JP2005216993A - Evaluation method for silicon wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an evaluation method for a silicon wafer by which a defect of shallow recession (pit) or the like usually difficult to detect as well as a crystal defect such as COP, is detected at high sensitivity to enable the evaluation of the silicon wafer in a highly precise and reliable manner. <P>SOLUTION: According to the evaluation method for the silicon wafer, an oxide film and an electrode are formed sequentially on the silicon wafer to form a MOS capacitor. An electric field is applied to the oxide film via the electrode to measure a dielectric breakdown characteristic of the oxide film. The evaluation method includes a pre-applying process of applying an electric field to the oxide film, and a dielectric breakdown characteristic measuring process of applying a certain current or a voltage to the oxide film of the MOS capacitor that has not broken down in the pre-applying process, prior to dielectric breakdown characteristic measuring process, to cause a secular dielectric breakdown in the oxide film and measure its dielectric breakdown characteristic. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for evaluating a silicon wafer. Specifically, a MOS (Metal Oxide Semiconductor) capacitor is fabricated on a silicon wafer, and the dielectric breakdown characteristics of the oxide film are measured to evaluate the silicon wafer. It is about how to do.

  As an effective method for evaluating crystal defects such as COP existing in silicon wafers, there is GOI (Gate Oxide Integrity) evaluation (see, for example, Non-Patent Document 1), which is widely used. Yes. When performing this GOI evaluation, for example, a silicon wafer is first oxidized to form a silicon oxide film (gate oxide film) which becomes an insulating film on its main surface, and a polysilicon method is formed on the formed oxide film by a CVD method or the like. After a silicon layer (polycrystalline silicon layer) is deposited, an electrode is formed on the polysilicon layer by photolithography and polysilicon etching. As a result, a capacitor (MOS capacitor) having a MOS structure on the silicon wafer is manufactured.

  The GOI can be evaluated by applying an electric field to the oxide film through the polysilicon electrode of the MOS capacitor and measuring the dielectric breakdown characteristics of the oxide film. At this time, as a method for measuring the dielectric breakdown characteristics of the oxide film, there is a TZDB (Time Zero Dielectric Breakdown) method for detecting the dielectric breakdown electric field strength of the oxide film.

  In this TZDB method, the current value flowing through the oxide film is monitored while applying an electric field to the oxide film while gradually increasing the electric field, and when the current value reaches the judgment current value, that is, the oxide film is The silicon wafer can be evaluated by measuring the electric field strength when the dielectric breakdown (breakdown) occurs. For example, a breakdown that is generally caused by an electric field strength of 3 to 8 MV / cm is called a B mode, and is known to be caused by a crystal defect called a COP (Crystal Originated Particle) in a silicon wafer. An oxide film having an electric field strength of 8 MV / cm or more (C mode) can be determined as a good product, and the quality of the silicon wafer can be evaluated based on the ratio of the good product to the total number of MOS capacitors measured.

  In the GOI evaluation of an oxide film, a TDDB (Time Dependent Dielectric Breakdown) method is sometimes used in which the dielectric breakdown of the oxide film is measured by causing dielectric breakdown over time. In this TDDB method, a constant current or voltage is continuously applied to the oxide film, and the electric field strength is detected at predetermined time intervals to determine the change over time, and the current until the oxide film reaches dielectric breakdown. Alternatively, the silicon wafer can be evaluated by determining the life of the oxide film and the quality of the film by measuring the voltage application time and the total amount of charge flowing in the oxide film.

  Furthermore, since such a TDDB method requires a long measurement time, for example, in Patent Document 1, the dielectric breakdown is evaluated to the same extent as the TDDB method by using the TZDB method that can be evaluated in a short time. An insulating layer evaluation method that can be used is disclosed. In the evaluation method of Patent Document 1, the operation of the TZDB method for detecting the breakdown electric field strength by stepping up the voltage to be applied is performed a plurality of times on the same insulating film of a semiconductor element such as a MOS capacitor. The evaluation equivalent to the TDDB method can be performed in a short time.

  And, in the silicon wafer evaluation by such TZDB method or TDDB method, there are oxide film pinholes, COPs, and oxygen precipitation nuclei as the main causes determined to be defective. Among these, if there is an oxide film pinhole, it conducts immediately after applying a voltage or current in the GOI evaluation and breaks down. Such a defect is called an A mode defect. COP, which is the cause of the aforementioned B-mode failure, is a cavity defect having an octahedral structure that occurs during crystal growth when a single crystal is grown.

  If such a COP exists on the main surface of the silicon wafer, for example, as shown in FIG. 6, when the oxide film 2 is formed on the main surface of the silicon wafer 3, the silicon oxide film 12 is also formed on the inner wall of the cavity of the COP. It is formed. The inner wall oxide film 12 of the COP has a thin oxide film at the corner (corner) of the octahedral structure. Therefore, by forming a polysilicon electrode 4 on the oxide film 2 and then applying an electric field, electrical stress is concentrated on the thinned portion of the oxide film, so that breakdown occurs at a low electric field strength. It is considered to be. Therefore, in the above-described TZDB method and TDDB method, by utilizing such a phenomenon, it is possible to evaluate the silicon wafer by detecting COP or the like existing in the silicon wafer with high sensitivity.

  As described above, an oxide film having a dielectric breakdown electric field strength of 8 MV / cm or more is called a C mode and is judged as a non-defective product. An electric field with an electric field strength of 8 MV / cm or more is applied to this good product. There is an oxide film that is destroyed by an oxide film and an oxide film that is not destroyed even when an electric field of 8 MV / cm or more is applied. One cause of the destruction of the oxide film that is destroyed by applying an electric field of 8 MV / cm or more is an oxygen precipitation nucleus (see, for example, Patent Document 2). Since the oxygen precipitation nuclei are considerably smaller in size than the COP, the oxide film in which the oxygen precipitation nuclei are present on the wafer surface breaks down on the electric field side higher than the electric field strength of 3 to 8 MV / cm at which the B mode failure occurs. Therefore, among the C-mode oxide films determined to be non-defective, an oxide film that is destroyed by applying an electric field of 8 MV / cm or more is generated.

  On the other hand, on the main surface of the silicon wafer, in addition to the COP and oxygen precipitation nuclei, there are concave defects (hereinafter referred to as pits) having a shallow depth. Such pits are thought to be mainly caused by damage during silicon wafer processing or COP that could not be completely eliminated by annealing performed on annealed wafers that have recently been used. Have. In recent years, with the high integration and miniaturization of semiconductor devices, it has been required to accurately evaluate the pits existing on the main surface of the wafer in order to improve the quality of the silicon wafer. However, even if such a concave pit is formed with an oxide film and an electrode on the wafer and subjected to the TZDB method or the TDDB method, dielectric breakdown that can be judged as defective is unlikely to occur. There was a problem that it was difficult to detect with high sensitivity with sufficient sensitivity and it was difficult to distinguish it from non-defective products.

JP-A-9-178800 Japanese Patent Laid-Open No. 2002-231777 M.M. Tamatsuka et al. , "Medium Field Breakdown Origin on Metal Oxide Semiconductor Capacitor Containing Growing-in Czochralski Silicon Crystal Defects," Jpn. J. et al. Appl. Phys. 37, 1236 (1998).

  Therefore, the present invention has been made in view of the above problems, and the object of the present invention is not only crystal defects such as COP but also shallow concave defects (pits) that have been difficult to detect conventionally. Is to provide a silicon wafer evaluation method that can detect silicon wafers with high sensitivity and evaluate silicon wafers with high accuracy and high reliability.

  In order to achieve the above object, according to the present invention, an oxide film and an electrode are sequentially formed on a silicon wafer to produce a MOS capacitor, and then an electric field is applied from the formed electrode to the oxide film. In the method of evaluating a silicon wafer by measuring the dielectric breakdown characteristics of the MOS capacitor, first, after performing a pre-application step of applying an electric field to the oxide film, the oxide film of the MOS capacitor that did not break down in the pre-application step The silicon wafer is evaluated by performing a dielectric breakdown characteristic measurement step of applying a constant current or voltage to the oxide film to cause dielectric breakdown over time in the oxide film and measuring the dielectric breakdown characteristics. A characteristic silicon wafer evaluation method is provided (claim 1).

  Thus, after manufacturing a MOS capacitor on a silicon wafer, a pre-application step of applying an electric field to the oxide film is performed, for example, an oxide film formed at the position of a concave pit existing on the wafer main surface Strain can be formed by applying stress due to electric field concentration on the uneven part of the film thickness, and then a constant current or voltage is continuously applied to the oxide film to cause dielectric breakdown over time in the oxide film. By performing a dielectric breakdown characteristic measurement process (hereinafter, simply referred to as a measurement process) for measuring the dielectric breakdown characteristics, it is possible to easily cause dielectric breakdown of an oxide film in which a strain is formed. Therefore, by evaluating the silicon wafer in this way, it becomes possible to detect a pit having a shallow concave shape with a high sensitivity, which has been difficult to detect in the past, and the silicon wafer can be evaluated with high accuracy. It can also be performed with high reliability.

At this time, in the pre-application step, it is preferable to apply an electric field to the oxide film with an electric field strength of 5 MV / cm or more and 15 MV / cm or less at maximum.
In this way, by setting the maximum electric field strength of the electric field applied to the oxide film in the pre-application process to 5 MV / cm or more, stress is applied to the oxide film at the position where the pit exists to surely form distortion. Can do. Further, by setting the maximum electric field strength of the electric field applied to the oxide film to 15 MV / cm or less, more preferably 8 MV / cm or more and 15 MV / cm or less, the oxide film destroyed in the conventional A, B, and C modes The oxide film that is not destroyed can be clearly and stably separated.

  In the pre-application step, it is preferable to apply an electric field by gradually increasing the electric field strength stepwise in the oxide film (Claim 3), and when applying an electric field in the pre-application step, The dielectric breakdown electric field strength of the oxide film in which the dielectric breakdown has occurred can be detected.

  In this way, by applying the electric field by gradually increasing the electric field strength of the electric field applied to the oxide film in the pre-application step in a stepwise manner, it is possible to stably apply the electric field having a desired strength in a short time. it can. If the electric field strength is gradually increased stepwise in this way and the electric field is applied, the dielectric breakdown electric field strength of the oxide film in which the dielectric breakdown has occurred can be easily measured. Defects can be detected with high accuracy, and silicon wafers can be evaluated with higher accuracy.

Furthermore, in the dielectric breakdown characteristic measurement step, it is preferable that an electric field is applied to the oxide film with an electric field strength of 5 MV / cm or more and 15 MV / cm or less in terms of electric field strength.
In this way, by applying an electric field with an electric field strength of 5 MV / cm or more and 15 MV / cm or less to the oxide film in the measurement process, the oxide film formed at the position of the pits existing on the main surface of the wafer is surely broken down. The dielectric breakdown characteristics can be measured with high accuracy.

In this case, in the dielectric breakdown characteristic measurement step, it is preferable to measure the dielectric breakdown characteristics of the oxide film by applying a current or voltage while the oxide film is heated.
As described above, in the measurement process, by applying a current or a voltage while the oxide film is heated, the oxide film at the place where the pit is present can be broken down more quickly, easily and stably. it can.

In the method for evaluating a silicon wafer according to the present invention, the dielectric breakdown characteristics are measured in the dielectric breakdown characteristics measurement step. When a constant current is applied to the oxide film, the change with time of the voltage applied to the oxide film is measured. Monitoring is performed by determining that dielectric breakdown occurs when a voltage change of 1 MV / cm or more occurs, and when applying a constant voltage to the oxide film, monitor the change with time of the current flowing in the oxide film. Thus, it is preferable to perform a dielectric breakdown when a current change of 0.01 mA / cm ² or more occurs.
In the evaluation method of the present invention, the dielectric breakdown can be determined in the measurement process in this way, and thereby the dielectric breakdown characteristics of the oxide film can be stably measured with high accuracy.

In particular, in the present invention, a plurality of MOS capacitors can be manufactured on the silicon wafer, and the dielectric breakdown characteristics of the oxide film can be measured using the manufactured MOS capacitors.
As described above, by fabricating a plurality of MOS capacitors on a silicon wafer and measuring the dielectric breakdown characteristics of the oxide film using these MOS capacitors, the quality of the oxide film can be determined stably by the plurality of MOS capacitors. Therefore, the silicon wafer can be evaluated with higher accuracy and higher reliability.

  According to the silicon wafer evaluation method of the present invention, an oxide film at a position where a pit having a shallow concave shape exists can be easily broken down. It becomes possible to detect a shallow concave pit, which has been difficult to detect, with high sensitivity as a defect, and it is possible to evaluate a silicon wafer with high accuracy and high reliability.

Hereinafter, although an embodiment is described about the present invention, the present invention is not limited to these.
In general, pits existing on the main surface of a silicon wafer have a shallow concave shape and a gentle shape unlike COP. Therefore, for example, as shown in FIG. 5, when a MOS capacitor is manufactured by forming the oxide film 2 and the polysilicon electrode 4 on the silicon wafer 3, the conventional oxide film in a portion where only such pits exist is used. Even if an electric field that causes dielectric breakdown occurs in the oxide film where the COP exists as in the GOI evaluation, there is no portion where electric field concentration occurs around the pit, and it is difficult to cause dielectric breakdown. It is thought that the detection of was difficult.

  Therefore, the present inventor has conducted extensive experiments and studies on a method that can detect such defects caused by pits existing on the main surface of the wafer by measuring the dielectric breakdown characteristics of the oxide film. As a result, in order to break down the oxide film formed at the position where such pits exist, electric field application for forming distortion in the oxide film and measurement of the breakdown characteristics of the oxide film are performed. It is considered that it is sufficient to apply electric field twice, and by applying such electric field twice, it is difficult not only to detect defects caused by COP and the like but also to detect by conventional GOI evaluation. The present inventors have completed the present invention by discovering that defects caused by concave pits having a shallow depth can be detected with high sensitivity and silicon wafers can be evaluated with high accuracy.

  That is, in the silicon wafer evaluation method of the present invention, an oxide film and an electrode are sequentially formed on a silicon wafer to fabricate a MOS capacitor, and then an electric field is applied from the formed electrode to the oxide film to insulate the oxide film. In a method of evaluating a silicon wafer by measuring breakdown characteristics, first, after performing a pre-application step of applying an electric field to the oxide film, a constant current or voltage is applied to the oxide film, and the oxide film It is characterized in that the silicon wafer is evaluated by performing a dielectric breakdown characteristic measuring step of measuring dielectric breakdown characteristics by causing dielectric breakdown over time.

  Hereinafter, although the silicon wafer evaluation method according to the present invention will be described in detail with reference to the drawings, the present invention is not limited to this. FIG. 1 is a flow chart showing an example of the silicon wafer evaluation method of the present invention.

(Production of MOS capacitor)
First, a silicon wafer to be evaluated is prepared, and the silicon wafer is oxidized to form an oxide film (gate oxide film). This oxide film can be easily formed by, for example, placing a silicon wafer on a boat, putting it in a horizontal heat treatment furnace or a vertical heat treatment furnace, and performing heat treatment in an oxygen atmosphere.

  Next, a polysilicon film (polycrystalline silicon film) is grown on the oxide film formed on the silicon wafer. This polysilicon film is grown by, for example, introducing a silicon wafer taken out of a heat treatment furnace into a CVD (Chemical Vapor Deposition) apparatus and introducing a growth gas such as monosilane into the reaction vessel of the apparatus under reduced pressure or normal pressure. Can be made. Then, after depositing the polysilicon layer as described above, impurities such as phosphorus are doped into the polysilicon layer using a thermal diffusion method or an ion implantation method to form a polysilicon layer having a low resistivity. It is also possible to form a low resistivity polysilicon layer by simultaneously doping impurities during the deposition of the polysilicon layer.

  After that, the low resistivity polysilicon layer formed as described above is subjected to a series of photolithography processes such as resist coating, exposure, and development, followed by an etching process, whereby a polycrystal is formed at a desired position on the oxide film. A silicon electrode can be formed. In this manner, a MOS capacitor in which an oxide film and a polysilicon electrode are formed on a silicon wafer can be manufactured.

(Pre-application process)
Next, as shown in FIG. 2, the silicon wafer 3 on which the oxide film 2 and the polysilicon electrode 4 are formed on the main surface is placed on the stage 5 of the electric field strength measuring apparatus 10 and then supported so as to be movable. The lower end of the probe 7 is brought into contact with the polysilicon electrode 4 of the MOS capacitor 1. The probe 7 is connected to one terminal of a variable power source 6 that can change the magnitude of applied voltage or current. On the other hand, the other terminal of the variable power source 6 is connected to the stage 5 of the electric field strength measuring device 10. It is connected. Further, a voltmeter 8 for measuring the applied voltage is connected in parallel to the variable power source 6, and an ammeter 9 is interposed between the probe 7 and the variable power source 6.

  When the probe 7 is brought into contact with the polysilicon electrode 4 using such an electric field strength measuring device 10, the variable power source 6 is turned on, and an electric field is applied to the oxide film 2 of the MOS capacitor 1 to perform a pre-application process. Can do. At this time, in the evaluation method of the present invention, it is important to apply an electric field with an electric field strength large enough to form a strain in a non-uniform thickness portion of the oxide film where pits exist. By applying an electric field in the pre-application process in this way, electric field concentration is caused by a non-uniform thickness portion (pit corner portion) of the oxide film formed at the position of the concave pit existing on the main surface of the wafer. A strain can be easily formed by applying stress.

  In this case, the magnitude of the electric field strength of the electric field applied to the oxide film is preferably 5 MV / cm or more and 15 MV / cm or less. If the electric field strength of the electric field applied to the oxide film in this pre-application process is less than 5 MV / cm, the stress applied to the oxide film is too small to form sufficient strain, and pits exist in the subsequent measurement process. It is conceivable that the oxide film does not break down at the positions where the Therefore, by setting the electric field strength of the electric field applied to the oxide film to 5 MV / cm or more in this way, sufficient strain is applied to the non-uniform thickness portion of the oxide film at the pit position to reliably form the distortion. be able to. Further, by setting the electric field strength of the electric field applied to the oxide film to 15 MV / cm or less, more preferably 8 MV / cm to 15 MV / cm, it is caused by A mode, B mode, and oxygen precipitation nuclei in the pre-application process. Thus, the oxide film destroyed in the C mode and the oxide film not destroyed can be clearly and stably separated.

  In the present invention, the method of applying an electric field to the oxide film in the pre-application process is not particularly limited. For example, as shown in FIG. 3A, the voltage is gradually increased stepwise using a variable power source. It is preferable to apply (in this case, the current flowing through the oxide film shows a transition as shown in FIG. 3B). By applying such a voltage in the pre-application process, the electric field can be applied to the oxide film in such a way that the electric field strength gradually increases stepwise, so that the electric field of the desired strength can be stabilized in a short time. Can be applied.

Then, if the electric field strength is gradually increased stepwise in this way and an electric field is applied, the dielectric breakdown electric field strength of the oxide film in which dielectric breakdown has occurred in the A, B and C modes can be easily detected, As a result, it is possible to accurately measure A-mode dielectric breakdown caused by short-circuits such as pinholes in the oxide film, B-mode caused by crystal defects such as COP, and dielectric breakdown caused by oxygen precipitation nuclei. The silicon wafer can be evaluated with higher accuracy.
When the application of the electric field with the desired electric field intensity is completed in the pre-application process in this way, the variable power source 6 is turned off and the probe 7 is separated from the electrode 4.

(Dielectric breakdown characteristic measurement process)
Subsequently, after performing the pre-application process as described above, a dielectric breakdown characteristic measurement process is performed in which the probe is brought into contact with the polysilicon electrode again to apply a constant current or voltage from the variable power source to the oxide film. For example, as shown in FIG. 4A, by continuously applying a constant current from the variable power source to the oxide film (in this case, the voltage of the oxide film is as shown in FIG. 4B). ) Since the oxide film at the position where the pit is present in the above pre-application step is distorted and the film quality is deteriorated, the oxide film at the position of the pit easily causes dielectric breakdown over time, The dielectric breakdown characteristics of the oxide film can be measured with high accuracy. In the present invention, after performing the pre-application step, the measurement step can be continuously performed without separating the probe from the electrode.

  At this time, the magnitude of the constant current or voltage applied to the oxide film is preferably 5 MV / cm or more and 15 MV / cm or less in terms of electric field strength. When the electric field strength of the electric field applied to the oxide film is less than 5 MV / cm, the stress applied to the oxide film is too small, and it takes time to cause dielectric breakdown in the oxide film at the position where the pit exists. There is a risk of increasing costs. Further, if the electric field strength of the electric field applied to the oxide film exceeds 15 MV / cm, the measurement accuracy of the determination voltage or the determination current may be lowered, and it may be difficult to accurately measure the dielectric breakdown characteristics of the oxide film. There is. Therefore, by applying an electric field to the oxide film with an electric field strength of 5 MV / cm or more and 15 MV / cm or less in terms of electric field strength in the measurement process, the oxide film at the pit position can be reliably broken down, and the insulation Destructive characteristics can be measured with high accuracy.

  Furthermore, when measuring the dielectric breakdown characteristics of the oxide film, it is preferable to apply a current or voltage while the oxide film is heated. If the current or voltage is applied while the oxide film is heated in this way, the oxide film at the location where the pits are present can be more easily broken down in a short time.

Further, in such a measurement process, the dielectric breakdown characteristics of the oxide film are such that, for example, when a constant current is applied to the oxide film, a change with time of the voltage applied to the oxide film is monitored, and a voltage of 1 MV / cm or more When a change occurs, it is determined that the dielectric breakdown occurs, and when a constant voltage is applied to the oxide film, a change in current flowing through the oxide film is monitored, and a current change of 0.01 mA / cm ² or more occurs. By determining the dielectric breakdown at the time, it is possible to measure stably with high accuracy.
When the measurement of the dielectric breakdown characteristics of the oxide film is completed, the variable power source 6 is turned off and the probe 7 is separated from the electrode.

  As described above, according to the present invention, an oxide film formed at a position where a pit having a shallow concave shape exists can be easily broken down. Therefore, an oxide film formed on a wafer can be obtained. By measuring the dielectric breakdown characteristics, it is possible to detect not only crystal defects such as COP and oxygen precipitation nuclei, but also defects such as shallow concave pits that have been difficult to detect with high sensitivity. Therefore, it is possible to discriminate between an oxide film which is defective due to the presence of crystal defects and pits such as COP and oxygen precipitation nuclei and a good oxide film which does not have crystal defects and pits and is not destroyed, more accurately than before. As a result, the silicon wafer can be evaluated with high accuracy and high reliability.

  In particular, in the present invention, a plurality of MOS capacitors can be fabricated on a silicon wafer, and the dielectric breakdown characteristics of the oxide film can be measured using the fabricated plurality of MOS capacitors. Therefore, the silicon wafer can be evaluated with higher accuracy and higher reliability.

EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited to these.
(Example)
First, as a sample to be evaluated, 100 P-type silicon wafers having a diameter of 200 mm and doped with boron were prepared. Next, these silicon wafers were placed on a boat and placed in a vertical heat treatment furnace, and heat treatment was performed in a dry oxygen atmosphere at 900 ° C. to form a 25 nm oxide film on the main surface of the wafer. Thereafter, the silicon wafer on which the oxide film was formed was put into a CVD apparatus, and a polysilicon layer was grown on the oxide film while doping phosphorus. At this time, the thickness of the grown polysilicon layer is 300 nm, and the resistance value is about 25 Ω / sq. Met.

Subsequently, after patterning the polysilicon layer by a photolithography process, the polysilicon layer is removed by wet etching with hydrofluoric acid in the etching process, so that the MOS capacitor using the polysilicon layer as an electrode is formed on the wafer surface. 101 pieces were produced. The electrode area of the MOS capacitor fabricated this time was set to 8 mm ² . Thereafter, in order to remove the silicon oxide film formed on the back surface of the silicon wafer, a resist was applied to the front surface side of the silicon wafer, and wet etching with dilute hydrofluoric acid was performed.

  Five wafers were extracted from the 100 silicon wafers on which the MOS capacitors were manufactured in this way, and a pre-application process was performed on each silicon wafer using a full-auto prober field strength measuring device. In this pre-application process, an electric field was applied to the oxide film by gradually increasing the electric field strength from 0 MV / cm to 15 MV / cm in steps of 0.25 MV / cm (in the case of an oxide film thickness of 25 nm). The voltage is applied in steps of 0.625 V up to 37.5 V). FIG. 7 shows an example of an IV curve of a MOS capacitor in which the silicon oxide film is not broken due to an accidental failure such as a pinhole in such a pre-application process.

Next, the dielectric breakdown characteristics of the oxide film were measured by continuously applying a constant current to the oxide film subjected to the pre-application process and monitoring the change with time of the voltage applied to the oxide film. In this measurement step, the current stress applied to the oxide film is 10 mA / cm ² (approximately 12 MV / cm when converted to electric field strength). Further, when applying the current, heating was performed so that the temperature of the oxide film became 100 ° C. in order to shorten the measurement time.

  Then, the pre-application process and the measurement process under the above conditions were performed on 99 of the MOS capacitors formed on the silicon wafer, and the dielectric breakdown characteristics of the oxide film of each MOS capacitor were measured (the remaining two MOSs). The capacitor was used for alignment when the wafer was placed on the stage of the electric field strength measuring device, so dielectric breakdown characteristics were not measured).

With respect to the measurement results of the dielectric breakdown characteristics of the oxide film thus obtained, the oxide film that was broken down immediately after applying the current in the measurement process was an initial failure, and the breakage occurred when the amount of electricity per unit area was less than 5 C / cm ^2. A silicon wafer is determined by determining the oxide film as a good product by the accidental failure of the oxide film that has been down, and the oxide film that has broken down due to the amount of electricity per unit area exceeding 5 C / cm ² and the oxide film that has not broken down. Was evaluated. FIG. 8 shows the wafer in-plane distribution of the evaluation results for one of the evaluated silicon wafers. In FIG. 8 and FIG. 9 described below, the star is a non-defective product, A is an initial failure, B is an accidental failure, and a blank indicates a location where dielectric breakdown characteristics were not measured.

  Furthermore, Table 1 shows the evaluation results of five silicon wafers evaluated under the same conditions. As shown in Table 1, all of the five silicon wafers evaluated by performing the pre-application process and the measurement process showed evaluation results with almost the same non-defective product rate.

(Comparative example)
Five other wafers were extracted from the 100 silicon wafers produced above, and the dielectric breakdown characteristics of the oxide films were measured by applying a constant current to the oxide films without performing a pre-application process on each of the silicon wafers. Only the measurement step to be performed was performed under the same conditions as in the above example. FIG. 9 shows the wafer in-plane distribution of the evaluation results for one of the evaluated silicon wafers. Table 2 shows the evaluation results of five silicon wafers evaluated under the same conditions. As shown in Table 2, all the five silicon wafers evaluated by performing only the measurement process showed evaluation results that yielded almost the same non-defective product rate.

  As is clear from Table 1 and Table 2 above, it can be seen from the evaluation results of the examples that the number of initial defects and accidental defects occurring in the wafer surface is increased compared to the comparative example. As a reason why the number of initial defects and accidental defects is increased in the evaluation results of the examples, an oxide film that is determined as a random defect in the comparative example is easily detected as an initial defect by the evaluation method of the present invention. In addition, it is conceivable that a defect caused by a pit or the like that has not been detected in the comparative example is likely to be detected as an initial defect or an accidental defect depending on its shape and size. From these results, it can be seen that according to the present invention, defects such as pits that have been difficult to detect in the past can be detected with high accuracy, and the wafer can be evaluated with higher accuracy.

  The present invention is not limited to the above embodiment. The above embodiment is merely an example, and the present invention has the same configuration as that of the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

  For example, in the above description, the case where the electric field is applied by gradually increasing the electric field strength when the electric field is applied to the oxide film in the pre-application process is described as an example, but the present invention is limited to this. For example, in the pre-application process, an electric field can be continuously applied to the oxide film with a constant electric field strength. In this case, for example, in the pre-application step, an electric field is applied to the oxide film with an electric field strength of about 10 MV / cm, and then in the dielectric breakdown characteristic measurement step, an electric field is applied to the oxide film with an electric field strength of about 12 MV / cm. It is only necessary to measure the dielectric breakdown characteristics, and thereby, the same effect as described above can be obtained.

It is a flowchart which shows an example of the evaluation method of the silicon wafer of this invention. It is a schematic explanatory drawing which represents roughly the structure of the electric field strength measuring apparatus used with the evaluation method of this invention. (A) is a graph showing the relationship between the electric field intensity applied in a pre-application process, and time, (b) is a graph showing the relationship between the electric current which flows into an oxide film, and time. (A) is a graph showing the relationship between the current applied to the oxide film in the dielectric breakdown characteristic measurement step and time, and (b) is a graph showing the relationship between the electric field strength applied to the oxide film and time. . It is a schematic explanatory drawing which illustrates roughly the state of the MOS capacitor formed in the location where a concave pit exists. It is a schematic explanatory drawing which illustrates roughly the state of the MOS capacitor formed in the location where COP exists. It is a graph showing the IV curve of the MOS capacitor which was not dielectrically broken by the pre-application process. It is a map figure which shows wafer surface distribution of the result of having evaluated the silicon wafer in the Example. It is a map figure which shows wafer surface distribution of the result of having evaluated the silicon wafer in the comparative example.

Explanation of symbols

1 ... MOS capacitor, 2 ... oxide film, 3 ... silicon wafer,
4 ... polysilicon electrode, 5 ... stage, 6 ... variable power supply,
7 ... probe, 8 ... voltmeter, 9 ... ammeter,
DESCRIPTION OF SYMBOLS 10 ... Electric field strength measuring device 12 ... The oxide film formed in the cavity inner wall of COP.

Claims (8)

  After forming an oxide film and an electrode sequentially on a silicon wafer to produce a MOS capacitor, the silicon wafer is evaluated by applying an electric field to the oxide film from the formed electrode and measuring the dielectric breakdown characteristics of the oxide film. In the method, first, after performing a pre-application step of applying an electric field to the oxide film, a constant current or voltage is applied to the oxide film of the MOS capacitor that did not break down in the pre-application step, and the oxide film A method for evaluating a silicon wafer, comprising: evaluating the silicon wafer by performing a dielectric breakdown characteristic measurement step of measuring dielectric breakdown characteristics by causing dielectric breakdown over time.   2. The method for evaluating a silicon wafer according to claim 1, wherein in the pre-application step, an electric field is applied to the oxide film with an electric field strength of 5 MV / cm to 15 MV / cm at the maximum.   3. The method for evaluating a silicon wafer according to claim 1, wherein, in the pre-application step, an electric field is applied to the oxide film by gradually increasing the electric field strength stepwise.   4. The silicon wafer according to claim 1, wherein when an electric field is applied in the pre-application step, a dielectric breakdown electric field strength of an oxide film in which dielectric breakdown has occurred is detected. 5. Evaluation methods.   5. The electric field is applied to the oxide film at an electric field strength of 5 MV / cm or more and 15 MV / cm or less in terms of electric field strength in the dielectric breakdown characteristic measurement step. The silicon wafer evaluation method described.   6. The dielectric breakdown characteristic measurement step of measuring a dielectric breakdown characteristic of the oxide film by applying a current or a voltage while the oxide film is heated in the step of measuring the dielectric breakdown characteristic. The method for evaluating a silicon wafer according to claim 1. In the measurement of dielectric breakdown characteristics in the dielectric breakdown characteristic measurement step, when a constant current is applied to the oxide film, a change with time of the voltage applied to the oxide film is monitored, and a voltage change of 1 MV / cm or more is observed. When it occurs, it is determined by dielectric breakdown, and when a constant voltage is applied to the oxide film, a change in current flowing through the oxide film is monitored, and a current change of 0.01 mA / cm ² or more is monitored. The method for evaluating a silicon wafer according to claim 1, wherein the evaluation is performed by determining that the dielectric breakdown occurs when the failure occurs.   8. A plurality of MOS capacitors are fabricated on the silicon wafer, and dielectric breakdown characteristics of an oxide film are measured using the fabricated plurality of MOS capacitors. The silicon wafer evaluation method described.

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