JP2009302240A - Preprocessing method for recombination lifetime evaluation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a preprocessing method for recombination lifetime evaluation in which a suitable recombination lifetime is measured by suppressing a recombination process on a wafer surface of a low-contamination and long-lifetime silicon wafer. <P>SOLUTION: Organic substances sticking on the silicon wafer surface are removed using a mixed liquid of ammonia and a hydrogen peroxide solution etc., a natural oxide film on the silicon surface is removed with an HF solution, and passivation is carried out by an iodine ethanol method, so that the recombination lifetime of a carrier is measured by a reflective microwave photoconduction attenuation method. For example, contaminations which are considered to be produced from a carrier box to stick and cannot be removed with the HF solution can be removed, so recombination of the carrier on the silicon surface is more suppressed to measure the recombination lifetime which is close to a bulk lifetime and from which the quality of the silicon wafer is evaluated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a pretreatment method for measuring a recombination lifetime of minority carriers of a silicon wafer by a reflection microwave photoconductive decay method, and more particularly to a pretreatment method by an iodine ethanol method.

An important property of silicon wafers that significantly affects device performance is the minority carrier recombination lifetime.
The recombination lifetime of minority carriers is shortened when the silicon wafer is contaminated with crystal defects or metal impurities, and is therefore generally used as a method for evaluating the amount of metal impurity contamination in a single crystal. The lifetime of minority carriers must be long for devices that require a small junction leakage current, such as DRAMs and CCDs, and short for devices that perform high-speed switching such as pn diodes. Is an important value that determines the switching speed and leakage current.
Thus, minority carrier recombination lifetime measurement is an important quality evaluation method for silicon wafers as device materials. In general, a silicon wafer having a long recombination lifetime is desired as a starting material for the device process.

  One method for measuring minority carrier recombination lifetime is the reflected microwave photoconductive decay method. This method is a method of irradiating a silicon wafer with a microwave, measuring the intensity of the reflected wave of the microwave, and measuring the time during which the intensity attenuates to a constant intensity. Since the intensity of the reflected wave is a function of the resistivity (or conductivity) of the silicon wafer, the time until the electrons excited by the microwave irradiation disappear in the recombination process by measuring this. That is, the recombination lifetime of the carrier can be measured.

  By the way, in order to appropriately evaluate the quality of a silicon wafer by measuring the recombination lifetime of minority carriers, the bulk lifetime inherent to the silicon wafer, that is, the recombination process in the recombination centers distributed inside the silicon wafer. It is appropriate to measure the lifetime. However, normally, in a silicon wafer, the periodicity of the crystal is broken at the surface, so that electrons localized on the surface become an eigenstate having a surface level, which becomes a recombination center and a recombination process occurs. The ratio of carrier recombination on the surface of the silicon wafer is large, which affects the measurement result of the minority carrier recombination lifetime.

  1 (A) to 1 (D) are diagrams showing the relationship between the bulk lifetime and the measurement lifetime for each surface recombination velocity of the silicon wafer, and FIG. In the range of 20000 μs, FIG. 1 (B) shows the bulk lifetime in the range of 0 to 10,000 μs, FIG. 1 (C) shows the bulk lifetime in the range of 0 to 5000 μs, and FIG. 1 (D) shows the bulk lifetime. It is a figure which shows the relationship between a bulk lifetime and a measurement lifetime in the range of 0-2000 microseconds. In either figure, the graph (plot) represented by the diagonal line from the origin (lower left) to the upper right shows that the measured lifetime and the bulk lifetime are approximately equal, and the further away from the diagonal line, that is, The graph (plot) closer to the x-axis (bulk lifetime change direction) indicates that the measured lifetime is different from the bulk lifetime.

This difference between the measurement lifetime and the bulk lifetime is the influence of the recombination process on the surface of the silicon wafer. Therefore, the smaller the surface recombination speed of the silicon wafer, the less the influence of the recombination process at the silicon wafer surface on the measured lifetime, and the smaller the difference between the measured lifetime and the bulk lifetime. Conversely, the difference between the measurement lifetime and the bulk lifetime increases as the surface recombination rate of the silicon wafer increases.
Thus, when measuring the recombination lifetime of minority carriers in a silicon wafer, the influence of the recombination process on the surface of the silicon wafer is significant, and in order to properly evaluate the quality of the silicon wafer, It is necessary to make the recombination rate of the substrate sufficiently low, in other words, to inactivate and passivate the surface of the silicon wafer. Specifically, from FIG. 1 (A) to FIG. 1 (D), in order to properly evaluate the quality of the silicon wafer, the surface recombination speed of the silicon wafer is required to be 10 cm / s or less. The

  There are various methods for inactivating the surface of the silicon wafer. For example, a method of controlling the surface recombination rate by thermally oxidizing the surface of the silicon wafer is known. However, the method of thermally oxidizing the wafer surface has a problem that the silicon wafer is contaminated during the thermal oxidation process or is affected by a change in potential of the wafer surface due to the formation of an oxide film on the silicon wafer surface.

As a method of inactivating and passivating the surface of a silicon wafer without thermally oxidizing the wafer surface, there is an iodine ethanol method (for example, see Patent Document 1). In the iodine ethanol method, the surface recombination velocity of the silicon wafer is sufficiently lowered by covering both sides of the silicon wafer with a passivation solution containing iodine and ethanol, and in this state, the carrier recombination life by the reflection microwave photoconductive decay method is used. This is a method for obtaining a recombination lifetime measurement value in which the influence of recombination on the silicon wafer surface is reduced by measuring the time. When the iodine ethanol method is used, a process of removing a natural oxide film on the surface of the silicon wafer with an HF solution is generally performed before the silicon wafer is covered with a passivation solution containing iodine and ethanol.
US Pat. No. 5,580,828

  As described above, when measuring the recombination lifetime of minority carriers by the reflected microwave photoconductive decay method, the pretreatment of removing the natural oxide film on the silicon wafer surface by HF solution and passivation by iodine ethanol method is performed. It has been implemented. Conventionally, in a state where the contamination level of the silicon wafer is high to some extent, the effect of the carrier recombination process on the surface of the silicon wafer on the lifetime measurement value can be sufficiently reduced by such pretreatment.

However, in recent years, the recombination lifetime has become high due to the improved contamination level of silicon wafers.
As shown in FIGS. 1A to 1D, the difference between the measurement lifetime and the bulk lifetime, that is, the influence of the recombination process on the silicon wafer surface is short in the bulk lifetime (measurement lifetime). It is smaller and larger as the bulk lifetime (measurement lifetime) becomes longer. Therefore, the influence of the carrier recombination process on the surface of the silicon wafer on the lifetime measurement value is relatively low due to the low metal contamination level of the silicon wafer and the long recombination lifetime (bulk lifetime) of the silicon wafer. Became bigger. As a result, the lifetime measurement value is measured significantly lower than the original value, which causes a problem that the quality level of the silicon wafer cannot be properly evaluated.

  The present invention has been made in view of such problems, and the object thereof is to further recombine processes on the surface of a silicon wafer when the recombination lifetime of minority carriers is measured by the reflected microwave photoconductive decay method. Before recombination lifetime measurement, which can reduce the impact of the product and obtain a measurement value close to the bulk lifetime, which enables proper evaluation of the quality of the wafer, even for high-lifetime silicon wafers with low contamination levels It is to provide a processing method.

When this inventor performed recombination lifetime evaluation through the iodine ethanol method with respect to the wafer after long-distance transportation in the summer, the value lower than a normal level was obtained. When the surface of this wafer was analyzed by GC / MS, organic matter contamination that was thought to be from the carrier box was observed, and it was found that this contamination was observed on the surface even after HF treatment. As a result, the inventor of the present application has the reason that the measured value of the maximum binding lifetime is low because the organic substance remains on the wafer surface before coating with the iodine ethanol solution, and the silicon surface is not sufficiently passivated with the iodine ethanol solution. I found out that it was. As a result of extensive research, the present inventor has found a method according to the present invention that effectively removes this organic contamination without leaving any damage on the surface layer.

  That is, the pretreatment method for recombination lifetime measurement according to the present invention is a pretreatment method for silicon wafer carrier recombination lifetime measurement by the reflected microwave photoconductive decay method, and comprises a natural oxidation of a silicon wafer to be measured. A natural oxide film removing step for removing the film, and a passivation step for passivating the silicon wafer from which the natural oxide film has been removed by an iodine ethanol method, and before removing the natural oxide film in the natural oxide film removing step It has an organic substance removal process which removes the organic substance adhering to the surface of the silicon wafer.

  Preferably, in the organic substance removing step, the surface of the silicon wafer is formed by wet treatment using a mixed solution of ammonia water and hydrogen peroxide solution, a mixed solution of sulfuric acid and hydrogen peroxide solution, or ozone water. It is characterized by removing attached organic matter.

  According to the present invention, when the recombination lifetime of minority carriers is measured by the reflection microwave photoconductive decay method, the influence of the recombination process on the silicon wafer surface is further reduced, and the high lifetime silicon having a low contamination level. Also for a wafer, it is possible to provide a pre-processing method for recombination lifetime measurement that can obtain a measurement value close to a bulk lifetime that can appropriately evaluate the quality of the wafer.

An embodiment of the preprocessing method for recombination lifetime measurement of the present invention will be described with reference to FIG.
FIG. 2 is a diagram showing the flow of the minority carrier recombination lifetime measurement process of the silicon wafer including the pretreatment according to the present invention.
As shown in FIG. 2, the recombination lifetime measurement method according to the present embodiment includes a pre-processing step (step S10) for cleaning the surface of a silicon wafer to be measured so that appropriate measurement can be performed, And a step of measuring the combined lifetime (step S20). As the pretreatment step S10, first, an organic substance removal process is performed (step S11), then a natural oxide film removal process using an HF solution is performed (step S12), and passivation by an iodine ethanol solution method is performed (step S13). Then, in a state where the wafer is passivated, the recombination lifetime of minority carriers is measured by the reflected microwave photoconductive decay method (step S20).

  In the organic substance removal processing S11, for example, organic contaminants emitted from a carrier box or the like and attached to the silicon wafer surface are removed so as not to leave damage on the silicon wafer surface layer. In this embodiment, this organic contaminant is removed by wet processing so as not to leave damage on the surface layer of the silicon wafer. Specifically, the organic substance adhering to the surface is removed by washing the silicon wafer to be measured with an appropriate treatment liquid. The treatment liquid may be any treatment liquid as long as it can remove organic matter. However, a mixed liquid of ammonia water and hydrogen peroxide water generally used in the silicon device process, sulfuric acid and hydrogen peroxide water, and the like. A mixed liquid or an ozone liquid is suitable. This is because these solutions have no possibility of damaging the silicon wafer and can appropriately remove organic substances.

  In addition, as the gist of the present invention, the method for removing organic substances is not limited to wet treatment. If no damage is left on the surface layer of the silicon wafer and the treatment does not affect the lifetime measurement value, the organic matter may be removed by a treatment other than the so-called wet treatment. However, the dry process generally known at present is not preferable because it causes damage to the surface layer of the silicon wafer, and the damage may reduce the lifetime measurement value. From such a point of view, a wet process using a solution as described above is suitable as a method generally known at present.

  In the natural oxide film removal process S12, the silicon wafer from which organic contaminants have been removed in step S11 is immersed in an HF solution to remove the natural oxide film on the wafer surface. The silicon wafer from which the natural oxide film has been removed with the HF solution is washed with pure water and used for the next step.

  In the passivation process S13, passivation by an iodine ethanol method is performed. That is, a silicon wafer is covered with a passivation liquid containing iodine and ethanol, and the silicon wafer surface is passivated (passivation). Then, the silicon wafer to be measured is subjected to the minority carrier recombination lifetime measurement process S20 in a state of being passivated with the iodine ethanol solution in this way.

  In step S20, the recombination lifetime of minority carriers of the silicon wafer to be measured is measured by the reflected microwave photoconductive decay method. That is, the silicon wafer is irradiated with microwaves, the intensity of the reflected wave of the microwave is measured, the attenuation curve of the intensity is detected, and the recombination lifetime, that is, the intensity of the reflected microwave is constant from the obtained attenuation curve. The time to decay to the intensity of is calculated.

  According to the method for measuring the recombination lifetime of minority carriers of the present invention, organic contaminants on the silicon wafer surface are removed before the natural oxide film removal process (step S12) with the HF solution. Therefore, it is possible to prevent a state in which these organic contaminants become recombination centers and promote the recombination process on the silicon wafer surface during the subsequent recombination lifetime measurement. As a result, it is possible to obtain a measurement value of the recombination lifetime of minority carriers that is close to the bulk lifetime and can appropriately evaluate the quality of the silicon wafer by the reflected microwave photoconductive decay method.

  In addition, the organic contaminant removal treatment is performed by wet treatment using a mixed solution of ammonia water and hydrogen peroxide solution, a mixed solution of sulfuric acid and hydrogen peroxide solution, or an ozone solution generally used in the silicon device process. Since it is performed by processing, there is no possibility of damaging the silicon wafer, and organic substances can be removed appropriately. As a result, the above effects can be obtained appropriately.

  In addition, this embodiment is described in order to make an understanding of this invention easy, and does not limit this invention at all. Each element disclosed in the present embodiment includes all design changes and equivalents belonging to the technical scope of the present invention, and various suitable modifications can be made.

An example of a method for measuring a minority carrier recombination lifetime of a silicon wafer including a pretreatment method according to the present invention will be described with reference to FIGS. 3 and 4 together with a comparative example.
An n-type 10 Ωcm silicon wafer (sample wafer) obtained by sample processing in an epi furnace (furnace A) with a high contamination level or an epi furnace (furnace B) with a low contamination level is subjected to the following four conditions (conditions 1 to 1). After passing through the condition 4), it was subjected to HF treatment, and then subjected to passivation treatment by iodine ethanol method, and recombination lifetime measurement was performed by reflection microwave photoconductive decay method.

Condition 1: Immediately after sample processing.
Condition 2: After sample processing, the sample silicon wafer is stored in a shipping case and exposed to an environment equivalent to long-distance transportation during the summer season.
Condition 3: The sample wafer of Condition 2 was cleaned with ammonia hydrogen peroxide solution and immediately after the cleaning.
Condition 4: The sample wafer of condition 2 is cleaned with sulfuric acid hydrogen peroxide solution, and immediately after the cleaning.

  That is, in the lifetime measurement of the silicon wafer according to the condition 1, immediately after sample processing of the n-type 10 Ωcm silicon wafer in the furnace A or B, the removal of the natural oxide film on the wafer surface with the HF solution and the pure rinse treatment are performed. The passivation treatment by the iodine ethanol method is performed, and the recombination lifetime measurement of minority carriers is performed by the reflection microwave photoconductive decay method.

  In addition, silicon wafer lifetime measurement according to Condition 2 is performed when a sample of an n-type 10 Ωcm silicon wafer is processed in furnace A or furnace B, then stored in a shipping case, and transported over long distances in summer After that, the wafer surface is removed with a natural oxide film and pure rinse treatment with HF solution, passivation treatment with iodine ethanol method, and minority carrier recombination lifetime by reflection microwave photoconductive decay method. The measurement is performed.

  In the measurement of the lifetime of the silicon wafer according to condition 3, a cleaning step using ammonia hydrogen peroxide is performed before the treatment with the HF solution in the treatment according to condition 2. That is, after sample processing is performed on an n-type 10 Ωcm silicon wafer in furnace A or furnace B, the sample wafer is stored in a shipping case and exposed to the same environment as when transported over a long distance in summer. After that, cleaning with ammonia hydrogen peroxide is first performed to remove organic contamination on the wafer surface, then the natural oxide film on the wafer surface is removed with a HF solution, and a pure rinse treatment is performed, followed by a passivation treatment using an iodine ethanol method. Minority carrier recombination lifetime is measured by reflected microwave photoconductive decay.

  In addition, the silicon wafer according to condition 4 is such that cleaning before HF treatment is performed using sulfuric acid hydrogen peroxide solution instead of ammonia hydrogen peroxide solution in the treatment of the silicon wafer according to condition 3. That is, after sample processing is performed on an n-type 10 Ωcm silicon wafer in the furnace A or the furnace B, the sample wafer is stored in a carrier box and exposed to the same environment as when transported over a long distance in summer. After that, cleaning with sulfuric acid and hydrogen peroxide is first performed to remove organic contamination on the wafer surface. Next, a natural oxide film on the wafer surface is removed with a HF solution, and a pure rinse treatment is performed, followed by a passivation treatment using an iodine ethanol method. Minority carrier recombination lifetime is measured by reflected microwave photoconductive decay.

  Five silicon wafers each treated in furnace A and furnace B were subjected to each treatment of minority carrier recombination lifetime measurement including conditions 1 to 4 and the recombination lifetime was measured. FIG. 3 shows the lifetime measurement results of five wafers subjected to the sample processing in the furnace A, and FIG. 4 shows the lifetime measurement results of five wafers subjected to the sample processing in the furnace B.

  As shown in FIG. 3, the lifetime measured value of the sample wafer with a high contamination level processed in the furnace A was a low value of around 1000 μs under any condition. Among them, it was confirmed that the lifetime measurement value of the silicon wafer that had undergone the treatment of condition 2 was slightly lower than that of the silicon wafer that had undergone the treatments of other conditions 1, 3, and 4. However, the difference is not large, and it is recognized that such a silicon wafer having a somewhat high contamination level has little influence on the recombination lifetime measurement value of minority carriers of organic substances attached to the silicon wafer.

  On the other hand, as shown in FIG. 4, the lifetime measurement value of the sample wafer with the low contamination level processed in the furnace B is a value of 5000 μs to 6000 μs for the silicon wafers subjected to the processing of the conditions 1, 3 and 4, and Even the processed silicon wafer has a value of about 2000 μs. From FIG. 4, it can be seen that, in a silicon wafer with a low contamination level, the adhesion of organic matter to the silicon wafer has a great influence on the recombination lifetime measurement of minority carriers. Then, by performing the organic contaminant removal process according to the present invention, that is, through the processes of conditions 3 and 4, the organic substance adhering to the silicon wafer is appropriately removed, and the minority carriers are recombined immediately after the sample process. It was found that the recombination lifetime of minority carriers can be measured without the influence of organic contamination similar to the measurement of lifetime.

It is a figure which shows the relationship between the bulk lifetime for every surface recombination velocity of a silicon wafer, and a measurement lifetime. It is a figure which shows the flow of the recombination lifetime measurement process of the minority carrier of the silicon wafer including the pre-processing process which concerns on embodiment of invention. It is a figure which shows the lifetime measurement result for every pre-processing condition with respect to the silicon wafer with a high contamination level. It is a figure which shows the lifetime measurement result for every pre-processing conditions with respect to the silicon wafer with a low contamination level.

Claims (5)

A pre-processing method for measuring the recombination lifetime of silicon wafer carriers by the reflected microwave photoconductive decay method,
A natural oxide film removing step for removing the natural oxide film of the silicon wafer to be measured;
A passivation step of passivating the silicon wafer from which the natural oxide film has been removed by an iodine ethanol method,
A pretreatment method for recombination lifetime measurement, comprising: an organic substance removing step of removing an organic substance adhering to the surface of the silicon wafer before removing the natural oxide film in the natural oxide film removing step.   The pretreatment method for recombination lifetime measurement according to claim 1, wherein in the organic matter removing step, the organic matter adhering to the surface of the silicon wafer is removed by wet treatment.   3. The recombination according to claim 2, wherein in the wet treatment in the organic matter removing step, the organic matter adhering to the surface of the silicon wafer is removed using a mixed solution of ammonia water and hydrogen peroxide solution. A pre-processing method for lifetime measurement.   3. The recombination life according to claim 2, wherein in the wet treatment in the organic matter removal step, the organic matter adhering to the surface of the silicon wafer is removed using a mixed solution of sulfuric acid and hydrogen peroxide solution. Preprocessing method for time measurement.   The pretreatment method for recombination lifetime measurement according to claim 2, wherein in the wet treatment in the organic matter removal step, the organic matter adhering to the surface of the silicon wafer is removed using ozone water.

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