JPH05343394A - Formation of thermal oxide film - Google Patents

Abstract

PURPOSE:To form a silicon thermal oxide film which ensures a longer life time and less amount of defects of silicon substrate by executing the final etching of the silicon substrate cleaning process with hydrofluoric acid and thereafter introducing the silicon substrate into an electronic furnace under the atmosphere where oxygen is reduced with nitrogen or inactive gas. CONSTITUTION:The wafer surface is changed to the hydrophobic state by processing a Si wafer after cleaning the surface thereof with ammonium, aqueous solution of hydrogen peroxide, water, hydrochloric acid, aqueous solution of hydrogen peroxide and water. The Si wafer of which surface is changed to hydrophobic state is then introduced into an electronic furnace under the atmosphere where oxygen is reduced by nitrogen or inactive gas. The Si thermal oxide film is formed by heating the Si wafer. In this case, it is prevented that air outside the quartz tube flows into the quartz tube by adjusting flow rate of gas in the quartz tube of the electric furnace, flow rate of gas in the quartz tube near the furnace and exhaust rate. Thereby, a purified thermal oxide film is formed on the silicon substrate and life time of the silicon substrate can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a clean silicon oxide film on a silicon substrate (silicon wafer) by a thermal oxidation method, and can be applied to an LSI manufacturing process.

[0002]

2. Description of the Related Art In the manufacturing process of LSI, thermal oxidation using an electric furnace and annealing are frequently used for silicon wafers. At that time, in order to prevent contamination of the silicon wafer, a high-purity quartz tube and a quartz boat for mounting the wafer are used, and purified high-purity nitrogen, oxygen, and hydrogen are used to anneal and heat the wafer. Oxidize. In this way, in the case of the horizontal electric furnace which is widely used, the purity of the quartz tube is usually used even though great attention is paid to the purity of the constituent materials of the electric furnace. Due to the reasons described below, unpurified air easily flows into the quartz tube in a large amount, which may cause contamination of the silicon wafer or affect the quality of the thermal oxide film formed on the silicon wafer.

The path through which air flows into the quartz tube is only on the furnace port side. The gas inlet side is connected to the quartz pipe from the gas purifier to the stainless steel SUS pipe up to the joint, and the joint is made of Teflon, both of which have good airtightness, and air cannot flow in. On the furnace opening side, in a normal horizontal electric furnace, when the wafer enters and leaves the quartz tube, the quartz cap is opened, and a large amount of air enters the quartz tube, or the wafer enters the quartz tube and the quartz cap is removed. Even when it is closed, air comes in reverse from the gas outlet provided in the quartz cap. The degree of inflow of this air depends on the flow rate of gas flowing in the quartz tube, the amount of exhaust air from a scavenger consisting of an exhaust port provided near the furnace opening, the degree of sealing of the quartz cap of the quartz tube, and the SUS outside the scavenger.
It is determined by the airtightness of the lid made of SUS (hereinafter referred to as SUS plate).

The problems caused by using the scavenger and the SUS plate will be described below.

The purpose of the scavenger is not only to simply exhaust the gas coming out of the furnace opening of the quartz tube, but also to dilute the gas coming out of the quartz tube with a large amount of air to a sufficiently low concentration to bring it into a safe state. After that, to exhaust the large amount of heat coming out of the quartz tube so that the temperature on the end station side of the furnace does not rise more than necessary, and not to the end station side, but to the scavenger side. Is. If the temperature of the furnace is raised to 1000 ° C or higher and the amount of heat from the furnace is discarded to the end station side, the quartz boat equipment on the end station side and parts related to it will become hot, It causes troubles in the operation of the device due to thermal deformation. Therefore, the scavenger's exhaust air volume cannot be made too small, and it can be taken sufficiently large by taking a margin. Moreover, the scavenger exhaust air volume adjustment is
Although it can be controlled by adjusting the opening and closing angle of the shutter,
High control, stability, and reproducibility cannot be expected.

The SUS plate is used to shield the heat radiation from the quartz tube on the outside of the scavenger on the throat side, and in the case of a hydrogen explosion accident when hydrogen is used by the hydrogen combustion method or the like. It is usually used for the safety of the oral area. When the wafer is completely inserted into the quartz tube and the quartz cap is closed, the SUS plate is also closed. When the SUS plate is completely sealed, the scavenger exhaust air flow and the flow rate of the gas coming out of the quartz tube are balanced in terms of flow rate. Since it is large, a negative pressure state is created near the outside of the gas outlet of the quartz tube, and air on the scavenger side flows into the quartz tube.

Conventionally, in order to prevent the inflow of air from the outside of the quartz tube, a structure in which the quartz tube is inclined obliquely downward to the furnace port side and a structure in which a nitrogen substitution chamber is provided in the furnace port have been devised. , Not a general structure. In addition, recently, in order to prevent these drawbacks, a vertical furnace with a load lock is being introduced, and the number of units used is rapidly increasing. However, the horizontal electric furnace, which has been widely used in the past, is still the mainstream in the production line. Especially in the temperature range higher than 1000 ° C., when the wafer moves in and out of the furnace, the horizontal electric furnace has a merit that the temperature change is gentler than that of the vertical electric furnace, so that the wafer is less likely to be distorted. Therefore, it is not in a situation where it can be completely replaced with a vertical electric furnace immediately.

As described above, the quartz tube, the quartz boat,
Although gas and other substances are of extremely high purity, in a normal horizontal electric furnace, air easily flows into the quartz tube from the furnace side, contaminating the wafer and degrading the oxide film quality. However, the time point at which the air flows in and the conditions for reducing the air flow have not been clear so far. By clarifying this, the gas flow rate is determined.

After the above gas flow rate is determined, the method of forming the silicon thermal oxide film needs to determine the gas species when inserting the wafer into the electric furnace and the precleaning conditions. This cannot be decided without evaluating the characteristics of the silicon thermal oxide film including the characteristics of the silicon substrate. As a conventional evaluation method,
Uniformity of thickness of formed oxide film, pinhole, or M
It has been common practice to evaluate the withstand voltage of the OS capacitor, the trap in the oxide film, and the interface characteristics, and feed back the results to the method of forming the thermal oxide film. However, since evaluation using a MOS capacitor requires a lot of man-hours for sample preparation, it takes time to feed back to device conditions. Further, since the electrode process affects the MOS capacitor characteristics, it is not possible to clarify the correspondence between the MOS capacitor characteristics and the detailed process level such as gas species when inserting the wafer into the electric furnace. Was very difficult.

[0010]

An object of the present invention is to suppress the inflow of air from the outside of the quartz tube, prevent contamination from entering from the outside, form a thermal oxide film in a clean atmosphere, and increase the lifetime of the silicon substrate.
And a silicon thermal oxide film with few defects is formed.

[0011]

[Example] As a method of evaluating the inflow of air, a method of measuring the dew point at each location from the furnace opening side in the quartz tube,
There is a method of measuring the oxide film thickness formed on a bare Si wafer in nitrogen. In the present invention, the inflow of air was evaluated by the latter method. After treatment in nitrogen in an electric furnace, Si
When the wafer is left in the air, oxygen and moisture in the air are adsorbed on the surface of the wafer with the lapse of time, and the film thickness measurement value by the ellipsometer increases. Therefore, in the following processing, after the wafer came out of the electric furnace, the measurement was performed immediately with an ellipsometer without waiting time.

The flow rate of nitrogen flowing in the quartz tube is 10 SLM.
At (liter / min), the oxide film formed on bare Si has a thickness of about 180 angstroms on the wafer near the furnace side of the quartz tube, as shown in FIG. As it goes inside, it drops rapidly and settles down to a constant film thickness of about 40 to 50 Å. Due to such a change in film thickness, the amount of inflowing air increases toward the furnace opening side (hereinafter referred to as intruding air), and a certain proportion of air (hereinafter referred to as residual air) is generated inside the quartz tube. You can see that it is mixed. The sum of the intruding air and the residual air will be referred to as inflow air. Since the inflowing air causes contamination of the wafer and the quartz tube, it is necessary to eliminate the oxide film thickness due to the intruding air or the residual air. However, in FIG. 1, the boat was put in and out at 900 ° C., and after the boat was put into the quartz tube, the temperature was raised to 1000 ° C. and stabilized, and then annealed at 1000 ° C. for 60 minutes.

As shown in FIG. 1, in order to prevent intruding air, it is necessary to increase the flow rate of gas flowing in the quartz tube to 20 SLM or more. It is not clear if the boat on board is in or out of the quartz tube, or after it has entered the quartz tube. When the flow rate of nitrogen in the quartz tube is increased, as shown in Fig. 1, the intruding air is pushed to the furnace side and the residual air also gradually decreases, until the inflowing air is completely expelled. As a result, the invading air disappears from the position of the wafer on the quartz boat, but the invading air remains in the quartz tube near the furnace opening. In order to eliminate this invading air, it is necessary to increase the gas flow rate in the quartz tube to a considerable amount, but this is not a preferable method in terms of temperature uniformity and temperature controllability in the quartz tube. Quartz bow
As will be described later, the residual air content in the oxide film thickness evaluation of the wafer on the wafer is oxidized even when the boat is taken in and out, so it is prevented only by increasing the gas flow rate or by the presence or absence of the air curtain or SUS plate. I can't do it either. Therefore, in the present invention,
Only the method of preventing invading air will be described.

As a method of preventing invading air, there is a method of filling the vicinity of the gas outlet on the furnace side of the quartz tube with nitrogen.
This method is usually called an air curtain. In addition, even though it is called air curtain, purified high-purity nitrogen is used instead of air. As shown in FIG. 2, when the nitrogen flow rate in the quartz tube is set to 155 SLM and the nitrogen flow rate of the air curtain is increased, 30 SLM is not effective yet.
50SLM can prevent invading air. However,
Also in this case, the step in which the invading air is occurring is not clear. Since air-caten is not a method of flowing a large amount of nitrogen directly in the quartz tube, it does not cause a problem with respect to temperature uniformity in the quartz tube. The horizontal electric furnace is designed to reduce the floor area,
Normally, several tubes are stacked one above the other, and if 50 SLM is flowed for each tube, for example, in the case of a four-stage 4-stage furnace, it is necessary to flow 200 SLM of purified high-purity gas only with an air curtain. However, there is a drawback that the gas flow rate is consumed too much.

Next, the exhaust air amount of the scavenger and the SUS
Describe the relationship between the plate and the invading air. FIG. 3 shows the relationship between the presence or absence of a SUS plate and the amount of scavenger exhaust air. Of course, the quartz cap is fully fitted. When there is no SUS board,
There is no invading air. If there is a SUS plate even with the same exhaust air amount, intruding air is generated, and the amount of intruding air increases as the scavenger exhaust air amount increases. In the case of the present embodiment, the quartz cap and the SUS plate are connected and move at the same time, and the SUS plate is closed at the same time when the quartz cap is closed.
The effect of the presence or absence of the S plate appears when the quartz cap is closed. In other words, in the case where the SUS plate is provided, when the quartz cap is closed, the SUS plate is also closed, a negative pressure is generated in the vicinity of the scavenger, and the intruding air is rapidly increased. That is,
It can be seen that the intruding air is generated after the quartz cap is closed.

In order to clarify the step in which the invading air is generated in the presence of the SUS plate by another method,
"When the boat is inserted" → "While the boat is set in the quartz tube and the quartz cap is closed" → "When the boat is pulled out" As shown in FIG. Nitrogen flow rate in 3 steps from 20 SLM to 10 SLM
→ When changing to 20SLM, there is intruding air and 10S
When changing from LM → 20SLM → 10SLM, there is no intruding air, so intruding air is generated even when the gas flow rate is small even after the quartz cap is closed. In other words,
The entering air is not determined by the magnitude of the gas flow rate when the boat is moving in and out, but is mainly determined by the gas flow rate after the quartz cap is closed in the quartz tube.

The degree of intruding air is also related to the amount of air flowing into the quartz tube when the boat is taken in and out. Figure 5
As shown in, even if the gas flow rate when the boat is inserted and the quartz cap is closed is set to 20 SLM, and the flow rate of the air carten is set to 0 SLM, the air car when loading and unloading the boat is set.
Increasing the Ten flow rate to 50 SLM reduces the amount of ingress air. From this, it can be seen that the air is expelled faster after the quartz cap is closed when the inflow of air into the quartz tube when the boat is taken in and out is reduced. Further, it can be seen that there is no invading air when the nitrogen in the second step is set to 15 SLM. In the case of the air curtain 0SLM and the nitrogen 15SLM, as shown in FIG. 1, there is intruding air, but it can be seen that intruding air disappears when the inflow degree of air at the time of putting in and out the boat is reduced. FIG. 6 shows the second step with nitrogen 10
When the SLM is used, the effect of the air curtain is obtained, but it is shown that the invading air cannot be eliminated even if the flow rate at the time of loading / unloading the boat is increased. As a result, even if the air flows in and out of the boat into the quartz tube, if the second step with the quartz cap closed is set to 20 SLM, the inflowing air can be expelled, but at 15 SLM, the ability to expel is reduced. However, it can be removed by reducing the amount of inflowing air when the boat is in and out. 10SLM
Then, even if the air inflow at the time of putting in and out the boat is reduced, it is understood that the air comes in when the quartz cap is closed.

The relationship between the various gas flow rates and the invading air is as follows.
In the case of wet oxidation by the hydrogen combustion method, the steam flow rate cannot be increased so much for the following reasons, which is particularly important. In the case of the hydrogen combustion method, if the hydrogen flow rate is increased too much to increase the flow rate of water vapor, the hydrogen flame becomes too large and the tip of the quartz nozzle of the hydrogen blowout port is deformed in a short time. Need to be done,
On the other hand, the deformation of the quartz nozzle changes the blowout state of the hydrogen flame, which affects the temperature distribution in the quartz tube. Further, if the hydrogen flame becomes too large, the hydrogen flame hits a certain position on the inner wall of the quartz tube, and the hydrogen flame selectively melts, causing a problem of forming holes. In the case of the present embodiment, for the above reason, the hydrogen flow rate is suppressed to 8 SLM due to the size of the hydrogen flame,
The oxygen flow rate was set to 11 SLM in order to keep the total gas flow rate at 15 SLM or more in order to prevent ingress air. As a result, the water vapor flow rate was 8 SLM and the oxygen flow rate was 7 SL in the quartz tube.
M, the total flow rate is 15 SLM. From FIG. 1, it is possible to reduce the amount of invading air by setting the total flow rate to 15 SLM. Further, under the condition of “O _{2 present} ” in the embodiment described later, a total of 22 SL of nitrogen 20 SLM and oxygen 22 SLM is obtained.
Set to M and set the total gas flow rate to 1 when the quartz cap is closed.
In the case of 5 SLM, as shown in FIG. 5, it is possible to prevent the invading air at the time of wet oxidation by setting the air curtain to 50 SLM only when the wafer is taken in and out.

As the removal speed of the boat is increased, the oxide film thickness due to the residual oxygen is gradually reduced as shown in FIG. 7, so that an oxide film is formed on the wafer even when the boat is installed and removed. It is understood that it is done. Since this is oxidized by air, it is desirable to put the boat in and out in the shortest possible time in order to achieve a cleaner atmosphere. However, if the loading / unloading speed is too high, the temperature difference between the front and back surfaces of the wafer, the center and the outer circumference becomes large, and large stress is applied to the wafer, and crystal defects such as slips easily occur. Then, it is not possible to prevent the formation of an oxide film at the time of taking in and out. Note that FIG. 7 shows an example without SUS.

The oxide film thickness due to the residual air is formed even when the boat is not taken in and out. As shown in FIG. 8, when the heat treatment temperature in the second step is increased from 800 ° C. to 1100 ° C., this film thickness is greatly increased from 20 Å to about 300 Å.
It can be seen that residual air is present in addition to the intruding air even after the quartz cap is closed. When the temperature is about 800 ° C., the oxide film thickness due to the intruding air and the residual air is small, so there is no obvious change, but at about 1100 ° C., the oxide film thickness itself due to the residual air increases. Therefore, 1000 ℃
Is easy to evaluate by separating contributions of intruding air and residual air.

As a result of the above, the gas flow rate capable of preventing the intruding air was clarified. Under the following oxidizing conditions, basically, the gas flow rate is set to 15 SLM or more to reduce the intruding air as much as possible. Needless to say, it is desirable to apply conditions that can prevent invading air by using an air caten at the time of inserting and removing the boat in order to carry out even more clean oxidizing conditions.

The wafer is pre-cleaned immediately after the bare Si is purchased and opened, and the electric furnace is kept in a standby state during the cleaning.
Thermal oxidization was carried out immediately without any time after washing, and after completion of the oxidation, the lifetime was measured by the non-contact lifetime evaluation method without any time. When the wafer is transferred by the end of the oxidation, only the periphery of the wafer is touched with tweezers, and the back surface of the wafer is not touched with chucks or stages. Only three steps of pre-cleaning → thermal oxidation → lifetime measurement were performed, and the results were obtained in an extremely short time.

The lifetime of the silicon substrate is the infrared ray
The laser current was kept constant by the non-contact evaluation method by the carrier injection by the laser and the time constant measurement of the attenuation of the reflected microwave. As the resistivity of the silicon wafer increases,
The lifetime increased, and in the case of the same specific resistance, the n-type wafer had a larger lifetime than the p-type wafer, and a clear difference in the lifetime was observed depending on the wafer manufacturer. , P-type (100) 3-5 Ωcm, Oi concentration 9-9.
5 × 10 ¹⁷ cm ⁻³ was used. A double-sided mirror is preferable because a double-sided mirror is not affected by the finished state of the back surface, but in this embodiment, a single-sided mirror normal wafer used in SLI manufacturing was used. The lifetime measured from the front side is about 8% larger than the lifetime measured from the back side. Immediately after removing SiO ₂ on the back surface with hydrofluoric acid, the lifetime measured from the front surface side was 60 compared with that before etching with hydrofluoric acid.
%, The lifetime measured from the back side is
It decreases to about 40%. In addition, the wafer from which SiO ₂ on the back surface is removed has an increased variation in lifetime and poor reproducibility. Therefore, in this example, the back surface SiO ₂ was left as it was, and the lifetime was measured from the front surface side (mirror side) of the wafer. A wafer with a long lifetime has a uniformly long lifetime in the central portion of the wafer, whereas a wafer with a short lifetime has a shorter lifetime in the central portion as compared with the peripheral portion. Here, the average value of the in-plane distribution of the wafer was taken as the effective lifetime.

After the wafer enters the quartz tube and the quartz cap is closed, when the wafer is oxidized, only O ₂ is used for dry oxidation in order to prevent or reduce intruding air.
In case of 0 SLM and wet oxidation, H ₂ is 8S as described above.
LM and O ₂ were set to 11 SLM. Oxidation temperature is 800-1
The temperature was 100 ° C. and the oxidation time was 1 to 4 hours. As the gas atmosphere at the time of inserting the wafer into an electric furnace, when the only nitrogen "O ₂ No", the case of oxygen diluted with nitrogen 'O ₂ present ",
The case of only oxygen will be referred to as "O ₂ flow". As an example, “no O ₂ ” is nitrogen 20 SLM, “O ₂ is present” is a mixture of nitrogen 20 SLM and oxygen 2 SLM, and “O ₂ flow” is oxygen 20 SLM. Pre-cleaning for thermal oxidation is generally known as a cleaning method for Si wafers, "ammonia / hydrogen peroxide solution" → water cleaning → "hydrochloric acid / hydrogen peroxide solution" → water cleaning → "presence or absence of hydrofluoric acid treatment" → In the process of washing with water, the cleaning including the process of hydrofluoric acid was performed with or without "HF".
The omitted condition is “without HF”. When HF is present, the wafer surface after pre-cleaning is hydrophobic. On the other hand, in the case of no HF, the cleaning is completed by washing with hydrochloric acid / hydrogen peroxide solution → pure water, so the wafer surface is hydrophilic. In addition, it was confirmed that the presence or absence of hydrogen anneal after thermal oxidation did not affect the measured value of lifetime by changing the level in the same process for both dry oxidation and wet oxidation. It is considered that the difference due to the level of lifetime in the example does not refer to the change in interface state density.

FIG. 9 shows that the oxidation time of dry oxidation is 1 / 60th.
Two examples will be given. Comparing the O ₂ flow and the O ₂ non-existence, the lifetime is almost the same at an oxidation temperature of 1000 ° C. or lower, but at 1100 ° C., the O ₂ flow has a lifetime of 1 or less.
It was on the extension line of the data below 000 ° C, and the lifetime was not deteriorated. On the other hand, if there is no O ₂ at 1100 ° C,
The lifetime deteriorates rapidly, and the difference from the O ₂ flow becomes extremely remarkable. The cause of the lifetime deterioration is that after inserting the boat and raising the temperature to 1100 ° C in an atmosphere containing only nitrogen, and the temperature at 1100 ° C is stabilized, oxygen 20 SLM is used.
Si _{x at} 1100 ° C until switching to
It is considered that a thin film of O _y N _z is formed, or SiO having a high vapor pressure is formed, so that fine irregularities are generated on the silicon surface. In fact, in the case of annealing bare Si only in nitrogen, surface roughness does not occur up to 1000 ° C., but surface roughness occurs at 1100 ° C., which is considered to correlate with a decrease in lifetime. Also, if O ₂ is present,
The result is equivalent to the _second stream.

At an oxidation temperature of 800 ° C., the lifetimes of HF-free and HF-free are almost the same, but as the oxidation temperature rises, the lifetime of HF-free significantly increases with respect to HF-free. This tendency is the same up to ° C. As a result, the condition with HF is good. As mentioned above, without HF, the surface after pre-cleaning is hydrophilic and
The low-grade silicon oxide generated on the Si wafer during the cleaning with the “hydrogen peroxide solution” and the “hydrochloric acid / hydrogen peroxide solution” is a factor that reduces the lifetime after the thermal oxidation is completed.
As a cause, the impurities contained in the chemicals are taken into the low-grade oxide even though it is of high purity, and this is considered to be the cause of the decrease in lifetime. It is considered that the lifetime of the wafer treated with hydrofluoric acid is increased because the impurities are removed. From this example, towards the O ₂ O ₂ flow than free will, who HF chromatic than HF-free is, the lifetime is significantly better.

FIG. 10 shows another embodiment of dry oxidation. The oxidation time is all 60 minutes, but hydrogen treatment is carried out after the oxidation. In the case of this embodiment, when the oxidation temperature is 900 ° C. or lower, the lifetimes of the O ₂ flow and the O ₂ -free are almost the same, but when the oxidation temperature is 1000 ° C. or higher, the lifetime of the O ₂ flow is not deteriorated but rather increased. In some cases, the lifetime was rapidly deteriorated in the absence of O ₂ . This example is similar to the example of FIG. 9 with respect to the effect of the O ₂ flow and HF, except that the respective differences are already large at 1000 ° C.

The difference in oxide film thickness with HF and without HF (Angstrom (A) unit) is almost the same when the oxidation temperature and the oxidation time are changed, as shown below. With HF and HF
There is a clear significant difference in the measured values of the lifetime even if the non-standard levels are separately oxidized. However, since there are large variations in the measured values of the lifetime in the first place, HF is added to clarify the significant difference. Wafers without and FH were alternately placed in adjacent grooves in the same boat and simultaneously oxidized, and it was confirmed that there was a clear significant difference. Oxidation temperature 800 ° C Oxidation time 30 minutes 60 minutes 120 minutes 240 minutes Oxide film thickness (A) +1.04 +1.15 +0.74 +0.12 Oxidation temperature 900 ° C Oxidation time 30 minutes 60 minutes 120 minutes 240 minutes Oxide film thickness (A) +0.29 -0.11 -0.23 -0.75 When oxidation temperature is 1000 ° C Oxidation time 30 minutes 60 minutes 120 minutes 240 minutes Oxide film thickness (A) -0.38 +3.34 -2.25 -2.68 When oxidation temperature is 1100 ° C Oxidation time 30 minutes 60 minutes 120 minutes 240 minutes Oxide film thickness (A) -0.35 -2.19 -3.11 -2.23

As shown in the figure, the lifetimes of the O ₂ flow and the O ₂ existence are almost the same. Since the oxide film continues to be formed in the boat in the O ₂ flow, the presence of O ₂ is more advantageous for the uniformity of the oxide film by dry oxidation.
In the above-mentioned Examples, in order to improve the film thickness uniformity, thermal oxidation is performed by raising or lowering the temperature by lowering the furnace temperature at the time of loading / unloading the boat by 100 to 200 ° C. with respect to the oxidation temperature. I took it. Therefore, O ₂ flow, O ₂ Yes, between the O ₂ free, film thickness uniformity are both good difference in thermal oxide film was not. However, in order to reduce the processing time, if the temperature inside the furnace during the loading and unloading of the quartz boat is set to the same or about the same as the thermal oxidation temperature, the boat will be inserted at the predetermined position in the quartz tube during the step of inserting the boat. The temperature at the furnace mouth side of the quartz tube is the lowest when the temperature goes up. As for the temperature at the position of the boat, there is almost no temperature drop inside the quartz tube, and the temperature drops a few tens of degrees Celsius at the furnace mouth side. In particular, in the case of the cantilever method or the soft landing method, since the quartz fork or the SiC fork enters the quartz tube, the temperature drop on the furnace port side is remarkable. It takes 10 to 20 minutes in this state, and the oxidation is continued until the temperature of the lowered part rises and the temperature stabilizes.
In the case of the O ₂ flow, the substantial oxidation temperature is different between the inner side and the inner side of the quartz tube if the substantial oxidizing time or the viewpoint is changed. Without O ₂ , this problem does not occur, but there is a problem of defects such as the above surface roughness. The presence of O ₂ has a great effect on the uniformity of the oxide film because only a small amount of the oxide film is formed during the temperature drop at the time of inserting the boat and the temperature stabilization. Here, an example of diluting nitrogen with O ₂ is shown, but the same applies to diluting with an inert gas.

FIG. 11 shows the results of wet oxidation 1000 to 11
An experimental example at 00 ° C. is shown. The gas flow rate of the wet oxidation was set to 8 SLM for hydrogen and 11 SLM for oxygen, and the total gas flow rate after hydrogen combustion was set to 15 SLM in consideration of pollution prevention. This is for the reason already explained regarding the gas composition ratio for preventing the invading air in the hydrogen combustion method. As with dry oxidation, O ₂ free the poor comparable or lifetime as compared to 1000 ° C. In O ₂ flow and O ₂ Yes, 110
At 0 ° C, no O ₂ is rapidly deteriorated. Therefore, the wet oxidation may be considered to be similar to the dry oxidation.

[0031]

According to the present invention, the air entering from the outside of the electric furnace is reduced to oxidize in a clean atmosphere, and the silicon wafer before oxidation is contaminated with impurities contained in the chemicals used for precleaning. The surface of the wafer is kept hydrophobic in the final step of the pre-cleaning process, and the temperature is already raised in the electric furnace until oxidation starts, and when the boat is inserted and the temperature is raised. By starting the formation of a thermal oxide film, a step of preventing the roughness of the silicon surface is taken, a clean thermal oxide film is formed on the silicon substrate, the lifetime of the silicon substrate is improved, and the silicon substrate characteristics are improved. Can be prevented from deteriorating.

When the present invention is applied to the thermal oxidation method of LSI,
Since defects in the thermal oxide film can be reduced and impurities can be reduced, it is possible to improve device characteristics and yield of LSI.

[Brief description of drawings]

FIG. 1 is a diagram showing the nitrogen gas flow rate dependency in a quartz tube, which is used for explaining the present invention.

FIG. 2 is a diagram showing the nitrogen gas flow rate dependency for an air curtain, which is used for explaining the present invention.

FIG. 3 is a diagram showing scavenger exhaust air velocity dependency, which is used for explaining the present invention.

FIG. 4 is a diagram showing the 3-step nitrogen gas flow rate dependency used for explaining the present invention.

FIG. 5 is a three-step nitrogen gas for explaining the present invention.
It is a figure which shows the air curtain gas flow rate dependence.

FIG. 6 is a three-step nitrogen gas for explaining the present invention.
It is a figure which shows the air curtain gas flow rate dependence.

FIG. 7 is a diagram showing the boat speed dependency, which is used for explaining the present invention.

FIG. 8 is a diagram showing temperature dependency of an air inflow amount, which is used for explaining the present invention.

FIG. 9 is a diagram showing the relationship between dry oxidation and lifetime for explaining the present invention.

FIG. 10 is a diagram showing the relationship between dry oxidation and lifetime (with hydrogen anneal) used for explaining the present invention.

FIG. 11 is a diagram showing the relationship between wet oxidation and lifetime for explaining the present invention.

Claims (2)

[Claims] 1. A silicon oxide film is formed on a silicon substrate.
In the method of forming by thermal oxidation, after the silicon substrate is subjected to a step using a hydrofluoric acid-based etchant in the final etching step of the pre-cleaning step, the atmosphere in the step of inserting the silicon substrate into an electric furnace is changed to oxygen. A method for forming a thermal oxide film, characterized in that the atmosphere is diluted with nitrogen or an inert gas. 2. A method of thermally oxidizing or annealing a silicon substrate in a semiconductor device manufacturing process, in order to prevent air outside a quartz tube of an electric furnace from flowing into the quartz tube, a gas flow rate in the quartz tube, A thermal oxidation or anneal method characterized by adjusting the gas flow rate and exhaust volume near the furnace opening of a quartz tube.