JP3158259B2 - Film formation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a film forming method in, for example, a semiconductor device manufacturing process.

[0002]

2. Description of the Related Art In a semiconductor device, a polysilicon film is widely used including a gate electrode of a transistor and the like. In particular, when a device is miniaturized and a deep submicron region is formed, a buried electrode for a contact hole is formed. It has become indispensable as a highly reliable material. For these reasons, it is necessary to further study the method of doping the polysilicon film with impurities.

Incidentally, as a method of forming a polysilicon film doped with an impurity such as phosphorus (P), conventionally, a method of implanting phosphorus into a polysilicon film by ion implantation and then performing an annealing treatment, or a method of forming a P2O5 film using a POCl3 gas. There is known a method of forming a film on the surface of a polysilicon film and then performing a diffusion treatment.

As a method utilizing HOT-WALL type low pressure CVD, for example, a doping gas obtained by diluting a phosphine (PH3) gas with a helium (He) gas, a monosilane (SiH4) gas or a disilane (Si2H6) gas is used.
There is known an in-situ method in which a film-forming gas diluted with, for example, helium gas is simultaneously supplied into a reaction tube and a film is formed while maintaining the inside of the reaction tube at a predetermined reduced pressure.

[0005]

However, the method of doping phosphorus by ion implantation has a disadvantage that crystals in the polysilicon film are destroyed by the impact of ion implantation.
Further, the method using the POCl3 gas has the disadvantage that the concentration uniformity in the depth direction is poor, and furthermore, a step of shaving the P2O5 film is required, so that the step is complicated.

On the other hand, the in-situ method does not have the above-mentioned drawbacks and has the advantage that the phosphorus concentration in the thin film can be controlled by adjusting the flow rate of the doping gas. Although widely used as an effective method for forming a film, in the in-situ method, the thickness of the peripheral portion of the wafer tends to be larger than that of the central portion, and the in-plane uniformity of the film thickness is reduced. There is a disadvantage that it is difficult to obtain. Here, with the demand for miniaturization and high reliability of the device, the in-plane uniformity of the film thickness must be further increased. For this reason, conventionally, for example, a dummy ring or an outer diameter having a larger outer diameter than the diameter of the wafer is required. Dummy disks are provided on a wafer boat, and wafers are placed on these wafers via claws. The presence of the dummy in the film formation area at the outer edge of the wafer boat allows the wafer thickness on the inner side to be reduced. In-plane uniformity is ensured.

However, in such a method, the jigs are complicated and the cost is high, and the distance between the surfaces is large.
There are drawbacks such as a reduced number of processed sheets per batch.

In general, when a polysilicon film is formed using a monosilane gas, a good coverage shape can be obtained. However, when a monosilane gas and a phosphine gas are allowed to flow at the same time, as shown in FIG. Since the film forming rate at the side wall and the bottom of the concave portion is smaller than the film forming speed at a portion other than the concave portion a, the entrance b of the concave portion a becomes narrower than the inside, so that the coverage shape is deteriorated. (Voi
d) c may be formed, and this tendency increases as the phosphorus concentration in the polysilicon film increases.

The present invention has been made under such circumstances, and an object thereof is to improve the in-plane uniformity of the film thickness when forming a thin film doped with impurities. An object of the present invention is to provide a method capable of obtaining good step coverage.

[0010]

The present invention SUMMARY OF] is a method for forming a thin film doped with an impurity on the target object, Sila
The film forming gas is a down-based gas is supplied into the treatment atmosphere
A first step of forming a first layer made of silicon , and supplying a doping gas into the atmosphere to be processed, or supplying a film-forming gas into the atmosphere to be processed together with the doping gas to form an impurity. a second step and, the line alternating Utotomoni or only the impurities to form the second layer of high density, a second step
After that, before performing the first step, the gas used in the second step is
A process of discharging from the atmosphere to be processed is performed, and thus a plurality of
A thin film having a structure in which a second layer is sandwiched between first layers
Obtaining and then heat-treating the thin film to diffuse impurities in the second layer into the first layer.

[0011]

By alternately performing the first step and the second step, a laminated body in which, for example, a first layer containing no impurities and a second layer containing only impurities are alternately obtained, is obtained.
This is subjected to a heat treatment so that impurities in the second layer are removed from the first layer.
And a thin film doped with impurities is obtained.
Here, it is considered that in-plane non-uniformity of the film-forming rate occurs when silicon is deposited on the impurity. If the film-forming gas and the doping gas are supplied to the atmosphere to be processed simultaneously, the film is always Since the impurities are scattered on the surface, in-plane non-uniformity of the film-forming speed occurs, and therefore, the in-plane uniformity of the film thickness of the thin film deteriorates.

On the other hand, according to the present invention, for example, when a film-forming gas and a doping gas are alternately flowed, the film is formed only at the initial stage of forming the first layer after forming the second layer. When the doping gas is supplied in addition to the film-forming gas in the first step, the amount of impurities is small, or the rate is not uniform in the second step. Even when the film gas is supplied, the amount of silicon is small, so that the degree of in-plane non-uniformity of the film formation rate during the film formation is small .

[0013]

FIG. 1 is a vertical sectional side view showing an example of a film forming apparatus for carrying out the method of the present invention. The heating furnace 1 is provided on a base plate 10, and is configured by providing a heater 13 on an inner peripheral surface of a heat insulating layer 12 laminated on an inner surface of a metal plate 11 so as to surround a reaction tube described later.

The heating furnace 1 includes an outer cylinder 21 made of, for example, quartz and having a closed upper end, and an inner cylinder 22 made of, for example, quartz concentrically arranged in the outer cylinder 21. A reaction tube 2 having a double tube structure for forming an atmosphere to be processed is provided.

The outer cylinder 21 and the inner cylinder 22 are respectively held at their lower ends by a cylindrical manifold 3 made of stainless steel or the like. The manifold 3 is fixed in a state inserted into the opening of the base plate 10. Have been. A cap 31 for hermetically sealing the opening is provided at the lower end opening of the manifold 3 so as to be openable and closable.

A rotary shaft 32 that can rotate in a hermetically sealed state is inserted through the center of the cap portion 31 by, for example, a magnetic seal. The lower end of the rotary shaft 32 is connected to a rotating mechanism (not shown) of the lift 4. The upper end is fixed to the turntable 33. A wafer boat 5 is mounted on the upper side of the turntable 33 via a heat retaining cylinder 34. The wafer boat 5 can store, for example, 150 semiconductor wafers W stacked at a predetermined interval. ing.

The elevating table 4 moves up and down along the elevating shaft 41 to load and unload the wafer boat 5 into and from the reaction tube 2.

On the lower side surface of the manifold 3, a film forming gas such as a monosilane (SiH 4) gas or a disilane gas (Si 2 H 6) gas is mixed with a carrier gas such as a helium gas and introduced into the reaction tube 2. A first gas introduction pipe 61 and a second gas introduction pipe 62 are inserted, and these gas introduction pipes 61 and 62 are connected to a separate gas supply source (not shown). The first gas introduction pipe 61 is bent upward in an L shape so that the gas outlet is located above the wafer boat 5, and the second gas introduction pipe 61
Is similarly bent in an L-shape such that the gas outlet is located near the lower end of the wafer boat 5. In this way, by adopting a structure in which the film-forming gas introduction pipes have two long and short lines, the film-forming gas can be more uniformly supplied to each wafer W on the wafer boat 5.

On the lower side surface of the manifold 3,
A third introduction pipe 7 for horizontally mixing a doping gas such as phosphine (PH3) with a carrier gas such as helium gas and introducing the mixed gas into the reaction tube 2 is provided.

On the other hand, on the upper side surface of the manifold 3,
An exhaust pipe 8 connected to a vacuum pump (not shown) for discharging the processing gas from the gap between the outer cylinder 21 and the inner cylinder 22 to set the inside of the reaction tube 2 to a predetermined reduced pressure atmosphere is connected.

Next, an embodiment of the method of the present invention using the above-described film forming apparatus will be described. First, the atmosphere to be processed in the reaction tube 2 is heated by the heater 13 so that the temperature of the center portion (central portion in the vertical direction) of the wafer boat 5 becomes, for example, 620 ° C. A wafer boat 5 containing 100 wafers W is loaded from the unit by the lift 4 at a pitch of, for example, 4.76 mm.

Next, a predetermined degree of vacuum, for example, 1 ×
After evacuating to 10-3 Torr, a 100% monosilane gas, which is a film forming gas containing a main component of the thin film, is supplied from the second gas introduction pipe 62 (the shorter gas introduction pipe) for 180 seconds.
While being introduced into the inner cylinder 22 at the flow rate of CCM, the reaction tube 2
The inside is evacuated to a pressure of 0.4 Torr, and the film is formed for 10 minutes while rotating the wafer boat 5 at a rotation speed of, for example, 3 rpm.

Then, the supply of the monosilane gas is stopped, and the gas (monosilane gas) in the reaction tube 2 is exhausted by, for example, a vacuum pump (not shown) connected to the exhaust pipe 8 for about 30 seconds.

Thereafter, a doping gas, for example, phosphine gas, is supplied from the third gas introduction pipe 7 with helium gas.
The mixed gas diluted to 0% is introduced into the inner cylinder 22 at a flow rate of 100 SCCM, and the inside of the reaction tube 2 is 0.5 Torr.
And the film is formed for, for example, one minute. Subsequently, the supply of the doping gas is stopped, and the gas in the reaction tube 2 is evacuated, for example, for about 30 seconds in order to exhaust the gas from the reaction tube 2, and then the first gas introduction tube 61 and the second gas introduction tube 62 100 SCCM each from monosilane gas
And at a flow rate of 80 SCCM into the inner cylinder 22 and exhaust the inside of the reaction tube 2 to 0.4 Torr.
For example, film formation is performed for 10 minutes. After this film formation, vacuum is drawn for about 30 seconds in order to further discharge the gas in the reaction tube 2.

By such an operation, first, a polysilicon layer is formed on the surface of the wafer W, and a phosphorus adsorption layer and a polysilicon layer are formed thereon in this order. In this embodiment, as shown in the sequence diagram of FIG. 2, each step after the step of forming the phosphorus adsorption layer, that is, the formation of the phosphorus adsorption layer →
By repeating the steps of evacuation → polysilicon layer formation → evacuation a plurality of times, for example, 20 times, a laminate 9 having a sandwich structure in which a phosphorus adsorption layer 92 is interposed between the polysilicon layers 91 as shown in FIG. 3 is formed. Is done. In this embodiment, a step of forming the polysilicon layer 91 with a monosilane gas and a step of forming a phosphorus adsorption layer 9 with a phosphine gas are described.
2 are formed by the first step and the second step of the present invention, respectively.
This corresponds to the step of

After performing such an operation, the inside of the reaction tube 2 is heated to 900 ° C. in a nitrogen (N 2) gas atmosphere, and the above-mentioned laminate 9 is annealed for 60 minutes. A polysilicon thin film having a concentration of 2 × 1020 / CC and a thickness of 4000 Å is obtained, and the in-plane uniformity of the polysilicon thin film is ± 2.1 to
It was 3.4%.

In the above embodiment, when the silicon layer 91 is formed, the temperature of the atmosphere to be processed is desirably 580 ° C. or lower, or 610 ° C. or higher. The reason is that silicon is in an amorphous state at 580 ° C. or lower and is in a polycrystalline state at 610 ° C. or higher. In a temperature range between the two, both states are mixed and the surface becomes rough. This is because the variation in the adsorption amount is large.

Next, a thin film was formed in exactly the same manner as in the above embodiment except that 40 phosphorus adsorption layers 92 were formed, and a thin film was formed in exactly the same manner as in the above embodiment except that eight phosphorus adsorption layers 92 were formed. Was formed, and the relationship between the deposition rate of each thin film and the height position of the wafer was examined. The result shown in FIG. 4 was obtained.
In FIG. 4, the vertical axis represents the film formation rate obtained by dividing the thickness of the thin film (laminate) having the sandwich structure shown in FIG. 3 by the time required for film formation (excluding the exhaust time in each process). The horizontal axis is the position counted from the bottom of the wafer boat. FIG. 4
The middle circles, triangles, and squares are data corresponding to 40, 20, and 8 phosphorus adsorption layers, respectively. The in-plane uniformity and the inter-plane uniformity of the film forming rate were evaluated under the respective conditions, and the results were as follows.

(In the case of eight phosphorus adsorption layers) In-plane uniformity: ± 1.3 to 1.9% Inter-plane uniformity: ± 3.
7% (when the phosphorus adsorption layer is 20 layers) In-plane uniformity: ± 2.1 to 3.4% Inter-plane uniformity: 2.3
% (When the phosphorus adsorption layer is 40 layers) In-plane uniformity: ± 3.6 to 4.7% Inter-plane uniformity: ± 2.
In the result of 9% or more, the phosphine gas and the monosilane gas were simultaneously introduced into the reaction tube to form an in-s film.
Given that the in-plane uniformity of the film thickness in the itu method is about 10%, the in-plane uniformity of the film thickness under each condition is in-s.
It can be understood that the improvement is remarkably improved as compared with the itu method.

The film formation rate is about 10 Å / min in the case of the in-situ method, but in the method of the present invention, as shown in FIG.
5 Angstroms / minute, so the method of the invention is considerably faster. In addition, the in-situ method has a film thickness uniformity of about 10% in the in-situ method, which indicates that the method of the present invention is superior to the in-situ method.

Further, the thin film obtained under each of the above conditions was also examined for the uniformity of the phosphorus concentration between the surfaces and the uniformity of the specific resistance between the surfaces. The results were about 5% and about 2 to 4%, respectively. It was a good result.

Further, in order to examine the relationship between the thickness of the polysilicon layer as the first layer and the phosphorus concentration of the thin film, the phosphorus concentration was measured while changing the thickness of the polysilicon layer in various ways. The result shown in FIG. However, the procedure was performed in the same manner as in the above example, except that the time for supplying the monosilane gas into the reaction tube was controlled. From this result, it can be seen that the relationship between the phosphorus concentration and the thickness of the polysilicon layer as the first layer is very linear, and that the phosphorus concentration can be controlled by controlling the thickness of the polysilicon layer.

Here, the reason why the in-plane uniformity of the film thickness of the thin film obtained by the conventional in-situ method is poor is considered. As already described in the section (action), in this case, It is considered that in-plane non-uniformity of the film forming rate occurs when silicon is attached. That is, when phosphorus is adsorbed on the film surface, monosilane is repelled by the phosphorus and cannot be adhered, and silicon is adsorbed exclusively through adhesion of SiH2 generated by a decomposition reaction of monosilane in a gas phase. However, since the probability of occurrence of a decomposition reaction (a reaction of forming SiH2) in the gas phase is small, the silicon adsorption rate is determined by the rate of supply of SiH2, so that the film forming rate on the peripheral side of the wafer is higher than that in the center. It is presumed that in-plane non-uniformity of the film thickness occurs. This is the same when, for example, a disilane (Si2H6) gas is used.

Although such a phenomenon always occurs in the in-situ method, in the above-described embodiment, it occurs only at the initial stage of forming the polysilicon layer after the formation of the phosphorus adsorption layer. It is considered that the uniformity is improved and the deposition rate is increased. Therefore, between the second step ( the step of forming the phosphorus adsorption layer) and the first step ( the step of forming the polysilicon layer), the gas in the reaction tube is evacuated as in the above-described embodiment, and the monosilane gas and the phosphorous gas are discharged. It is desirable to prevent film formation from proceeding in a state where the fin gas is mixed. Further, according to the method of the above-described embodiment, since the non-doped polysilicon films are alternately laminated even in the concave portions, the shape of the step coverage is also changed as shown in FIG. It was very good as in the case of forming a silicon film, and no void was present in the concave portion a.

As described above, in the present invention, in the first step, a first gas having a low impurity concentration may be formed by mixing a film-forming gas and a doping gas, In the step, a second gas having a high impurity concentration may be formed by mixing a film forming gas and a doping gas.

[0036] Then when heat-treating the laminate of the sandwich, if even without annealing treatment, followed by oxidation, diffusion, such as the CVD process is performed,
These heat treatments can be substituted.

In the present invention, the impurities are not limited to phosphorus .

[0038]

According to the present invention, the first layer having zero or low concentration of impurities and the second layer having all or high concentration of impurities are alternately stacked, and then the impurities are doped by heat treatment. I have a thin film. Therefore, as described above, the degree of in-plane non-uniformity of the film formation rate can be reduced as compared with the case where a thin film containing a predetermined concentration of impurities is grown while simultaneously supplying the doping gas and the film formation gas. As a result, a thin film having high in-plane uniformity of the film thickness can be obtained, and a good step coverage shape can be obtained.

[Brief description of the drawings]

FIG. 1 is a vertical sectional side view showing an example of an apparatus used in an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an example of a gas supply sequence.

FIG. 3 is an explanatory view showing a laminate before a heat treatment.

FIG. 4 is a characteristic diagram showing an inter-plane distribution of a film forming speed.

FIG. 5 is a characteristic diagram showing a relationship between a thickness of a polysilicon layer and a phosphorus concentration.

FIG. 6 is an explanatory diagram of a step coverage shape obtained by the method of the present invention.

FIG. 7 is an explanatory diagram of a step coverage shape obtained by a conventional method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Reaction tube 3 Manifold 4 Lifting table 5 Wafer boat 61, 62, 7 Gas introduction tube 8 Exhaust tube 91 Polysilicon layer 92 Phosphorus adsorption layer

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Reiji Shinno 2-3-1 Nishi-Shinjuku, Shinjuku-ku, Tokyo Inside Tokyo Electron Limited (72) Inventor Yoshiyuki Fujita 2-3-1 Nishi-Shinjuku, Shinjuku-ku, Tokyo No. Tokyo Electron Co., Ltd. (72) Inventor Hiroshi Suzuki 52 Imatsudane, Iwayado, Esashi City, Iwate Prefecture Inside Tokyo Electron Tohoku Co., Ltd. (56) References JP-A-1-186615 (JP, A) JP JP-A-4-5824 (JP, A) JP-A-5-62904 (JP, A) JP-A-64-44013 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21 / 205 H01L 21/225

Claims (1)

(57) [Claims] In a method for forming a thin film doped with an impurity on an object to be processed, a film-forming gas which is a silane-based gas is supplied into an atmosphere to be processed to form a first layer made of silicon. A first step of forming, and supplying a doping gas into the atmosphere to be processed, or supplying a film-forming gas together with the doping gas into the atmosphere to be processed to form a second impurity containing only impurities or a high concentration of impurities. after the second row and steps, the alternating Utotomoni, second step of forming a layer, line a first step
The gas used in the second step before the process
Performing a second step between the plurality of first layers.
A film forming method comprising: obtaining a thin film having a structure in which layers are sandwiched; and thereafter heat-treating the thin film to diffuse impurities of a second layer into the first layer.