Batch CVD method and apparatus for semiconductor process

A batch CVD method repeats a cycle including adsorption and reaction steps along with a step of removing residual gas. The adsorption step is preformed while supplying the source gas into the process container by first setting the source gas valve open for a first period and then setting the source gas valve closed, without supplying the reactive gas into the process container by keeping the reactive gas valve closed, and without exhausting gas from inside the process container by keeping the exhaust valve closed. The reaction step is performed without supplying the source gas into the process container by keeping the source gas valve closed, while supplying the reactive gas into the process container by setting the reactive gas valve open, and exhausting gas from inside the process container by setting the exhaust valve to gradually decrease its valve opening degree from a predetermined open state.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims the benefit of priority from Japanese Patent Application No. 2009-171557, filed on Jul. 22, 2009 in the Japan Patent Office, the disclosure of which is incorporated herein in the entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a batch CVD (chemical vapor deposition) method and apparatus, and particularly to a semiconductor processing technique for forming a product film on target objects, such as semiconductor wafers. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target object, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target object.

2. Description of the Related Art

In manufacturing semiconductor devices for constituting semiconductor integrated circuits, a target object, such as a semiconductor wafer, is subjected to various processes, such as film formation, etching, oxidation, diffusion, and reformation. Film formation processes of this kind are performed in film formation apparatuses of the single-substrate type, such as an apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 09-077593, and film formation apparatuses of the batch type, such as an apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-006801.

FIG. 5is a structural view schematically showing a conventional batch CVD apparatus. For example, where a silicon oxide film is formed, a wafer boat4holding target objects or semiconductor wafers W at intervals in the vertical direction is loaded into a vertical process container2. The wafers W are heated to a predetermined temperature, such as about 600° C., by a heater6disposed around the process container2. A silicon source gas and a reactive gas, such as ozone, are supplied from a gas supply system8. These gases are delivered into the process container2from a number of gas spouting holes8A and10A formed on distribution nozzles8and10vertically extending inside the process container2. Further, the inner space of the process container2is vacuum-exhausted by a vacuum exhaust system14including a vacuum pump16through an exhaust port12formed at the lower side of the process container2. Under these conditions, a process for forming a silicon oxide film is performed inside the process container2with a predetermined pressure kept therein.

In recent years, owing to the demands of increased miniaturization and integration of semiconductor integrated circuits, it is required to alleviate the thermal history of semiconductor devices in manufacturing steps, thereby improving the characteristics of the devices. For vertical processing apparatuses, it is also required to improve semiconductor processing methods in accordance with the demands described above. For example, there is a CVD method for a film formation process, which performs film formation while intermittently supplying a source gas and so forth to repeatedly form layers each having an atomic or molecular level thickness, one by one, or several by several. In general, this film formation process is called ALD (Atomic layer Deposition) or MLD (Molecular Layer Deposition), which allows a predetermined process to be performed without exposing wafers to a very high temperature.

Where a silicon oxide film is formed by ALD or MLD using the apparatus shown inFIG. 5, operations are performed as follows. Specifically, a switching valve8B for a silicon source gas and a switching valve10B for ozone serving as an oxidizing gas are operated to alternately supply the silicon source gas and oxidizing gas. Further, the exhaust valve14B of the vacuum exhaust system14is operated to adjust its valve opening degree to control the pressure inside process container2.

FIG. 6is a graph showing the relationship between the valve states and the pressure inside the process container where a silicon oxide film is formed by ALD using the apparatus shown inFIG. 5.FIG. 6, (A), shows the state of the source gas switching valve8B,FIG. 6, (B), shows the state of the reactive gas switching valve10B,FIG. 6, (C), shows the state (valve opening degree) of the exhaust valve14B of the vacuum exhaust system, andFIG. 6, (D), shows the pressure inside the process container.

According to the method shown inFIG. 6, a cycle comprising an adsorption step T11, an exhaust step T12, a reaction step T13, and an exhaust step T14in this order is repeated a plurality of times. In the adsorption step T11, as shown inFIG. 6, (A), the source gas switching valve8B is set open to supply the silicon source gas, so that this gas is adsorbed on the surface of the wafers W. In the reaction step T13, as shown inFIG. 6, (B), the reactive gas switching valve10B is set open to supply the reactive gas or ozone, so that the ozone reacts with the source gas adsorbed on the surface of the wafers, thereby forming a thin SiO2film. In the exhaust steps T12and T14, the exhaust valve14B is set open to exhaust gas from inside the process container2, without supplying either of the source gas and reactive gas.

By performing one cycle, a thin film having an atomic or molecular level thickness is formed. Thin films formed by respective times in repetition of the cycle are laminated so that a product film having a predetermined thickness is formed. In one cycle, the time length of each of the adsorption step T11and reaction step T13is about 60 seconds, and the time length of each of the exhaust steps T12and T14is about 10 seconds. This batch CVD method allows the process to be performed without exposing wafers to a very high temperature. However, as described later, the present inventors have found that batch CVD methods of this kind have room for improvement in terms of some characteristics thereof concerning the film quality, throughput, and source gas consumption.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a batch CVD method and apparatus having improved characteristics concerning the film quality, throughput, and source gas consumption.

According to a first aspect of the present invention, there is provided a batch CVD (chemical vapor deposition) method for a semiconductor process in a batch CVD apparatus, the apparatus comprising a vertically long process container configured to accommodate a plurality of target objects, a holder configured to support the target objects at intervals in a vertical direction inside the process container, a source gas supply system configured to supply a source gas into the process container, the source gas supply system including a source gas valve for adjusting supply of the source gas, a reactive gas supply system configured to supply a reactive gas into the process container, the reactive gas supply system including a reactive gas valve for adjusting supply of the reactive gas, and an exhaust system configured to exhaust gas from inside the process container, the exhaust system including an exhaust valve for adjusting an exhaust rate, the method being preset to repeat a cycle a plurality of times to laminate thin films formed by respective times and thereby to form a product film having a predetermined thickness on the target objects, the cycle comprising: an adsorption step of adsorbing the source gas onto the target objects, while supplying the source gas into the process container by first setting the source gas valve open for a first period and then setting the source gas valve closed, without supplying the reactive gas into the process container by keeping the reactive gas valve closed, and without exhausting gas from inside the process container by keeping the exhaust valve closed; then, a first intermediate step of removing residual gas from inside the process container, without supplying either of the source gas and the reactive gas into the process container by keeping both of the source gas valve and the reactive gas valve closed, while exhausting gas from inside the process container by setting the exhaust valve open; then, a reaction step of causing the reactive gas to react with the source gas adsorbed on the target objects, without supplying the source gas into the process container by keeping the source gas valve closed, while supplying the reactive gas into the process container by setting the reactive gas valve open, and exhausting gas from inside the process container by setting the exhaust valve to gradually decrease its valve opening degree from a predetermined open state; and then, a second intermediate step of removing residual gas from inside the process container, without supplying either of the source gas and the reactive gas into the process container by keeping both of the source gas valve and the reactive gas valve closed, while exhausting gas from inside the process container by setting the exhaust valve to have a valve opening degree larger than that at end of the reaction step.

According to a second aspect of the present invention, there is provided a computer readable storage medium containing program instructions for execution on a processor, which, when executed by the processor, control a batch CVD apparatus to perform a method according to the first aspect.

According to a third aspect of the present invention, there is provided a batch CVD (chemical vapor deposition) apparatus for a semiconductor process, the apparatus comprising: a vertically long process container configured to accommodate a plurality of target objects; a holder configured to support the target objects at intervals in a vertical direction inside the process container; a source gas supply system configured to supply a source gas into the process container, the source gas supply system including a source gas valve for adjusting supply of the source gas; a reactive gas supply system configured to supply a reactive gas into the process container, the reactive gas supply system including a reactive gas valve for adjusting supply of the reactive gas; an exhaust system configured to exhaust gas from inside the process container, the exhaust system including an exhaust valve for adjusting an exhaust rate; and a control section configured to control an operation of the apparatus, wherein the control section is preset to execute a batch CVD method, which repeats a cycle a plurality of times to laminate thin films formed by respective times and thereby to form a product film having a predetermined thickness on the target objects, the cycle comprising an adsorption step of adsorbing the source gas onto the target objects, while supplying the source gas into the process container by first setting the source gas valve open for a first period and then setting the source gas valve closed, without supplying the reactive gas into the process container by keeping the reactive gas valve closed, and without exhausting gas from inside the process container by keeping the exhaust valve closed, then, a first intermediate step of removing residual gas from inside the process container, without supplying either of the source gas and the reactive gas into the process container by keeping both of the source gas valve and the reactive gas valve closed, while exhausting gas from inside the process container by setting the exhaust valve open, then, a reaction step of causing the reactive gas to react with the source gas adsorbed on the target objects, without supplying the source gas into the process container by keeping the source gas valve closed, while supplying the reactive gas into the process container by setting the reactive gas valve open, and exhausting gas from inside the process container by setting the exhaust valve to gradually decrease its valve opening degree from a predetermined open state, and then, a second intermediate step of removing residual gas from inside the process container, without supplying either of the source gas and the reactive gas into the process container by keeping both of the source gas valve and the reactive gas valve closed, while exhausting gas from inside the process container by setting the exhaust valve to have a valve opening degree larger than that at end of the reaction step.

DETAILED DESCRIPTION OF THE INVENTION

In the process of developing the present invention, the inventors studied problems with regard to conventional batch CVD methods and apparatuses for a semiconductor process of this kind. As a result, the inventors have arrived at the findings given below.

The method described above with reference toFIGS. 5 and 6is arranged in accordance with a conventional concept such that, when a silicon source gas is supplied in the adsorption step T11, it is preferable to form flows of the source gas on or above the surface of the wafers because the amount of source gas to be adsorbed on the surface of wafers is thereby increased. For this reason, in the adsorption step T11, as shown inFIG. 6, (C), the exhaust valve14B of the vacuum exhaust system14is set open at some level, such as a valve opening degree of about 30%, so as to exhaust gas to a certain degree to form flows of the source gas on or above the surface of the wafers. In this case, since the source gas is set at a flow rate of about 10 to 500 sccm (standard cubic centimeter per minutes), the pressure inside the process container2is gradually and linearly increased, as shown inFIG. 6, (D).

On the other hand, in the reaction step T13, the exhaust valve14B is set more open than the adsorption step T11described above, such as a valve opening degree of about 60%, so as to efficiently cause the reaction. In this case, since the ozone is set at a flow rate of about 20 slm (standard liter per minute), which is far larger than the flow rate of the source gas, the pressure inside the process container2is first increased in an instant and saturated for a while by the ozone supply, and is then gradually decreased. Further, in the exhaust steps T12and T14, the exhaust valve14B is set at a valve opening degree of 100% to perform forcible vacuum-exhaust, so as to swiftly remove residual gas from inside the process container2. The vacuum pump16is kept running through all the steps.

However, according to the conventional method described above, the exhaust valve14B is set open to a certain degree to keep the silicon source gas flowing during the adsorption step T11. Consequently, the source gas is consumed by a large amount, and the running cost becomes significantly higher particularly where the source gas is expensive.

Further, in the reaction step T13, the pressure inside the process container2is first increased by the ozone supply, to a point P1shown inFIG. 6, (D), which is excessively high for the ozone such that the ozone can be easily deactivated while losing its activity. Consequently, the film formation proceeds sufficiently on the peripheral part of the wafers, but proceeds insufficiently on the central part of the wafers, due to inadequate supply of the ozone, thereby deteriorating the planar uniformity of the film thickness. Particularly, where the surface of the wafers includes a trench structure, this brings about a prominent loading effect that drastically increases the ozone consumption while making the film thickness very small on the wafer central part. In this respect, there may be a countermeasure of setting the pressure inside the process container2much lower in the reaction step T13, but this countermeasure is undesirable because the deposition rate drops to a great extent along with a decrease in the pressure, thereby lowering the throughput.

An embodiment of the present invention achieved on the basis of the findings given above will now be described with reference to the accompanying drawings. In the following description, the constituent elements having substantially the same function and arrangement are denoted by the same reference numerals, and a repetitive description will be made only when necessary.

FIG. 1is a structural view schematically showing a batch CVD apparatus according to an embodiment of the present invention. This apparatus is designed to form a thin film or silicon oxide film (SiO2) by use of a source gas comprising 3DMAS (trisdimethylamino silane), which is a Si-containing organic material, and a reactive gas comprising ozone, which is an oxidizing gas, (practically a gas mixture of O3and O2).

As shown inFIG. 1, this film formation apparatus20includes a process container22, in which a process field23is defined to accommodate and process a plurality of semiconductor wafers (target objects) stacked at intervals in the vertical direction. The process container22has a double tube structure comprising an inner tube24and an outer tube26, both of which are vertically long tubes. The inner tube24is shaped as a cylindrical column with a ceiling and an opened bottom, in which the process field23is defined. The outer tube26is shaped as a cylindrical column with a ceiling and an opened bottom and surrounds the inner tube24with a predetermined gap interposed therebetween. The inner tube24and outer tube26are made of quartz.

The process container22has a diameter of, e.g., about 400 to 500 mm for accommodating wafers W having a diameter of 300 mm. The inner volume of the process container22depends on the number of wafers W to be loaded therein, and is set at, e.g., about 200 liters for accommodating 150 wafers W at most.

The bottom of the outer tube26is airtightly connected through a seal member30, such as an O-ring, to a cylindrical manifold28made of, e.g., stainless steel. The bottom of the outer tube26is supported by the manifold28, while the manifold28is supported by a base plate (not shown). The manifold28includes a ring support32extending from its inner surface, on which the bottom of the inner tube24is mounted. A holder or wafer boat34made of quartz is placed inside the inner tube24of the process container22.

The wafer boat34is moved up and down through the bottom port of the manifold28, so that the wafer boat34is loaded/unloaded into and from the process field23of the process container22. A number of target objects or semiconductor wafers W are stacked on the wafer boat34. For example, in this embodiment, the wafer boat34can support, e.g., about 50 to 100 wafers having a diameter of 300 mm at essentially regular intervals in the vertical direction.

The wafer boat34is placed on a table42through a heat-insulating cylinder40made of quartz. The table42is supported by a rotary shaft44, which penetrates a lid36made of, e.g., quartz or stainless steel, and used for opening/closing the bottom port of the manifold28. The portion of the lid36where the rotary shaft44penetrates is provided with, e.g., a magnetic-fluid seal46, so that the rotary shaft44is rotatably supported in an airtightly sealed state. A seal member38, such as an O-ring, is interposed between the periphery of the lid36and the bottom of the manifold28, so that the interior of the process container22can be kept sealed.

The rotary shaft44is attached at the distal end of an arm50supported by an elevating mechanism48, such as a boat elevator. The elevating mechanism48moves the wafer boat34and lid36up and down together. Further, when the rotary shaft44is driven by a rotating mechanism built in the arm50, the wafer boat34is rotated, and so the wafers on the wafer boat34are also rotated about a vertical axis extending through their center. However, the table42may be fixed to the lid36, so that wafers W are processed without rotation of the wafer boat34.

A heat insulating casing51is disposed around the process container22. The heat insulating casing51is provided with a heater52on the inner surface for heating the atmosphere and wafers W inside the process container22. For example, the heater52is formed of a carbon wire, which causes no contamination and has good characteristics for increasing and decreasing the temperature. A thermocouple (not shown) is disposed inside the inner tube24of the process container22to control the heater52.

A gas supply section is connected to the side of the manifold28to supply predetermined process gases to the process field23of the process container22. Specifically, the gas supply section includes a source gas supply circuit54, a reactive gas supply circuit56, and a purge gas supply circuit58. The source gas supply circuit54is arranged to supply 3DMAS gas as a silicon source gas. The reactive gas supply circuit56is arranged to supply ozone gas (practically a gas mixture of O3and O2) as a reactive gas or oxidizing gas. The purge gas supply circuit58is arranged to supply an inactive gas, such as N2gas, as a purge gas. Each of the source and reactive gases is mixed with a suitable amount of carrier gas, such as N2gas, as needed. However, such a carrier gas will be mentioned only when necessary, hereinafter, for the sake of simplicity of explanation. The purge gas or carrier gas may be a rare gas, such as Ar or He, in place of N2gas.

More specifically, the source gas supply circuit54, reactive gas supply circuit56, and purge gas supply circuit58include gas distribution nozzles60,64, and68, respectively, each of which is formed of a quartz pipe which penetrates the sidewall of the manifold28from the outside and then turns and extends upward inside the inner tube24(seeFIG. 1). The gas distribution nozzles60,64, and68respectively have a plurality of gas spouting holes60A,64A, and68A, each set of holes being formed at predetermined intervals in the longitudinal direction (the vertical direction) over all the wafers W on the wafer boat34. Each set of the gas spouting holes60A,64A, and68A deliver the corresponding gas almost uniformly in the horizontal direction, so as to form gas flows parallel with the wafers W on the wafer boat34, as needed.

The nozzles60,64, and68are connected to gas sources54S,56S, and58S of 3DMAS gas, O3gas, and N2gas, respectively, through gas supply lines (gas passages)62,66, and70, respectively. The gas supply lines62,66, and70are provided with flow rate controllers62A,66A, and70A, such as mass flow controllers, and switching valves62B,66B, and70B, respectively. With this arrangement, 3DMAS gas, O3gas, and N2gas can be supplied at controlled flow rates and selectively supplied and stopped.

The nozzles60,64, and68are gathered on one side inside the inner tube24(thoughFIG. 1shows the nozzle68as being disposed on the opposite side to the other nozzles60and64due to limitations of drawing space). The sidewall of the inner tube24has a plurality of gas through holes72having a large diameter, which are formed therein on the opposite side to the nozzles60,64, and68and are arrayed in the vertical direction. Each of the gases flowing between the wafers in the horizontal direction passes through the gas through holes72into the gap74between the inner tube24and outer tube26.

The manifold28has an exhaust port76formed therein and communicating with the gap74between the inner tube24and outer tube26. The exhaust port76is connected to a vacuum exhaust system78configured to vacuum-exhaust the interior of the process container22. The vacuum exhaust system78has an exhaust line80connected to the exhaust port76, which is provided with a vacuum pump82for vacuum-exhausting gas from inside the process container22and an exhaust valve80B for adjusting the pressure inside the process container22by changing the valve opening degree. The exhaust valve80B is configured not only to adjust the valve opening degree to an arbitrary vale but also to switch between the totally open state and the totally closed state to completely shut off the exhaust.

The film formation apparatus20further includes a main control section84formed of, e.g., a computer, to control the entire apparatus. The main control section84can conduct a batch CVD process as described below in accordance with process recipes stored in the storage section86thereof in advance, with reference to the film thickness and composition of a film to be formed. In the storage section86, the relationship between the process gas flow rates and the thickness and composition of the film is also stored as control data in advance. Accordingly, the main control section84can control the elevating mechanism48, gas supply circuits54,56, and58, exhaust system78, heater52, and so forth, based on the stored process recipes and control data. Examples of a storage medium are a magnetic disk (flexible disk, hard disk (a representative of which is a hard disk included in the storage section86), etc.), an optical disk (CD, DVD, etc.), a magneto-optical disk (MO, etc.), and a semiconductor memory.

Next, an explanation will be given of a batch CVD method (so called ALD or MLD film formation) performed in the apparatus shown inFIG. 1. In this batch CVD method, a silicon oxide film is formed on semiconductor wafers by ALD or MLD. In order to achieve this, 3DMAS gas serving as a silicon source gas and ozone (O3) gas serving as an oxidizing reactive gas are selectively supplied to the process field23accommodating wafers W. This selective supply is used to alternately repeat a plurality of times an adsorption step of adsorbing 3DMAS gas on the surface of the wafers W and a reaction step of causing ozone gas to react with 3DMAS gas adsorbed on the surface of the wafers W to form a thin film of silicon oxide. Specifically, a film formation process is performed along with the following operations.

When the film formation apparatus20is set in standby with no semiconductor wafers W loaded therein, the process field23is maintained at a temperature lower than the process temperature. When the process is started, at first, the wafer boat34set at room temperature, which supports a number of, such as 50, semiconductor wafers W, is loaded from below into the process field23set at a predetermined temperature (the process container22is in a hot wall state). Further, the bottom port of the process container22is closed by the lid36to airtightly seal the process container22.

Then, the process field23is vacuum-exhausted to set the process field23at a predetermined process pressure. Further, the power applied to the heater52is increased to heat the process field23to the process temperature for the film formation process. After the process field23is stabilized at the process pressure and process temperature, predetermined process gases necessary for film formation are supplied to the process field23. In this embodiment, 3DMAS gas and ozone gas are supplied at controlled flow rates respectively from the nozzles60and64of the gas supply circuits54and56to the process field23. Further, the valve opening degree of the exhaust valve80B of the vacuum exhaust system78is adjusted to control the pressure inside the process container22.

The 3DMAS gas is supplied from the gas spouting holes60A of the gas distribution nozzle60to form gas flows parallel with the wafers W on the wafer boat34. While being supplied, the 3DMAS gas is activated by the heating temperature to the process field23, and molecules of the 3DMAS gas and molecules and atoms of decomposition products generated by decomposition are adsorbed on the wafers W.

On the other hand, the ozone gas is supplied from the gas spouting holes64A of the gas distribution nozzle64to form gas flows parallel with the wafers W on the wafer boat34. While being supplied, the ozone gas is activated by the heating temperature to the process field23, and molecules of the ozone gas and molecules and atoms of decomposition products generated by decomposition react with decomposition products and so forth derived from the 3DMAS gas and adsorbed on the wafers W. Consequently, a thin film of silicon oxide is formed on the wafers W. Alternatively, when decomposition products and so forth derived from the 3DMAS gas flow onto decomposition products and so forth derived from the ozone gas and adsorbed on the wafers W, the same reaction is caused, so a thin film of silicon oxide is formed on the wafers W.

The exhaust steps are performed between the adsorption step and reaction step described above by exhausting gas from inside the process container2without supplying either of the 3DMAS gas and ozone gas. Further, in each of the exhaust steps, N2gas serving as a purge gas is supplied at a controlled flow rate from the nozzle68of the gas supply system58, as needed. Gas components flowing through the process field23passes through the gas through holes72into the gap74between the inner tube24and outer tube26. Then, these gas components are exhausted by the vacuum exhaust system78through the exhaust port76located at the bottom of the outer tube26.

FIG. 2is a graph showing the relationship between the valve states and the pressure inside the process container where a silicon oxide film is formed by ALD using the apparatus shown inFIG. 1.FIG. 2, (A), shows the state of the switching valve62B for the source gas,FIG. 2, (B), shows the state of the switching valve66B for the reactive gas,FIG. 2, (C), shows the state of the exhaust valve80B of the vacuum exhaust system (valve opening degree), andFIG. 2, (D), shows the pressure inside the process container.

As shown inFIG. 2, the film formation method according to this embodiment is arranged such that a cycle comprising an adsorption step T1, an exhaust step T3, a reaction step T2, and an exhaust step T4in this order is repeated a plurality of times. The adsorption step T1is arranged to set the switching valve62B for the source gas open, as shown inFIG. 2, (A), to perform supply of the 3DMAS gas to adsorb decomposition products and so forth derived from this gas onto the surface of the wafers W. The reaction step T3is arranged to set the switching valve66B for the reactive gas open, as shown inFIG. 2, (B), to perform supply of the ozone gas to cause this gas to react with decomposition products and so forth adsorbed on the surface of the wafers, so as to form a thin SiO2film. Each of the exhaust steps T2and T4is arranged to exhaust gas from inside the process container2without supplying either of the 3DMAS gas and ozone gas. The vacuum pump82is kept running through all the steps T1to T4.

A thin film having an atomic or molecular level thickness is formed by one cycle described above. Thin films formed by respective times of repetition of the cycle are laminated so that a product film having a predetermined thickness is formed. In one cycle, the time length of each of the adsorption step T1and reaction step T3is 2 to 120 seconds, such as about 60 seconds. The time length of each of the exhaust steps T2and T4is 2 to 20 seconds, such as about 10 seconds. The exhaust steps T2and T4may be excluded to improve the throughput of the film formation process.

More specifically, the adsorption step T1is arranged, as shown inFIG. 2, (A), such that the switching valve62B for the source gas is first set open for a predetermined period t1and is then set closed, so that the source gas is supplied only for the predetermined period t1. At this time, as shown inFIG. 2, (C), the exhaust valve80B of the vacuum exhaust system is kept closed throughout the entirety of the adsorption step T1. This adsorption step T1differs from the adsorption step T11of the conventional method, in which the exhaust valve14B is kept open at a predetermined valve opening degree to vacuum-exhaust gas, as shown inFIG. 6, (C). Consequently, in this embodiment, the adsorption step T1proceeds such that the source gas (3DMAS gas) is confined inside the process container22not to flow out, while the silicon source gas is adsorbed onto the surface of the wafers W. In this case, even if the flow rate of the source gas is low, adsorption of the source gas onto the surface of the wafers is promoted.

The time length of the predetermined period t1described above is set to be 1 to 50%, and preferably of 5 to 30%, of the time length of the adsorption step T1(for example, t1=2 to 60 seconds within the range relative to T1). If the predetermined period t1is larger than 50%, the effect of reducing the source gas consumption becomes undesirably smaller. Further, the source gas is wasted because adsorption of the source gas onto the surface of the wafers is saturated. The flow rate of the source gas at this time is set to be about 10 to 500 sccm. In the adsorption step T1, the pressure inside the process container22is first quickly increased, and then becomes constant when the switching valve62B for the source gas is closed. The pressure at this time is about 667 Pa, for example, though it depends on the flow rate of the source gas.

Then, after the adsorption step T1is finished, the first exhaust step T2starts. In this step, without supplying any of the gases, the exhaust valve80B of the vacuum exhaust system is set totally open to quickly exhaust residual gas from inside the process container22. However, at this time, N2gas serving as a purge gas may be supplied to promote removal of the residual gas. In the exhaust step T2, the pressure inside the process container22is quickly decreased (FIG. 2, (D)).

Then, the reaction step T3starts. In this step, as shown inFIG. 2, (B), the switching valve66B for the reactive gas is set open to supply the reactive gas or ozone throughout the entirety of the reaction step T3. At this time, as shown inFIG. 2, (C), the exhaust valve80B of the vacuum exhaust system is first set open and then is set to gradually decrease its valve opening degree with progress of the reaction step T3. This reaction step T3differs from the reaction step T13of the conventional method, in which the exhaust valve14B of the vacuum exhaust system is kept open at a valve opening degree of about 60%, as shown inFIG. 6, (C).

Consequently, the reactive gas or ozone gas reacts with the source gas or silicon source gas adsorbed on the surface of the wafers, and a thin film or silicon oxide film is thereby formed. In general, the ozone gas described above is generated from oxygen gas in an ozonizer, wherein O3gas is contained in O2gas by about 10 vol %, and such a gas mixture is practically used as an ozone-containing gas. In the reaction step T3, the exhaust valve80B of the vacuum exhaust system is first kept totally open for a first predetermined period t3from the exhaust step T2, and is then set to decrease its valve opening degree instantaneously to a predetermined level, such as 50%, and to further decrease its valve opening degree gradually and linearly to a predetermined level, such as 20%. In this case, the flow rate of the reactive gas or ozone is set at about 20 slm, which is far larger than the flow rate of the source gas described above.

In the case of the conventional method, as indicated by the point P1shown inFIG. 6, (D), a quick pressure increase appears and brings about a prominent loading effect. On the other hand, the reaction step T3of the method according to this embodiment is arranged such that the exhaust valve80B of the vacuum exhaust system is set totally open only for the first predetermined period t3, and is then set to decrease its valve opening degree instantaneously to about 50% and to further decrease it gradually. Consequently, the pressure inside the process container22is prevented from rendering a quick increase, and is almost linearly and gradually increased, as shown inFIG. 2, (D). The maximum pressure during the reaction step is about 133 Pa (1 Torr), for example.

As described above, the reaction step T3is arranged such that the exhaust valve80B of the vacuum exhaust system is first set totally open, and is then set to gradually decrease its valve opening degree. In this case, the pressure is increased without causing a prominent loading effect, because it is possible to prevent a quick pressure increase, such as that indicated by the point P1, which brings about such a loading effect. Consequently, the reaction of the reactive gas or ozone with the silicon source gas adsorbed on the surface of the wafers is promoted. In other words, since there is no high pressure state, the ozone is prevented from being deactivated and prolongs its service life. Thus, the loading effect is suppressed and the deposition rate is kept high. Further, the planar uniformity of the film thickness on the surface of the wafers is kept high, and the throughput is improved.

The time length of the predetermined period t3described above is set to be 1 to 50%, and preferably of 5 to 30%, of the time length of the reaction step T3(for example, t3=2 to 60 seconds within the range relative to T3). If the predetermined period t3is larger than 50%, an increase in the pressure inside the process container22is suppressed too much, and so the ozone reaction is deterred and the deposition rate becomes undesirably lower. It should be noted that the valve opening degrees 50% and 20% appearing halfway in the operation of the exhaust valve80B of the vacuum exhaust system (FIG. 2, (C)) are mere examples. The optimum value of the valve opening degree is determined in light of the reaction rate of the ozone and silicon source gas. However, in the reaction step T3, the valve opening degree of the exhaust valve80B is preferably set to have a maximum value (such as 100%) the same as the valve opening degree of the exhaust valve80B used in the exhaust step T2, and a minimum value of 2% or more.

Then, after the reaction step T3is finished, the second exhaust step T4starts. In this step, without supplying any of the gases, the exhaust valve80B of the vacuum exhaust system is set totally open to quickly exhaust residual gas from inside the process container22. However, at this time, N2gas serving as a purge gas may be supplied to promote removal of the residual gas. In the exhaust step T2, the pressure inside the process container22is quickly decreased (FIG. 2, (D)). By doing so, one cycle of the film formation is finished. This cycle is repeated a plurality of times to form a thin film or silicon oxide film having a necessary film thickness.

In the embodiment described above, the valve opening degree80B of the vacuum valve includes the totally open state (valve opening degree=100%) and the totally closed state (valve opening degree=0%). However, in the exhaust valve80B practically used, where the valve opening degree is 90% or more, the exhaust conductance for the process container22is not changed so much but is almost the same as that obtained by the totally open state. Further, where the valve opening degree is 2% or less, the exhaust conductance for the process container22is not changed so much but is almost the same as that obtained by the totally closed state. Accordingly, in the embodiment described above, “the totally open state” of the exhaust valve80B can be replaced with “a valve opening degree of 90 to 100%”. Further “the totally closed state” of the exhaust valve80B can be replaced with “a valve opening degree of 0 to 2%”.

Further, in the embodiment described above, as shown inFIG. 2, (C), the exhaust valve80B is set totally open only for the predetermined period t3in the reaction step T3.FIGS. 3A and 3Bare views showing two modifications of the reaction step, in terms of a change in the valve opening degree of the exhaust valve of the vacuum exhaust system. According to the modification shown inFIG. 3A, the valve opening degree is linearly changed from 100% to 50% in the predetermined period t3. According to the modification shown inFIG. 3B, the valve opening degree is linearly changed from 100% to 20% over the entire period of the reaction step. These modifications concerning a change in the valve opening degree are also mere examples.

Further, the valve opening degree may be changed along a curved or stepped line, such that the degree is changed stepwise a plurality of times, for example. One of the important points resides in that the valve opening degree of the exhaust valve80B is set large at the beginning of the reaction step T3, so that the pressure inside the process container22is prevented from increasing at the beginning so as not to cause the loading effect. If necessary, the reaction step T3may include supplying an inactive gas, such as N2gas or a rare gas, as a carrier gas together with the reactive gas.

As described above, according to the embodiment of the present invention, by use of the film formation apparatus20including the process container22configured to accommodate a plurality of target objects, such as semiconductor wafers W, the adsorption step and reaction step are alternately repeated with an intermediate period (the exhaust period) interposed therebetween to form a thin film on the target objects. In the adsorption step, while the exhaust valve80B of the vacuum exhaust system78is kept closed, the switching valve62B of the source gas supply system54is first set open for a predetermined period and then immediately set closed. By doing so, a source gas, such as 3DMAS gas serving as a silicon source gas, is temporarily supplied into the process container22to adsorb the source gas onto the target objects. In the reaction step, while the switching valve66B of the reactive gas supply system56is kept open to supply the reactive gas into the process container22, the exhaust valve80B of the vacuum exhaust system78is first set open and is then set to gradually decrease its valve opening degree, so as to cause the reactive gas, such as ozone, react with the source gas. Consequently, it is possible to remarkably reduce the source gas consumption while maintaining the film quality at a high level and preventing the throughput from being lowered.

In order to assess the batch CVD method according to the embodiment described above, an experiment was performed as follows. In a present example PE, a silicon oxide film was formed on wafers in the apparatus shown inFIG. 1in accordance with the method shown inFIG. 2. In the present example PE, 3DMAS gas was used as the silicon source gas, and ozone gas (gas mixture of oxygen with 10 vol % ozone) was used as the reactive gas. The process temperature was set at 550° C., and the process pressure (the maximum value) was set at 1.2 kPa. The time length of the adsorption step T1was set at different values, but the predetermined period t1to set the switching valve62B for the source gas open was fixed at 7 seconds. On the other hand, the time length of the reaction step T3was fixed at 7 seconds, and the predetermined period t3to set the exhaust valve80B of the vacuum exhaust system totally open was fixed at 2 seconds. The time length of the adsorption step T1was used as a parameter to change the time to set the switching valve62B for the source gas closed in the adsorption step T1, i.e., the holding time H(=T1−t1).

Further, in a comparative example CE, a silicon oxide film was formed on wafers in the apparatus shown inFIG. 1in accordance with the method shown inFIG. 6, under conditions the same as those of the present example PE except for the flow rate of 3DMAS gas and the operations of the valves. Specifically, in the comparative example CE, the flow rate of 3DMAS gas was set at 4 times lager than that of the present example PE, and the time length of the adsorption step was set at 30 seconds.

FIG. 4is a graph showing results of this experiment. InFIG. 4, the horizontal axis denotes the holding time, and the vertical axis denotes the deposition rate per cycle. As shown inFIG. 4, the comparative example CE, in which the 3DMAS gas was supplied at X grams, rendered a deposition rate of about 0.13 nm/cycle. On the other hand, the present example PE rendered a deposition rate linearly improved with an increase in the holding time, even though the 3DMAS gas was supplied at a smaller value of X/4 grams. In this case, when the holding time was about 40 seconds, the deposition rate was almost the same as that of the comparative example CE. Accordingly, it was confirmed that, even though the source gas flow rate was decreased to ¼, the deposition rate was obtained at a value almost equal to or higher than that of the comparative example CE by setting the holding time at 40 seconds or more. In other words, it is possible to remarkably reduce the flow rate of the source gas while maintaining the deposition rate at a value almost equal to that of the comparative example CE, by operating the respective valves as in the method according to the embodiment of the present invention.

Further, the present example PE and comparative example CE described above were examined in terms of their loading effect, using patterned wafers. The term “a patterned wafer” means “a wafer provided with projections and recesses on the surface by forming part of circuit patterns”. Since the gas consumption of patterned wafers is larger than flat wafers, the loading effect can prominently appear. As a result of this examination, the comparative example CE rendered a planar uniformity of about ±4.3% in film thickness on the patterned wafers. The present example PE rendered a planar uniformity of about ±3.8% in film thickness on the patterned wafers. Accordingly, it has been found that the planar uniformity in film thickness on patterned wafers can be improved by the method according to the embodiment of the present invention.

In the embodiment described above, the silicon source gas is 3DMAS gas. Alternatively, the silicon source gas may be selected from other amino silane organic gases, such as BTBAS (bistertialbutylamino silane), 4DMAS (tetrakisdimethylamino silane), and DIPAS (diisopropylamino silane).

In the embodiment described above, the reactive gas is ozone gas serving as an oxidizing gas. Alternatively, the reactive gas may be selected from other oxidizing gases, such O2, N2O, and NO. Further, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-175441, oxygen radicals and hydroxyl radicals generated under a low pressure of 133 Pa or less may be used.

In the embodiment described above, a silicon oxide film is formed by a batch CVD method. Alternatively, the present invention may be applied to a batch CVD method for forming another film, such as a silicon nitride film (SiN film) or silicon oxynitride film (SiON film). Where a silicon nitride film is formed, the silicon source gas may be DCS (dichloro silane), HCD (hexachloro disilane), or TCS (tetrachloro silane), and the reactive gas may be NH3, for example. Where a silicon oxynitride film is formed, the silicon source gas may be an amino silane organic gas, and the reactive gas may be O3, O2, N2O, NO, or NH3, for example.

In the embodiment described above, a batch CVD method is performed in the batch processing apparatus that comprises the process container22having a double tube structure. Alternatively, the present invention may be applied to a batch processing apparatus that comprises a process container having a single tube structure.

As a target object, the semiconductor wafer mentioned here encompasses a silicon substrate and a compound semiconductor substrate, such as GaAs, SiC, or GaN. Further, the present invention may be applied to another target object, such as a glass substrate for LCD devices or a ceramic substrate.