Abstract:
An improved method for depositing material on a substrate is shown. The material to be deposited is energized by irradiation with light in a chamber in which a CVD method is carried out. The energy induced by the irradiation remains in the molecules of the material even after the molecules have lain on the substrate. With the residual energy, the molecules can wander on the substrate even to a hidden surface. Due to this wandering, the deposition can be performed also on the inside of a deep cave. A semiconductor device is thereafter formed on the inside of the cave.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of co-pending U.S. application Ser. No. 06/909,203, filed Sept. 19, 1986, now U.S. Pat. No. 4,735,821. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for forming a film coat on the inside of a depression, and in particular for a photo CVD process carried out so that deposition is effectively performed on insides of depressions in a semiconductor device. 
     There is broadly known and used a low pressure CVD method, a plasma CVD method, and a chemical vapor deposition method for semiconductor processing. In the semiconductor processing method, depressions such as a hole, a trench, a cave (a sub-surface re-entrant opening having surfaces out of line-of-sight) or so on are configured on whose surfaces are placed a product created by the CVD method to form a buried field insulating layer or an electric element such as a capacitance in the depression, or to fill an over-etching region in the form of a depression. When a formation of a layer on the depression is desired, it is inevitable that the thickness &#34;ds&#34; of the layer on an inner surface (depression) and the thickness &#34;dt&#34; of the layer on an upper surface result in dt/ds&gt;1. One of the problems of researching to obtain a finely configured semiconductor in the VLSI field is how an inverse ratio, namely ds/dt, can be increased near 1. Further, in the case where the depth of a cave has a measure more than the measure of the opening of the cave, it was impossible to form a uniform layer throughout the inside of the cave. Such caves are formed, e.g., with a known trench method which can dig a cave of depth having a measure 3 to 5 times as large as the measure of the width of the opening thereof. Anyhow, existing methods are not suitable to perform a deposition in such a cave. 
     Namely, according to an existing CVD method, atoms or molecules are deposited on a substrate in an excited condition which are diffused into vapor after being decomposed or after undergoing a reaction caused by thermal energy. The existing process can be performed effectively when it is carried out under negative pressure, since the active molecules have a relatively long mean free path in the vapor under a negative pressure, compared with that under the atmospheric pressure. For example, on a substrate with a trench of 2 micron meters in width and also in depth, a depression resulted in a layer 1 of micron meter thickness on the upper surface, a layer with at most a thickness of 0.7 micron meter on the side wall of the trench and a layer with the thickness of 0.6 micron meter on the bottom of the trench. In any case, a step coverage ds/dt is expected only up to about 0.7. 
     According to another known method, step coverages are no more than that of the above method. Normally, ds/dt=0.3-0.5. A plasma enhanced CVD alone is comparable with the above LPCVD. 
     SUMMARY OF THE INVENTION 
     This application is directed to a capacitor which can be used for finely densed capacitances formed on a semiconductor memory, and to a method for coating the side surface of a trench other than its bottom. The opening of the coating on the bottom can be used for forming a capacitance, a transistor, or so forth. 
     The invention has been conceived on the basis of the discovery by the inventor of a phenomenon in which active atoms, (molecules) excited by irradiation with light can preserve their active energy, referred to hereafter as a first energy, for a relatively long time. The active energy of the atom partially remains after the atoms are laid on the substrate, the residual energy being referred to hereafter as the second energy. 
     While undergoing a CVD method, atoms with the first energy are overlying the atoms with the second energy which are already deposited. According to experiment by the Applicant, the overlying atoms can wander on the substrate so that the sum of the first energy and the second energy takes less value. The wandering atoms can reach a bare surface of the substrate on which they are stabilized. This means that it is possible to form a layer even on a hidden surface from a principal surface. Further, the phenomenon makes it possible that a layer can be formed according to a new formation theorem which is entirely different from the exciting theorem. According to experiments described in detail infra, the phenomenon appears as if material is poured into a trench, like liquid. 
     It is therefore an object of the invention to provide an improved CVD method capable of performing deposition on a depression region. 
     It is another object of the invention to provide an improved CVD method according to which a thick layer is formed even on an interior surface of a depression region. 
     It is further object of the invention to provide an improved CVD method according to which a deposition is established also in the inside of a deep cave. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a CVD device used in methods of the invention. 
     FIG. 2 is a section view of the semiconductor device formed in accordance with a first embodiment of the invention. 
     FIGS. 3(A) to 3(C) are fragmentary section views showing a method for depositing a layer on a substrate in accordance with a second embodiment of the invention. 
     FIGS. 4(A) to 4(C) are fragmentary section views showing a third embodiment of the invention. 
     FIGS. 5(A) to 5(C) are fragmentary section views showing a fourth embodiment of the invention. 
     FIGS. 6(A) to 6(C) are fragmentary section views of a novel transistor formed according to the present invention. 
     FIG. 7 is a schematic view illustrating a reaction chamber usable in the present invention. 
     FIGS. 8(A) to 8(D) are top elevational views of a substrate in which different caves are formed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an example of a CVD apparatus which is preferably used for the invention is shown. In the figure, a CVD device comprises a reaction system 10, an exhausting system 11 and a reactant gas supply system 12. The reaction system 10 includes a prechamber 14 and a reaction chamber 13, both being evacuated in advance of deposition. First, a plurality of substrates 30 are prepared on which a deposition will be carried out. The substrates are supported on the holder 17 so as to be arranged apart from each other. Then, the plurality of substrates are transferred to the reaction chamber 13 from the prechamber 14 together with the holder 17 through a gate valve 31. In the reaction chamber 13, the substrates 30 are irradiated with ultraviolet light of 184 nm or 254 nm wavelength from a low pressure mercury lamp 21 in a light source chamber 22. The substrate 30 is irradiated with light from the lower side of the substrate 30 and heated from the upper side by a halogen lamp heater 16 provided in a heating chamber 15. 
     Reactant gas is introduced to the reaction chamber 13 from a gas supply (not shown) via a flow rate meter 28, a valve 29 and a nozzle 18. In the reaction chamber 13, the gas constitutes flows designated with references 19 and 19&#39; in which the gas is excited, made active and decomposed by the ultraviolet light emitted from the lamp 21, and a resultant product is deposited on the substrate 30 as a layer. 
     As reactant gases, a mixed gas of polysilane and polyfluorosilane is used for the silicon layer. In addition to this reactant gas, an amount of ammonia gas can be further mixed to make a silicon nitride layer. When a p-type or an n-type semiconductor layer is desired, a suitable impurity or impurities may be blended. Such reactive gas is introduced with a carrier gas, if necessary. 
     Further, the reactive gas for the silicon layer is mixed with oxide gas to form a silicon oxide layer, a phosphorus glass or a boron glass. Instead of a silicon layer, an alkylmetal can be introduced to form a metal layer, or an alkylmetal and a polysilane can be introduced to form a layer composed of a metal and a silicide. 
     Referring now to FIG. 2, a section view of an experimental product formed by the above method is described to show a first embodiment of the invention. This product was fabricated using a monocrystalline silicon substrate 30&#39; of 15 mm long×20 mm wide×380 micron meters thick. The substrate 30&#39; was irradiated with ultraviolet light at 3 torr. Then, as shown in the figure, a silicon nitride layer 33 was formed on the substrate 30&#39; with a thickness of 1000 A on the bottom and also with a thickness of 1000 A on the side wall. The noticeable result of the experiment is that the silicon nitride layer 33&#34; was observed also on the upper surface as an extension 35 of 5 mm, whose thickness was measured of 1000 A on the edge portion having 2 mm width from the side wall of the substrate 30&#39;. 
     What is further of interest is that the wandering of the deposited materials seems to depend little on the temperature of the substrate 30&#39;. A silicon nitride layer can be formed at a temperature less than 400° C. Needless to say, no layer is formed without irradiation even at about 300° C. 
     FIGS. 3(A) to 3(C) are fragmentary cross sectional views showing a second embodiment according to the invention. On a silicon semiconductor substrate 1 is a silicon nitride layer 2 which was etched to prepare an opening as a mask for a trench 3. The trench 3 was created by etching with the nitride layer 2 as a mask. 
     After removing the silicon nitride layer 2, a silicon oxide layer was deposited on the substrate 1 in accordance with the method explained above in conjunction with FIG. 1. In FIG. 3(B), broken lines 4 are plotted to explain how the silicon nitride layer 36 was grown. The broken lines 4-1 to 4-4 show contours in sequence of the layer growing. The thickness of each layer deposited on each step 35-1 and 35-1&#39;, 35-2 and 35-2&#39; . . . or 35-4 was observed to be uniform throughout the deposited surface including the inside of the trench 3. 
     Since the uppermost surface of the layer 3 just over the trench tends to be finished in the form of a concave, the upper portion of the layer may be removed by isotropic etching to level the surface as shown in FIG. 3(C). 
     Although this experiment was carried out to form a silicon oxide layer, other layers of silicide such as a silicon nitride layer can be formed according to a similar process, such as silicon nitride. Also, after completion of a layer different from a silicon oxide layer, the surface of the layer may be oxidized to form a surface of silicon oxide. 
     Contrary to existing technique, the CVD according to the invention is carried out at relatively low temperature (about 300° C.) since the bottom of a trench is likely to produce lattice defects therein at a high temperature. The layer thus formed, however, has a very fine structure comparable with a layer conventionally formed at higher than 1000° C. 
     Referring to FIGS. 4(A) to 4(C), a third embodiment is shown. The embodiment includes an extrinsic semiconductor. The process is substantially identical to the preceding embodiment so that redundant descriptions will not be repeated. On a silicon semiconductor are a silicon oxide or a silicon nitride layer 4, a polysilicon or amorphous silicon layer 5 which is doped with phosphorus as which is an impurity or a metallic conductive layer 5 such as of titanium chloride or tungsten, and a silicon oxide layer 6. In this configuration, a capacitance is constituted between the semiconductor substrate 1 and the conductive layer 6. 
     A fourth embodiment of the invention is shown in FIGS. 5(A) to 5(C). The experiment was made to form a semiconductor device in which a cave is formed in a trench to increase the capacitance formed in the trench. 
     The trench was formed with the depth of 5 micron meters and with the width of 2.5 micron meters at the upper portion and of 1.5 micron meters at the bottom portion, with a silicon nitride layer 2 and a silicon oxide 2&#39; as a mask as shown in FIG. 5(A). Thereafter a silicon nitride layer 45 and 45&#39; was deposited by a CVD method using light irradiation. 
     Then the silicon nitride layer was let undergo an anisotoropic etching to remove selectively the upper layer 45&#39; and the part of the layer 45 formed on the bottom portion 39 of the trench 3. Further, for the substrate 1, an anisotropic etching was carried out to perform a lateral etching so that a cave 40 is formed. 
     After removing the silicon oxide layer 2&#39; and the silicon nitride layer 2, a silicon oxide layer 41 was deposited on the inside walls of the cave 40, the trench 3 and the surface of the semiconductor substrate 1 according to the method which is the same as that discussed in the preceding. Further on the layer 41 is formed a titanium silicide layer or silicon layer 42 which is doped with phosphorus, an insulating layer 4 such as silicon oxide or silicon nitride and a conductive layer 5 such as a polycrystalline silicon or titanium silicide layer which is doped with phosphorus, each layer being fabricated by a CVD method according to the present invention. Thereafter, a silicon oxide layer 46 was superimposed on the laminate over the trench 3 so as to completely stop the trench. During the process, contacts 44 and 43 were defined by photolithgraphy. Consequently, an improved semiconductor device was obtained with large capacitance. 
     In FIGS. 6(A), 6(B), and 6(C), a substrate 100 is made of a p-type semiconductor and provided with an n-type drain region 103 on the upper edge of a trench 105. A silicon oxide film 107 is formed on the substrate surface, including along the inside of the trench 105 as a gate insulating film as well as on the upper surface of the substrate 100. The silicon oxide film on the bottom portion of the trench 100 is removed by anisotropic etching. Through the exposed bottom, an impurity is diffused into the substrate 100 to form an n-type source region 109. The source region 109 is isolated by an insulator 111. A uniform conductive film 113 is formed on the inside of the trench over the gate insulating film 107 as a gate electrode. Over the substrate, a passivasion film 115 is formed. While not illustrated in the figure, contacts have to be formed on the drain region and on the source region. The drain contact can be formed by simply forming an electrode on the drain region. The source contact can be formed through a contact n-type region which is formed from the substrate surface and merged with the end of the source region. 
     The etching process illustrated in FIGS. 5(A) and 5(B) will be explained with reference to FIG. 7. Inside a reaction chamber, a substrate to be etched is disposed between a pair of electrodes. As an etchant gas, CF 4 , CF 3  Br, CCl 4 , or the like is inputted to the reaction chamber at a negative pressure, an alternating voltage, biassed if necessary, is applied to the pair of electrodes to initiate discharge in between, whereupon a chemical vapor reaction takes place. Then, the chemical vapor reaction generates a plasma gas in the reaction chamber by virtue of the energy of the discharge. Under the alternating electric field induced by the applied voltage, the plasma particles reciprocate (resonate) in a vertical direction between the electrodes, and collide with the horizontal surface of the substrate perpendicularly to the direction of the electric field in the vicinity of the substrate. This etching process is called RIE (Reaction Ion Etching) 
     The film 2 somewhat hides the side wall of the case 3. The reason why the side wall is partially removed adjacent to the bottom of the case 3 is that the side wall is slightly inclined and therefore not perfectly free from etching, that the anisotropy of RIE is not perfect, and that the effect of the hiding by the film 2 is lessened as departing from the film 2. 
     The direction of anisotropy etching depends on the crystal orientation, having planes designated conventionally as (110) and (111), and the etchant. Some etchant attacks on the (111) planes at a low rate as compared with the other planes, while another etchant attacks on the plane (110) at a high rate as compared with the other planes. 
     Speaking about FIG. 5(C), the cave is desirably formed, in order to increase the inner surface area, like the low portion of a wine-glass as illustrated in the sketches 8(A) and 8(B). Of course, the lower profile of the cave may be elongated only in one horizontal direction as shown in the sketches 8(C) and 8(D), by appropriately choosing the orientation of the crystal and the etchant. Anisotropic etching itself is well-known in the art. 
     While the present invention has been described with reference to several preferred embodiments thereof, many variations and modifications will now occur to those skilled in the art. The scope of the present application is limited solely by the scope of the appended claims and not by the specific embodiments disclosed herein.