. However, during these reactions, SiO₂, P₂O₅, H₂SiO₃, and similar products (hereinafter referred to as by-products) are formed and deposited on the end portions or outlets of the gas inlet tubes, and these by-products give the following disadvantages. Namely, referring again to Fig. 1, the problem of the particles of the by-products occurs in the end portions of each of gas inlet tubes 2 and 3. Unavoid­ ably, some of these by-products are formed and deposited in the end portions of the inlet tubes and the deposited by-products are sometimes dislodged and the particles thereof adhere as dust to the wafer surface. In addi­ tion, the by-products deposited on the outlets of the inlet tubes cause variations in the amount of gases introduced into the reaction chamber, thereby adversely affecting the uniform growth of the PSG coatings on the wafer. Similar disadvantages also arise in the CVD reactor illustrated in Fig. 2. It was found that during the CVD reactions in the reactor, products such as SiO₂, P₂O₅ and H₂SiO₃ are formed and deposited on the orifice-side surface of the gas outlet 8 and particles thereof are sometimes dislodged and adhere to the surface of the wafer l5 opposite the gas outlet 8. Experiments by the present inventors showed that about ten particles of the by-products, having a diameter of l µm were adhered to a wafer having a diameter of l0 cm (4 inches). In U.S. Patent No. 4,625,678 issued December 2, l986, assigned to Fujitsu Limited, Shioya et al teach a plasma CVD apparatus for forming a deposited film such as silicide or metal film on a base body, which includes a gas feeding pipe and a heating device capable of heating at least a part of the gas feeding pipe inside the reaction chamber, thereby preventing the gas from condensing or solidifying. The heating of the gas feeding pipe as taught by Shioya et al can provide satisfactory results, but similar results cannot be obtained in the prior art CVD methods discussed above, because there are essential differences in the solidifi­ cation or particle deposition mechanisms of these CVD methods. In the prior art CVD methods discussed in the U.S. Patent, a problem arose in that a reaction gas maintained at a high temperature is condensed and solidified immediately after introduction into a reaction chamber, due to a sudden reduction of the temperature thereof. An object of this invention is to provide an improved process for forming a PSG coating by using the CVD method, in which the problem of the formation of the by-product particles discussed above does not occur, and a satisfactory step coverage is obtained. According to this invention, there is provided a process for the formation of PSG coatings on a substrate by the CVD method, in which a reaction gas comorising monosilane (SiH₄), phosphine (PH₃), and oxygen (O₂) is introduced into a reaction chamber in admixture with ammonia (NH₃) gas. In the practice of this invention, the three components which make up the reaction gas may be introduced into the reaction chamber separately or in combination, and all or part of the gas flows may contain NH₃ gas. The volume ratio of NH₃ gas to SiH₄ gas in the reaction gas can be widely varied, depending upon the results desired. However, to attain the best results, preferably the NH₃/SiH₄ volume ratio is within the range of from: 0.1:1 to 100:1. Further, preferably the CVD reactions of this invention are carried out at temperatures of from 300°C to 450°C. The process according to this invention can be used in the production of semiconductor devices such as IC, LSI and VLSI circuits, and the resulting PSG coatings are effective as an electrical insulating layer, a passivation layer, as a gettering source, and the like. For a better understanding of the invention and to show how the same can be carried into effect, reference will now be made, by way of example only, to the accompanying drawings wherein the figures not hitherto referred to herein denote the following: Fig. 3 is a schematic view of the atmospheric-­ pressure CVD reactor Fig. 4 is a graph comparing the deposition rate of the PSG coating of the process according to this inven­ tion with that of the prior art process; Fig. 5 is a graph showing a relationship between the deposition rate of the PSG coating and the NH₃/SiH₄ ratio; Fig. 6 is a graph showing a relationship between the number of particles of by -products distributed on the wafer surface and the NH₃/SiH₄ ratio; and, Figs. 7A and 7B are cross-sectional views through PSG coatings deposited on Al, comparing the step coverage of the process according to this invention with that of the prior art process. Figure 3 shows an atmospheric-pressure CVD reactor suitable for carrying out the process of this invention, in which ll is a reaction chamber, 8 is a gas outlet with orifices, l5 is a wafer, 9 is a susceptor, l0 is a heater, 2la to 2lf are valves, 22a to 22f are mass flow controllers, 23a to 23f are valves, l6 is a vacuum gauge, l9 is a mechanical pump, and 20 is a rotary pump. The CVD reactor is designed to carry out the deposition of a PSG coating on the wafer in accordance with the low-temperature CVD method and by using a reaction gas (SiH₄ + PH₃+ O₂). The operation of the CVD reactor of Fig. 3 is as follows. SiH₄ gas, PH₃ gas and N₂ gas are fed through the valves 2lb, 2lc, and 2ld, the mass flow controllers 22b, 22c and 22d, and the valves 23b, 23c and 23d, respectively. The addition of N₂ gas is not essential to the process of this invention, but in the illustrated CVD method, N₂ gas is introduced into the reaction chamber to control the progress of the reaction. These gas flows are then combined, and to the combined gas flow is added NH₃ gas which is supplied through the valve 2la, the mass flow controller 22a, and the valve 23a, and O₂ gas which is supplied through the valve 2le, the mass flow controller 22e, and the valve 23e. The mixture of reaction gases thus obtained is then led through the feed pipe l2 to the gas outlet 8. The gas outlet 8 is maintained at a lower temperature than that of the wafer l5 on the susceptor 9, because the outlet 8 is farther away from the heater l0 for heating the wafer l5. The CVD reactions are carried out in the reaction chamber ll. During the reactions, the chamber ll is exhausted by the mechanical pump l9 and the rotary pump 20 connected in series. Exhaust ports l3 and l4 of the reaction chamber ll are connected with an exhaust pipe l7, and the exhaust pipe l7 is connected with the pump l9. The exhaust port l4 is provided with a vacuum gauge l6, which is used to observe the pressure and variations thereof in the chamber ll. N₂ gas is supplied from a feed pipe l8 to the exhaust pipe 17 positioned between the vacuum gauge l6 and the pump l9, to control the pressure in the chamber ll. Namely, the pressure in the chamber ll can be controlled by con­ trolling a flow rate of the N₂ gas supplied from the pipe l8, by changing the exhaust pressure of the system. Various experiments by the inventors, using the reactor of Fig.3, showed that the addition of NH₃ gas into the reaction gas (SiH₄ + PH₃ + O₂) effectively prevented the formation of particulate by-products such as SiO₂ , P₂O₅ and H₂SiO₃ and the adhesion thereof on the wafer surface, as well as a nonuniform deposition of the PSG coating due to the formation of by-products, and notably improved the step coverage. Similarly, satis­ factory results were obtained when other types of the CVD reactors such as a reduced-pressure reactor or the like were used instead of the reactor of Fig. 3. The inventors could not ascertain the exact details of the mechanisms for the prevention of the formation of the particulate by-products and the improvement of the step coverage, but consider that, since the CVD reactions near the outlet 8 for the reaction gas are effectively inhibited because that area has a relatively low temper­ ature - because it is remote from the heater - and because the reaction gas contains a suitable amount of inert NH₃ gas, undesirable formation of the particulate by-products is avoided. When the reaction gases approach the heated wafer, active CVD reactions occur because that area has a high temperature and the effect of the NH₃ gas on the CVD reactions is now negligible. In addition, it should be noted that, surprisingly, excel­ lent effects could be obtained only with the use of NH₃ gas at a predetermined NH₃/SiH₄ ratio, and that good results could not be obtained when other gases such as CO₂, Ar and N₂0 were added to the reaction gas instead of NH₃ gas. Since undesirable particulate by-products were not produced near the outlets of the reaction gas, the reaction gas was uniformly guided to and subjected to the CVD reactions on the wafer surface. Accordingly, the uniform PSG coatings without defects were deposited on the wafer surface. The rate of deposition of the PSG coatings was stable (for example, l0⁻⁷ /min. for the CVD reactions in the reactor of Fig. 2) and higher than that of the prior art CVD reactions in which NH₃ gas was not introduced into the reaction gas. Figure 4 is a graph showing the relationship between the pressure (linear) in the reaction chamber and the rate of deposition of the PSG coating on the wafer surface. A solid line A indicates the rate for the process of this invention, in which NH₃ gas at an NH₃/SiH₄ volume ratio of 2:1 was introduced into the reaction gas (SiH₄ + PH₃ + O₂), and a dotted line B indicates the rate for the prior art process in which NH₃ gas was not added to the reaction gas. From this graph, it will be appreciated that, when NH₃ gas is not added to the reaction gas to be introduced into the reaction chamber, particulate products such as SiO₂, P₂O₅ and H₂SiO₃ are formed and deposited on the outlet portions of the gas outlet from the following reactions: SiH₄ + O₂ → SiO₂ + H₂O (or H₂) PH₃ + O₂ → P₂O₅ + H₂O (or H₂) These particles adversely affect the amount of the reaction gas to be supplied toward the wafer, thereby causing a reduction of the rate of deposition of the PSG coating. The rate of deposition of the PSG coating in this case is slower than that where NH₃ gas is added. In addition, the rate of deposition of the PSG coating is inversely proportional to the pressure in the reaction chamber. In contrast, when NH₃ gas is added to the reaction gas before or during introduction of the same into the reaction chamber, the reaction gas is stably supplied into the chamber without the adverse effect of the particulate by-products described above, because these by-products are not formed in the outlet portions of the gas outlet. As a result, a stable deposition of the PSG coating on the wafer surface can be attained. A stable deposition of the PSG coating on the wafer could be obtained by adjusting the amount of NH₃ in SiH₄ + PH₃ to be from 2 or 3 to 12 or 13% by vol and having an O₂SiH₄ volume ratio to from 1.5 to 10:1, while maintaining the pressure within the chamber at 26.6Pa (0.2 Torr). Under such conditions, only five or less particles having a diameter of about 1 µm were found as dust on the surface of a wafer having a size of 10 cm (4 inches) square. Figure 5 shows the dependence of the rate of deposi­ tion of the PSG coating upon the NH₃/SiH₄ ratio. The graph of Fig. 5 indicates that the rate of deposition of PSG is reduced when the NH₃/SiH₄ ratio is increased and, unexpectedly, the curves at 350°C and 400°C each have a threshold point at which the deposition rate is suddenly reduced. These are important factors when determining the conditions for carrying out the CVD methods according to this invention. Figure 6 shows the dependence of the number of particulate by-products (about l µm diameter) found on the wafer surface, upon the NH₃/SiH₄ volume ratio. From this graph, it will be appreciated that, to prevent the adhesion of the particulate by-products to the wafer surface, preferably NH₃ gas is added to the reaction gas so that the NH₃/SiH₄ volume ratio is within the range of from 0.l:1 to 100:1. The improvement in the step coverage will be easily understood by referring to Fig. 7A (prior art) and Fig. 7B (this invention), in which 30 is a silicon (Si) wafer, 3l is an SiO₂ layer, 32 is an Al layer, 33 is a PSG coating, and 34 is a plasma SiN coating. When NH₃ gas is introduced into the reaction gas during the CVD process, the step coverage of the PSG coating can be remarkably improved. The results of these drawings as well as those of Figs. 4 to 6 were obtained by using the CVD reactor illustrated in Fig. 3. As described hereinabove in detail, according to the process of this invention, it is possible to reduce the number of particulate by-products adhering as dust on the wafer surface to one half or less, in comparison with the prior art CVD process for forming the PSG coatings. In addition, since a stable deposition of the PSG coatings can be attained, and the step coverage also can be improved, the process of this invention will effec­ tively increase the yield in the production of the semiconductor devices. Further, the process of this invention has been described with regard to a batch system, with reference to Fig. 3. In this system, even if the input and output of the wafers was carried out repeatedly and continuously, no particulate by-products were adhered to the wafer surface. Thus, a considerable reduction in the labor and time necessary for cleaning the deposition chamber can be realized.

Claims: 1. A process for formation of a coating of phosphosilicate glass on a substrate by a chemical vapor deposition method, in which a reaction gas comprising monosilane, phosphine and oxygen is introduced into a reaction chamber in admixture with ammonia gas. 2. A process according to claim 1, in which the gases making up the reaction gas are introduced separately or in combination into the reaction chamber. 3. A process according to claim 1 or 2, in which the volume ratio of ammonia gas to monosilane gas in the reaction gas is within the range of from 0.1:1 to 100:1. 4. A process according to claim 3, wherein the amount of NH₃ in relation to SiH₄ and PH₃ is from 2 to 13% by vol. and the 0₂/SiH₄ volume ratio is from 1.5:1 to 10:1. 5. A process according to any one of claims 1 to 4, in which the chemical vapor deposition reactions are carried out at a temperature of from 300 to 450°C. 6. A process according to any one of claims 1 to 5 which is used in the production of semiconductor devices. 7. A process according to any preceding claim, which is carried out using a hot-wall reduced pressure chemical vapor deposition reactor. 8. A process according to any one claims 1 to 6, which is carried out using an atmospheric pressure chemical vapor deposition reactor.

Inventors: KOYAMA, KENJI, NISHIMURA, MASAHIDE, TAKASAKI, KANETAKE, TSUKUNE, ATSUHIRO
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: C23C16
IPC codes: H01L21, C23C16