JP2010114282A - Method for evaluation of metal pollution in semiconductor substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for simply and highly accurately analyzing metal impurities that have adverse effects on the characteristics of a semiconductor substrate. <P>SOLUTION: A method for evaluation of the metal contamination in the semiconductor substrate includes a step of forming a roughened area on one surface out of both sides of the semiconductor substrate, a step of heating the semiconductor substrate after forming the roughened area on one surface and a step for analysis of a metal component in a solution after scanning the roughened area with the solution. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for evaluating metal contamination of a semiconductor substrate, and more particularly, to a method for evaluating metal contamination of a semiconductor substrate capable of accurately and simply evaluating a metal impurity concentration in a semiconductor substrate that adversely affects the characteristics of the semiconductor substrate. Is,

  In the semiconductor manufacturing field, with the high integration of circuits and the miniaturization of devices, it has become an important issue to reduce the amount of metal impurities present in a semiconductor substrate that adversely affects device performance. Therefore, it is desired to establish an analysis method capable of easily and accurately evaluating metal impurities contained in a semiconductor substrate.

The following methods are known as methods for evaluating metal impurities in a semiconductor substrate.
(1) A method in which a semiconductor substrate is dissolved in a large amount of a mixed solution of hydrofluoric acid and nitric acid, and the acid solution is diluted or concentrated, and quantitative analysis is performed with an atomic absorption photometer (AAS) or the like.
(2) A method in which a semiconductor substrate is decomposed with acid vapor, the decomposition residue is post-treated with an acid, and quantitative evaluation is performed with an AAS or an inductively coupled plasma mass spectrometer (ICP-MS) (see Patent Document 1).
(3) A method for evaluating Cu contamination in a semiconductor substrate processing process using a silicon wafer having a polysilicon layer (see Patent Document 2).
(4) A method in which a silicon substrate is heat-treated and Cu is diffused outwardly on the surface to analyze Cu on the substrate surface (see Patent Document 3).
Japanese Patent No. 3832204 JP 2007-227516 A JP-A-9-64133

Since the method (1) uses a large amount of acid for dissolving the substrate, the lower detection limit is inevitably increased. Therefore, there is a problem of insufficient sensitivity in a semiconductor substrate that requires high cleanliness.
In the above method (2), since a large amount of silicon residue is present in the sample, when an acid solution is introduced as it is into ICP-MS, which is a highly sensitive analyzer, accurate analysis becomes difficult due to interference of silicon molecules. Further, in order to suppress the interference of the silicon molecules, a pretreatment that requires a long time is required to remove the silicon residue outside the analysis sample system.
On the other hand, although the method (3) is simpler than the methods (1) and (2) because it is not necessary to decompose the substrate, a polysilicon layer for capturing Cu in the semiconductor substrate is used as the semiconductor. A step of forming on the substrate surface, a step of removing the polysilicon layer on one side of the semiconductor substrate in order to diffuse Cu in the semiconductor substrate only on one side, a Cu layer in the semiconductor substrate being diffused into the polysilicon layer and trapped in the polysilicon layer A heat treatment step for quantitatively evaluating Cu trapped in the polysilicon layer, dissolving a polysilicon layer of a semiconductor substrate with a mixed acid of hydrofluoric acid and nitric acid, and quantifying a Cu component contained in the mixed acid; It takes a lot of time and labor. Furthermore, it is necessary to install and maintain an expensive heat treatment furnace for forming a polysilicon layer on the semiconductor substrate. Further, in the process of dissolving the polysilicon layer with a mixed acid of hydrofluoric acid and nitric acid, cleaning of the fluororesin plate used and management of cleanliness are complicated, and there is a problem in work efficiency.
On the other hand, the method (4) is a simpler method than the methods (1) to (3), but the detection sensitivity is a metal contamination analysis method for a semiconductor substrate that is required to analyze trace metal components with high sensitivity. And the measurement accuracy was insufficient.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a means for analyzing metal impurities that adversely affect semiconductor substrate characteristics in a simple and highly accurate manner.

As a result of intensive studies to achieve the above object, the present inventors have conducted heat treatment on a semiconductor substrate having one surface roughened, and the metal impurities in the semiconductor substrate are damaged by the surface roughening. As a result of diffusion into the strain field, the metal impurities can be recovered on the roughened substrate surface, and the recovery rate of the metal impurities by this method is the same as the method (4) in which the heat treatment is performed without roughening the semiconductor substrate. It has been newly found that it can be remarkably improved.
The present invention has been completed based on the above findings.

That is, the above object was achieved by the following means.
[1] forming a roughened region on one of the front and back surfaces of the semiconductor substrate;
Heat-treating the semiconductor substrate after formation of the roughened region, and after scanning the solution on the roughened region, analyzing a metal component in the solution;
Method for evaluating metal contamination of semiconductor substrate including
[2] The method according to [1], wherein the semiconductor substrate is a silicon substrate containing boron.
[3] The method according to [1] or [2], wherein the metal to be analyzed is Cu and / or Ni.
[4] The method according to any one of [1] to [3], wherein the roughened region is formed by grinding.
[5] The method according to any one of [1] to [4], wherein the semiconductor substrate is a substrate for grasping process contamination.

  ADVANTAGE OF THE INVENTION According to this invention, the metal contamination of the semiconductor substrate which has a bad influence on a semiconductor substrate characteristic can be evaluated highly sensitively and simply. By making the semiconductor substrate to be evaluated a semiconductor substrate for grasping the process contamination, it is possible to grasp the metal contamination in the process, thereby providing a high-quality semiconductor substrate.

[Method for evaluating metal contamination of semiconductor substrates]
The method for evaluating metal contamination of a semiconductor substrate according to the present invention includes the following steps.
(1) forming a roughened region on one of the front and back surfaces of the semiconductor substrate;
(2) Heat-treating the semiconductor substrate after forming the roughened region, and (3) scanning the solution on the roughened region, and then analyzing a metal component in the solution.
Hereinafter, the details of each process will be described sequentially.

Process (1)
Step (1) is a step of forming a roughened region on one of the front and back surfaces of the semiconductor substrate. In the present invention, the roughened region means a region having a rough surface compared to the surface opposite to the surface forming the roughened region. By heat-treating the semiconductor substrate having a roughened region formed on the surface in the step (1), metal impurities in the substrate can be captured on the roughened region with a high recovery rate. The inventors presume this reason as follows.
By roughening the substrate surface, a strain field is formed on the roughened substrate surface, and then heat treatment is performed, whereby metal impurities in the substrate are diffused toward the strain field. On the other hand, when the heat treatment is performed without forming the roughened region as in the above method (4), the metal impurities in the substrate diffuse toward the both surfaces of the substrate. Metal components must be recovered from. If a roughened region is formed on one side of the substrate, metal impurities can be selectively diffused toward the surface including the roughened region, so that high sensitivity can be obtained by recovering metal components from one side of the substrate. Analysis becomes possible. Moreover, it is considered that the diffusion rate toward the roughened region is significantly higher than the diffusion rate toward the mirror surface, which contributes to the improvement in the recovery rate of the metal component in the present invention.

  The roughened region is formed on either the front or back surface of the substrate. From the viewpoint of the recovery rate of the metal component, the other surface is preferably a mirror surface. Further, although a roughened region may be formed on a part of the substrate surface, it is preferable to roughen the entire surface in order to increase detection sensitivity.

  The roughened region can be formed by surface treatment such as polishing, grinding, or cutting. In order to impart suitable roughness, it is preferable to form a roughened region by grinding. Grinding conditions can be controlled by the abrasive grain size of the grinding wheel. From the viewpoint of the recovery rate of the metal component, the grinding depth is preferably 50 to 500 nm.

The semiconductor substrate to be evaluated is preferably a silicon substrate. As a silicon substrate, a silicon substrate (so-called p ⁺⁺ substrate) having a dopant concentration such as boron of about 10 ¹⁹ atms / cm ³ and a resistance value of about 1 to 10 mΩ · cm, a dopant concentration of 10 ¹⁸ atms / cm ^A silicon substrate (so-called p ⁺ substrate) having a resistance value of about 10 mΩ · cm to about 1000 mΩ · cm, a dopant concentration of about 10 ¹⁵ atms / cm ³ , and a resistance value of about 1 Ω · cm There is a silicon substrate (so-called p ⁻ substrate), and the evaluation method of the present invention can be applied to any of the above substrates. However, it is preferably applied to p ⁺⁺ substrate and p ⁺ substrate for the following reasons. ⁺⁺ More preferably applied to the substrate.
Conventionally, among silicon substrates, a silicon substrate containing a dopant at a high concentration (low resistance substrate) has been difficult to analyze metal impurities. This is because the higher the dopant concentration in the substrate, the higher the solid solubility of the metal component, particularly Cu, and the lower the diffusion coefficient of Cu. On the other hand, according to the present invention, by forming the roughened region on the substrate surface, the recovery rate of the metal impurities on the substrate surface can be increased, so that the metal impurities can be accurately detected even on a low resistance substrate Can be analyzed.

  Examples of the metal to be evaluated in the evaluation method of the present invention include various metals known as contaminating metals of the semiconductor substrate, and Cu and Ni are preferable from the viewpoint of the recovery rate.

Process (2)
Step (2) is a step of heat-treating the semiconductor substrate on which the roughened region is formed. By the heat treatment, metal impurities can be selectively diffused from the inside of the substrate toward the roughened region. The heat treatment can be performed by a method of holding the substrate in a heating furnace for a predetermined time, a method of bringing the substrate into contact with a heating source such as a hot plate, or the like. The heating conditions can be set according to the dopant concentration and the like in the substrate, the heating temperature is, for example, 200 to 1250 ° C., and the heating time is, for example, 30 minutes to 24 hours. The heating temperature refers to the atmospheric temperature in the heating furnace for the method of using the heating furnace, and the temperature of the heating source for the method of contacting with the heating source. In the method using a heating source, it is preferable that the surface opposite to the surface including the roughened region is brought into contact with the heating source. This is because the recovery rate of the metal component in the roughened region is increased by forming a temperature gradient in the substrate. Specifically, the semiconductor substrate is preferably held on the hot plate for a predetermined time with the surface on which the roughened region is formed facing up. The atmosphere during the heat treatment is not particularly limited, and can be, for example, the air, an oxidizing atmosphere, an inert gas atmosphere such as nitrogen, or the like.

  Step (2) can also be performed in a heat treatment furnace actually used in the production line. For example, when a semiconductor substrate with a roughened region formed on one side is introduced as a sample substrate into a production line and subjected to heat treatment in a heat treatment furnace used for production of product substrates, contaminated metals in the heat treatment furnace are exposed inside the substrate. Then, it diffuses toward the roughened region. Thereafter, by subjecting the sample substrate taken out from the heat treatment furnace to the step (3), it is possible to qualitatively and / or quantitatively analyze the contaminated metal of the substrate caused by metal contamination in the heat treatment furnace.

Step (3)
After the step (2), the substrate is cooled by cooling or the like as necessary, and then the step (3) is performed. It is considered that the metal impurity in the substrate is precipitated as an oxide or the like on the roughened region of the substrate surface by the step (2). Step (3) is a step of analyzing a metal component in the solution after scanning the solution on the roughened region in order to collect the deposited metal impurities from the roughened region. The scanning of the solution can be performed automatically or manually by a method of rotating the substrate while tilting so that the solution dripped onto the surface of the substrate is adapted to the entire surface. An acidic solution can be used as the recovery solution. As the acidic solution, a mixed solution of hydrofluoric acid, hydrogen peroxide solution and water, specifically, a mixed solution of 2% by mass hydrofluoric acid / 2% by mass hydrogen peroxide solution / water can be used. According to the present invention, the metal component can be easily recovered by surface scanning with the above weak acid solution without dissolving the substrate or the polysilicon layer as in the above-described methods (1) to (3). . The amount of solution supplied and scanned on the substrate surface is preferably set as appropriate so that the metal component on the substrate surface can be recovered at a high recovery rate.

  The metal component recovered in the recovery solution can be analyzed by various analysis methods usually used for quantitative analysis and / or qualitative analysis of the metal component in the solution. Examples of such methods include atomic absorption spectroscopy (AAS method) and inductively coupled plasma mass spectrometry (ICP-MS method: Inductively Coupled Plasma Mass Spectrometry).

  An AAS method, particularly a graphite furnace heating type AAS method (GF-AAS: Graphite Furnace AAS) is widely used because the apparatus is simple and easy to operate. In the GF-AAS method, a sample solution is introduced into a graphite electric furnace, and then a solvent is vaporized at a relatively low temperature, and then heated to 2000 to 2800 ° C. to atomize a metal element. Next, the atomized metal is quantified by measuring the absorption ratio of light unique to each element irradiated from an external light source. Usually, a hollow cathode lamp is used as the light source. Although it is necessary to change the light source for each measurement element, this GF-AAS method can obtain an analytical sensitivity of up to several tens of ppt (pg / ml) at a concentration in the sample solution. Further, detection sensitivity can be further improved by applying a magnetic field during absorption measurement and correcting the background by using the Zeeman effect.

In the ICP-MS method, a sample solution is gasified or aerosolized by a nebulizer and introduced into argon plasma by high frequency power applied by an inductive coupling coil. The sample is heated to about 6000 to 7000 K in atmospheric pressure plasma, and each element is atomized and further ionized with an efficiency of 90% or more. After passing through the skimmer (interface), the ions are focused by the ion lens unit, and then guided to a mass spectrometer maintained in a high vacuum state of <10 ⁻⁶ Pa for mass analysis. Thereby, the metal component in a solution can be quantified. According to the ICP-MS method, trace metal components can be analyzed with high sensitivity. Although it is difficult to analyze a strong acid solution in the ICP-MS method, according to the present invention, the metal component can be recovered with a weak acid, and therefore the metal component can be easily analyzed by the ICP-MS method. Can do.

  Metal impurities are easily diffused into the substrate during various heat treatments such as oxidation and diffusion during the manufacturing process of the semiconductor substrate. There is a possibility that the lifetime of the carrier is reduced, the leakage current is increased, and the dielectric breakdown voltage of the oxide film is deteriorated. Therefore, in order to reduce metal contamination in manufacturing processes such as heat treatment processes, the process contamination amount of the heating furnace to be used is usually evaluated on a trial basis using an evaluation substrate before heat treating the product. After improving the contamination based on this, full-scale product heat treatment is performed. In addition, in order to grasp daily process contamination, process contamination is also evaluated by sampling, for example, one substrate per lot, one substrate per day, or one substrate per week. Yes. The evaluation method of the present invention can be used as a substrate evaluation method for grasping the process contamination as described above.

  Hereinafter, the present invention will be described with reference to examples. However, this invention is not limited to the aspect shown in the Example.

1. Example / comparative example of high-concentration known metal contamination semiconductor substrate evaluation

[Example 1]
Ni and Cu uniformly contaminated the semiconductor substrate (boron-doped silicon substrate, substrate resistance value: 0.010 Ω · cm) uniformly at a surface concentration of 1 × 10 ¹³ atoms / cm ² . After the entire surface of one surface of the semiconductor substrate is roughened by applying grinding damage, the surface with the grinding damage is turned upward and the substrate is horizontally placed on a hot plate heated to 300 ° C. for 1 hour. The heat treatment was performed. Thereafter, the surface of the semiconductor substrate to which grinding damage was added was scanned with an acidic aqueous solution (2% by mass hydrofluoric acid / 2% by mass hydrogen peroxide / water), and then the metal components in the acidic solution were quantified by ICP-MS.

[Comparative Example 1]
A polysilicon layer was added to both sides of a semiconductor substrate contaminated with a known amount of NiCu in the same manner as in Example 1. Thereafter, the semiconductor substrate from which the polysilicon layer was removed only on one side was placed horizontally on a hot plate heated to 300 ° C. with the polysilicon layer facing upward, and heat-treated for 1 hour. After the heat treatment, the surface of the polysilicon layer was scanned with the same acidic solution as in Example 1, and then the metal components in the acidic solution were quantified by ICP-MS.

[Comparative Example 2]
After the heat treatment, the polysilicon layer was dissolved in a mixed solution of hydrofluoric acid and nitric acid, and then the metal components in the solution were quantified by ICP-MS.

[Comparative Example 3]
The same operation as in Example 1 was performed except that grinding damage was not added to the substrate surface.

[Comparative Example 4]
The same operation as in Example 1 was performed except that the heat treatment was not performed on the hot plate.

The quantitative results of Cu and Ni by ICP-MS in Example 1 and Comparative Examples 1 to 4 are shown in FIG.
In Comparative Example 2, the recovery rate was good, but the evaluation procedure was complicated. Moreover, it can be seen from the results of Comparative Example 1 that the metal impurities in the semiconductor substrate cannot be recovered only by scanning the recovered liquid on the surface of the polysilicon layer.
On the other hand, as shown in FIG. 1, according to Example 1, the contaminated metal could be quantified with a high recovery rate comparable to that of Comparative Example 2 by a simple method.
On the other hand, in the semiconductor substrate to which no grinding damage was applied in Comparative Example 3, the recovery rate was about 10% with respect to the amount of contamination, and the recovery rate was low. In addition, in the semiconductor substrate to which grinding damage of Comparative Example 4 was added but not subjected to the heat treatment by the hot plate, the metal impurity in the semiconductor substrate does not diffuse to the grinding damage capturing site, so that contamination metal cannot be detected. It was.

2. Examples and comparative examples of low-concentration known metal contamination semiconductor substrate evaluation

[Example 2]
The same operation as in Example 1 was performed, except that the semiconductor substrate (boron-doped silicon substrate, substrate resistance value: 0.010 Ω · cm) was contaminated with NiCu at a surface concentration conversion of 1 × 10 ¹¹ atoms / cm ² .

[Comparative Example 5]
The same operation as in Comparative Example 1 was performed except that the semiconductor substrate was contaminated with NiCu at a surface concentration conversion of 1 × 10 ¹¹ atoms / cm ² .

[Comparative Example 6]
The same operation as in Comparative Example 2 was performed except that the semiconductor substrate was contaminated with NiCu at a surface concentration equivalent of 1 × 10 ¹¹ atoms / cm ² .

[Comparative Example 7]
The same operation as in Comparative Example 3 was performed, except that the semiconductor substrate was contaminated with NiCu at a surface concentration equivalent of 1 × 10 ¹¹ atoms / cm ² .

[Comparative Example 8]
The same operation as in Comparative Example 4 was performed except that the semiconductor substrate was contaminated with NiCu at a surface concentration equivalent of 1 × 10 ¹¹ atoms / cm ² .

The quantitative results of Cu and Ni by ICP-MS in Example 2 and Comparative Examples 5 to 8 are shown in FIG.
As shown in FIG. 2, in Example 2, it was possible to recover metal impurities in the substrate at a recovery rate of almost 100% with respect to the amount of contamination.

  From the above results, it was shown that according to the present invention, simple and highly sensitive analysis is possible for both a semiconductor substrate contaminated with a high concentration metal and a semiconductor substrate contaminated with a low concentration metal.

3. Example of evaluating metal contamination of unknown concentration

[Example 3]
Prepare two mirror-polished boron-doped silicon substrates (substrate oxygen concentration: 1.0 × 10 ¹⁸ atoms / cm ³ (ASTM F-121 1979) substrate resistance: 0.008 Ω · cm) with different semiconductor manufacturing processing lines did.
In order to apply grinding damage to one side of the two semiconductor substrates, the mirror-polished surface of the semiconductor surface was ground at a grinding depth of about 200 nm with a grinding apparatus having a grinding wheel of about # 8000. .
With the ground surface facing up, the semiconductor substrate was held horizontally for 1 hour on a hot plate heated to 300 ° C. and subjected to heat treatment. After heating the hot plate, the semiconductor substrate was placed on a fluororesin plate with the grinding damage face up, and allowed to cool until the temperature of the semiconductor substrate reached room temperature.
After standing to cool, 1 ml of 2% hydrofluoric acid, 2% hydrogen peroxide (mass%) aqueous solution was dropped on the substrate surface to which grinding damage was added. The dropped acidic aqueous solution was scanned on the surface of the semiconductor substrate, and metal impurities diffused from the semiconductor substrate to the surface of the semiconductor substrate were collected.
Cu and Ni in the acidic recovery solution were quantitatively analyzed by ICP-MS. The results are shown in Table 1. As shown in Table 1, it can be understood from the comparison of the results of line 1 and line 2 that metal contamination is present in line 1.

4). Example of grasping process contamination

[Example 4]
Two mirror-polished boron-doped silicon substrates (substrate oxygen concentration: 1.0 × 10 ¹⁸ atoms / cm ³ (ASTM F-121 1979) substrate resistance value: 0.010 Ω · cm) were prepared.
In order to add grinding damage to one side of the two semiconductor substrates, the mirror-polished surface of the semiconductor surface was ground at a grinding depth of about 300 nm with a grinding machine having a grinding wheel of about # 8000. .
The semiconductor substrate to which the grinding damage was added was subjected to heat treatment in a nitrogen atmosphere in a heat treatment furnace 1 and a heat treatment furnace 2 for manufacturing semiconductors one by one in a nitrogen atmosphere.
1 ml of 2% hydrofluoric acid, 2% hydrogen peroxide (mass%) aqueous solution was dropped on each of the two heat-treated substrate surfaces subjected to grinding damage. The dropped acidic aqueous solution was scanned on the surface of the semiconductor substrate, and metal impurities diffused from the semiconductor substrate to the surface of the semiconductor substrate were collected.
Cu and Ni in the acidic recovery solution were quantitatively analyzed by ICP-MS. The results are shown in Table 2. The metal impurities detected here correspond to the amount of metal impurities diffused from the heat treatment furnace into the semiconductor substrate by the heat treatment. From the results in Table 2, it can be determined that Ni contamination exists in the heat treatment furnace 2 and that the heat treatment furnace needs to be improved.

  The present invention is useful in the field of manufacturing semiconductor substrates.

The evaluation result of Example 1 and Comparative Examples 1-4 is shown. The evaluation result of Example 2 and Comparative Examples 5-8 is shown.

Claims (5)

Forming a roughened region on one of the front and back surfaces of the semiconductor substrate;
Heat-treating the semiconductor substrate after formation of the roughened region, and after scanning the solution on the roughened region, analyzing a metal component in the solution;
Method for evaluating metal contamination of semiconductor substrate including The method of claim 1, wherein the semiconductor substrate is a silicon substrate containing boron. The method according to claim 1 or 2, wherein the metal to be analyzed is Cu and / or Ni. The method according to claim 1, wherein the roughened region is formed by grinding. The method according to claim 1, wherein the semiconductor substrate is a substrate for grasping process contamination.