GB2326648A

GB2326648A - Growth of polycrystalline silicon film by raising temperature during deposition

Info

Publication number: GB2326648A
Application number: GB9812887A
Authority: GB
Inventors: Akio Tanikawa; Taishi Kubota
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1997-06-24
Filing date: 1998-06-15
Publication date: 1998-12-30
Anticipated expiration: 2018-06-15
Also published as: KR19990006994A; GB2326648B; GB9812887D0; JPH1116838A

Description

GROWTH METHOD OF POLYCRYSTAL SILICON FILM BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a growth method of a polycrystalline silicon film and a CVD apparatus for use in the growth method.

2. Description of the Prior Art A polycrystalline silicon (poly-Si) film on an amorphous insulating film which can be used in a semiconductor device such as an LSI can be grown and deposited by a chemical vapor deposition method (CVD), but heretofore, a film growth has been made, maintaining constant conditions during the growth.

Furthermore, the conventional CVD apparatus which can be used for the film growth slowly responds to a temperature change, and it is difficult to alter film growth conditions during the growth of a thinner film and to repeatedly change film forming conditions during film formation.

In recent years, with the miniaturization of the LSI, the roughness of the surface and interface of the poly-Si film and the concentration distribution of a dopant have been required to be controlled more precisely and more finely, and by the conventional film formation technique under the constant growth conditions, it has been impossible to optimize many factors such as work function, diffusion properties and the roughness of the surface and interface of the film which are required for the film.

SUMMARY OF THE INVENTION A first object of the present invention is to provide a growth method of a polycrystalline silicon (poly-Si) film which permits the optimization of many factors such as work function, diffusion properties and the roughness of a film and interface.

A second object of the present invention is to provide a CVD apparatus for use in the effective and prompt practice of the above-mentioned growth method.

In a first aspect, the present invention provides a method of growing a polycrystalline silicon film on a substrate of an amorphous insulating film using chemical vapour deposition, said method comprising the step of raising a film formation temperature during growth after the lapse of a predetermined time period in order to grow the film to a predetermined film thickness.

The term "amorphous insulating film" includes physical and chemical equivalents thereof.

A preferred embodiment of this aspect of the present invention is a growth method of a polycrystalline silicon (poly-Si) film which comprises growing the polycrystalline silicon (poly-Si) film on a substrate of an amorphous insulating film or the like by a CVD method, said method comprising the step of raising a film formation temperature up to a predetermined temperature higher than a start temperature after the lapse of a predetermined time from a growth start point, or the step of repeating the raise of the film formation temperature from a low temperature, while the film growth is continued, thereby growing the film to a desired film thickness.

In a second aspect, the present invention provides apparatus for depositing a thin film on a substrate using chemical vapour deposition, said apparatus being equipped with a movable thermalradiation screening means insertable between a heat source and the substrate.

A preferred embodiment of the second of the present invention is an apparatus for use in the effective and prompt practice of the above-mentioned growth method, and this apparatus is a CVD apparatus, for depositing a thin film on a substrate by a CvD method, which is equipped with a movable thermal radiation-screening plate or tube which can be inserted between a heater as a heat source and the substrate.

An Si film grown on an amorphous layer by the CvD method becomes a poly-Si or a-Si under the influence of a growth temperature. The poly-Si obtained by crystallizing the a-Si film by a heat treatment is different from a film grown as the poly-Si film from a start point in orientation and the diameter of grains. Furthermore, also in the film grown as the poly-Si film from the start point, the orientation and the diameter of grains depend on growth conditions.

In the method of the present invention, such a change is positively caused so as to occur in a depth direction of the poly-Si film, whereby the optimization of many factors such as work function, diffusion properties and the roughness of a film surface and interface can be realized for the poly-Si film. In this method, if the operation of raising the film formation temperature from a low temperature is repeated to form the film, it is possible to intricately cause the above-mentioned change in the depth direction of the poly-Si film.

In order to promptly cause the change of the abovementioned film forming conditions, particularly the film growth temperature by the CVD apparatus without impairing a temperature uniformity, a thermal-radiation-screening tube or plate can be inserted between a heater as a heat source and a substrate, whereby the thermal radiation from the heater can'be immediately screened. That is to say, the occurrence of the film growth temperature change can be effectively realized by the CVD apparatus of the present invention, in which the thermal-radiationscreening tube or plate is equipped between the heater and the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows schematic views for the explanation of Example 1 regarding the present invention; and (a), (b) and (c) show a film growth state at a growth temperature of 510 C, a film growth state at a growth temperature between 5100C and 6200C, and a film growth state at a final growth point, respectively.

Fig. 2 shows a schematic view for the explanation of a conventional technique.

Fig. 3 shows a schematic view for the explanation of a reaction tube type low pressure CVD apparatus according to the present invention; and (a) and (b) show a state of a thermal-radiation-screening tube installed at such a position as to avoid a uniform heat band, and a state of the screening tube at such a position as to cover the uniform heat band, respectively.

Fig. 4 shows a schematic view for the explanation of a back surface heater type ultra-high vacuum CVD apparatus according to the present invention; and (a) and (b) show a state of'a thermal-radiation-screening plate installed at such a position as to avoid a substrate, and a state of the screening plate at such a position as to cover the substrate, respectively.

Fig. 5 shows a schematic view for the explanation of a conventional reaction tube type low pressure CVD apparatus.

PREFERRED EMBODIMENTS OF THE INVENTION In a preferable embodiment of a method according to the present invention, for example1 a film is grown at a low growth temperature (substrate temperature) which is amorphous silicon growth conditions or polycrystalline silicon growth conditions for such a certain time that the film of 1 nm or more is deposited from the start of the film growth, and while the film formation is continued, the growth temperature (substrate temperature) is immediately or + tinuously) hanged to a higher temperature which is the polycrystalline silicon growth conditions to grow the film to a desired film thickness.

According to another preferable embodiment, when the film growth at the above-mentioned high temperature has reached rye termined film thickness, the growth temperature (subs ate temperature) is immediately or c AFtinuously changed to a lower temperature which is the amorphous silicon growth conditions or the talline silicon growth conditions, while the film growth is continued, and the film formation at the low temperature is then carried out until a predetermined film thickness has been reached. Afterward, the film formation at the above-mentioned high temperature is done again until the predetermined film thickness has been reached. The operation of raising the film formation temperature from the lower temperature may be repeated several times as needed.

According to still preferred embodiment of the present invention, during the continuation of the above-mentioned film growth, a flow rate of a source gas for the film formation is changed to a flow rate different from an initial flow rate at a growth start after the lapse of a predetermined time from the growth start point, or such a change of the flow rate is repeated, and this operation is used together with the above operation of raising the film formation temperature from the low temperature to grow the film until the predetermined film thickness has been reached.

According to a preferable embodiment of a CVD apparatus of the present invention, for example, in the case of a reaction tube type LP-CVD apparatus a space is formed between a heater and the reaction tuber and for example, a movable thermal-radiation-screening tube made of tantalum is disposed therein. In the case that the above-mentioned method of the present invention is performed by the use of this apparatus, at the start of the film growth, the screening tube is put at such a position as to cover a uniform heat band which is a film growth position, and when it is needed to rapidly raise the film formation temperature after the lapse of a predetermined time, the screening tube is upwardly or downwardly moved to such a position as to avoid the uniform heat band. When it is needed to form the film by repeating the rise of the film formation temperature from the lower temperature, such a temperature rise can be promptly carried out by repeating the movement of the screening tube.

Furthermore, for example, in the case that the device of the present invention is a back surface heater type UHV-CVD apparatus, a movable thermal-radiationscreening plate is disposed between the heater and the substrate. In the case that the above-mentioned method of the present invention is carried out by the use of this apparatus, at the start of a film growth, the screening plate is put at such a position as to cover a substrate, and after the lapse of a predetermined time, the screening plate is moved to such a position as to avoid the substrate, whereby a film formation temperature can be promptly raised. By repeating the movement of the screening plate, the rise of the film formation temperature from the lower temperature can be repeated to form the film.

It can be easily understood that, by the use of the CVD apparatus of the present invention, a poly-Si film can be grown at the start of a film growth, and after the lapse of a predetermined time, a film formation temperature can be effectively and promptly lowered to grow an a-Si film thereon.

Next, examples of the present invention will be described in detail with reference to drawings.

Example 1 In the first place, reference will be made to an example in which a method of the present invention is carried out by the use of a conventional low pressure CVD device.

On an SiO2 film 2 on the surface of an Si substrate 1, a film growth was started at a silane flow rate of 800 sccm at a film formation temperature of 5100C [refer to Fig. l(a)]. After the lapse of 5 minutes, the conditions were reset to a silane flow rate of 300 sccm and a film formation temperature of 6200C, and in about 20 minutes, an actual growth temperature reached 62()0C [refer to Fig.

1(b)). While the conditions were maintained at a silane flow rate of 640 sccm and a film formation temperature of 6200C, the film growth was continued for 30 minutes to complete the film formation [refer to Fig. 1(c)). In the above-mentioned steps, an a-Si film 3 was deposited as thick as 10 nm when the temperature was 5100C [refer to Fig. l(a)], and while the a-Si film was crystallized during the temperature rise, a transition layer 5 was deposited [refer to Fig. l(b)]. At a final point when the film growth had been continued at 6200C for 30 minutes, a poly-Si film (a poly-Si I film 4 + the transition layer 5 + a poly-Si II film 6) having a thickness of about 300 nm was formed, as shown in Fig.

l(c). That is to say, in the formed poly-Si films (4+5+6), the transition layer 5 having a depth of 200 to 250 nm was present, and under and on the transition layer 5, there were the layers of the poly-Si I film 4 and the poly-Si II film 6 having different orientations.

In comparison with a poly-Si film 9 (refer to Fig.

2) obtained by the conventional technique, i.e., in the case that the film formation was continued at a constant temperature of 6200C, the poly-Si film obtained in this example was more excellent. This reason is that an roughness of the interface between the poly-Si film 9 and the SiO2 film 2 in the conventional method can be reduced and the amount of a dopant which reaches the SiOz film 2 at the time of ion implantation into the poly-Si film 9 can be decreased.

The growth conditions just described are required to be optimized in consideration of kinds of apparatus and source gas, but usually between increase/decrease of the conditions and the quality of the film, the same tendency as described above is observed.

With regard to the poly-Si film formed in the above manner, neither a barrier nor a low conductive layer for disturbing the movement of a carrier is present between the respective layers, and hence, if this poly-Si film is used as a gate electrode of an MOS transistor, an MOS transistor having all of excellent yield, performance and reliability can be manufactured.

This example uses the conventional low pressure CVD apparatus, but in the case that a more rapid change of conditions is necessary, the following CVD apparatus of the present invention can be used.

Example 2 An example in which a reaction tube type low pressure CVD apparatus is used is shown in Fig. 3.

A space was formed between a heater 10 and a reaction tube 13, and a movable thermal-radiationscreening tube 14 made of tantalum was disposed so as to be introduced into the space.

In the case that a method of the present invention is carried out by the use of this apparatus, at the start of a film growth, the screening tube 14 is placed at a position shown in Fig. 3(b), and when it is required that a growth temperature is promptly raised, the screening tube 14 is moved to a position shown in Fig. 3(a). By repeating the above movement of the screening tube 14 between the positions shown in Fig. 3(b) and Fig. 3(a) as needed, the prompt change .of the film growth temperature can also be repeated.

Example 3 An example of a back surface heater type ultra-high vacuum CVD apparatus is shown in Fig. 4.

A movable thermal-radiation-screening plate 15 made of tantalum was disposed between a heater 10 and a substrate 12 was arranged.

In the case that a method of the present invention is carried out by the use of this apparatus, at the start of a film growth, the screening plate 15 is placed at a position shown in Fig. 4(b), and when it is required that a growth temperature is promptly raised, the screening plate 15 is moved to a position shown in Fig. 4(a). As in the case of Example 2, by repeating the above movement of the screening plate 15 between the positions shown in Fig. 4(b) and Fig. 4(a), the prompt change of the film growth temperature can also be repeated.

In the conventional reaction tube type vacuum CVD apparatus shown in Fig. 5, several tens of minutes are taken for the rise and drop of the growth temperature, but according to the above-mentioned CVD apparatus the grown temperature can be changed in a time of one minute or less. In consequence, for example, there can be grown a poly-Si film having a thickness of 150 nm and comprising a lower layer having a structure shown in Fig. 1 and an upper layer having a structure other than that shown in Fig. 1. Furthermore, for example, a poly-Si film having a thickness of 150 nm, wherein a formation of a poly-Si film having the structure of Fig. 1 and having a thickness of 50 run is repeated three times by the above-described growth method, can be obtained by using the above-mentioned CVD apparatuses.

As described above, in the manufacture of a semiconductor device, a poly-Si film can be obtained in which the optimization of many factors such as work function, diffusion properties and the roughness of a film surface and interface is realized.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

The description of the invention with reference to the drawings is by way of example only.

The text of the abstract filed herewith is repeated here as part of the specification.

A growth method of a polycrystalline silicon film comprises the steps of raising a film formation temperature up to a predetermined temperature higher than a start temperature after the lapse of a predetermined time from a growth start point, or the step of repeating the rise of the film formation temperature from a low temperature, while the film growth is continued, thereby growing the film to a desired film thickness. A CVD apparatus is equipped with a movable thermal-radiation- screening plate or tube which can be inserted between a heater as a heat source and the substrate.

The growth method can permit the optimization of many factors such as work function, diffusion properties and the roughness of a film surface and interface, and the CVD apparatus can provide effective and prompt practice of this growth method.

Claims

CLAIMS:

1. A method of growing a polycrystalline silicon film on a substrate of an amorphous insulating film using chemical vapour deposition, said method comprising the step of raising a film formation temperature during growth after the lapse of a predetermined time period in order to grow the film to a predetermined film thickness.

2. A method according to Claim 1, comprising the steps of lowering and raising again the film formation temperature during growth.

3. A method according to Claim 1 or 2, wherein said film formation temperature is raised to a predetermined temperature.

4. A method according to any preceding claim wherein film growth is conducted at a first growth temperature to deposit amorphous silicon or polycrystalline silicon to a thickness of 1 nm or more, the growth temperature then being immediately or gradually increased to a second temperature to deposit polycrystalline silicon to attain said predetermined film thickness.

5. A method according to Claim 4 wherein when the film growth at the second temperature has reached the predetermined film thickness, the growth temperature is immediately or gradually decreased to a third temperature to deposit amorphous silicon or polycrystalline silicon, the film growth at the third temperature being conducted until the film attains a second predetermined thickness, the growth temperature then being raised to the second temperature and film growth conducted until the film attains a third predetermined film thickness.

6. A method according to Claim 5 wherein the growth temperature is repeatedly increased and decreased to attain the desired film structure.

7. A method according to any preceding claim, wherein during film growth, a flow rate of a source gas for film formation is changed after the lapse of said predetermined time period.

8. Apparatus for depositing a thin film on a substrate using chemical vapour deposition, said apparatus being equipped with a movable thermal-radiation screening means insertable between a heat source and the substrate.

9. Apparatus according to Claim 8, wherein the screening means comprises a screening plate or tube.

10. A method according to any of Claims 1 to 7 conducted using the apparatus according to Claim 8 or 9, said screening means being moveable to raise or lower the film formation temperature.

11. A method of growing a polycrystalline silicon film substantially as herein described.

12. Apparatus for depositing a thin film on a substrate substantially as herein described with reference to Figure 3 or Figure 4 of the accompanying drawings.