JPH1083970A - Method for determining thickness of masking photoresist in semiconductor-ion-implantation process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to determine the application thickness of photoresist in the minimum test, by performing ion implantation at a SIM analysis after ion implantation is performed at the constant voltage and ion-current quantity, and determining the thickness of the photoresist, wherein the transmission thickness corresponds to zero, from the functional relationship between the thickness of each photoresist and the depth of the transmission. SOLUTION: For plural kinds of wafers having the different application thicknesses of photoresist, for example, the primary ion beam of cesium is used, and ion implantation is performed at the constant voltage and the constant ion-current quantity. Then, the chart of the relationship of an impurity detecting level with respect to the transmission depth from the surface is formed by SISM (secondary-ion mass spectroscopy) of the wafer. Thus, the transmission depth from the surface of a sample in correspondence with the constant detecting level of an analyzer is determined. Then, the functional relationship between the thickness of each photoresist and the transmission depth in correspondence with the constant detecting level of the analyzer is determined. By utilizing the functional relationship, the thickness of the photoresist in correspondence with the zero thickness is determined.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for determining the thickness of a masking photoresist in a semiconductor ion implantation process.
The present invention relates to a method for determining a thickness of a masking photoresist in a semiconductor ion implantation process using Mass Spectrometry.

[0002]

2. Description of the Related Art In a process of manufacturing a semiconductor device, an ion implantation method is often used as a method for forming an individual device by forming a well with impurities in a predetermined region of a semiconductor substrate or a specific layer. . In this case, impurities must be implanted only in a certain region of the semiconductor. Therefore, after applying a photoresist to the wafer, a pattern is exposed with a photomask to form a window only in the corresponding region, and the photoresist remains in other regions to serve as a blocking mask. . In this state, an ion beam is scanned on the front surface of the wafer, and as a result, impurities are implanted only into the corresponding region, and depending on the type of ions to be implanted, N-well and P-well are used.
1 is formed.

At this time, the impurity wale must be formed at an appropriate depth on the surface of the wafer to be processed in the ion implantation process. Therefore, the magnitude of the voltage for accelerating the ions must be selected according to the depth. Also, the thickness of the photoresist applied must be determined appropriately. That is, if the photoresist is applied very thinly, it cannot serve as a sufficient blocking mask, so that the photoresist must be applied sufficiently.

On the other hand, when the photoresist is applied very thickly, the slop problem that occurs when developing a thick photoresist and the contamination after the photoresist stripping become a source of particles. Due to the problem of Residue or the like in which the occurrence of defects increases, the probability of occurrence of defects in the process increases.

[0005] Therefore, it is important to determine an appropriate photoresist coating thickness depending on the process, and many experiments and a lot of trial and error have been required for the determination.

The most important and universal measurement method used in the experiment for determining the coating thickness of the photoresist is SIMS analysis. In this SIMS analysis, unlike an electron microscope or the like, a method is used in which ions that are not electrons are accelerated to have a constant energy and then collide with the surface of the sample.

In the collision, secondary ions are generated by the used primary ion beam. The generated secondary ions are weighed through an attached mass spectrometer. Therefore, the composition ratio of the material forming the surface of the sample can be determined. Also, 1
Since the surface is subsequently lost by the scanning of the secondary ions, the composition ratio at a constant depth from the surface can also be determined by analyzing and weighing the secondary ions generated over time with a mass spectrometer. .

A conventional method for determining an appropriate photoresist coating thickness in an ion implantation process using the SIMS analysis will be examined. First, a photoresist is applied to the front surface of the wafer and ion implantation is performed. The ion implantation generally includes a certain acceleration voltage required in the process and a certain amount of ion current (or Dose). Since the wafer is charged up while using the SIMS analysis, the photoresist of the semiconductor is removed by a method such as stripping to prevent the measured value from being distorted.

When the SIMS analysis is used, N-
In the well, phosphorus (Phosphorus) is detected as an impurity. Also, in the case of SIMS analysis, primary ion scanning is performed with a constant acceleration voltage and a constant ion current amount. In the analysis, the concentration of phosphorus is measured by the depth at the surface of the sample, that is, the semiconductor wafer. Then, on the surface, for example, CAMECA ims4f
1E1 corresponding to the minimum effective detection sensitivity of SIMS analysis equipment
At a level of 5 atoms / cc, the analysis is repeated while the photoresist coating thickness is gradually increased until the degree of detection of the impurity phosphorus appears at the noise level.

[0010]

SUMMARY OF THE INVENTION Such a conventional SI
In the case of the MS method, when detecting phosphorus as an impurity, a primary ion beam uses cesium (Cs) ions.
However, especially in an environment of high humidity, hydrated silicon (SiH) having almost the same mass number as phosphorus (P: mass number 31) is generated by secondary ions and causes interference, so that the accurate concentration of phosphorus is reduced. An unknown problem occurs.

Further, an SI for separating whether or not impurities can pass through at a depth within 100 to 200 angstroms of the surface.
In the MS analysis, elements such as hydrogen on the surface that collided with the ion beam were immersed in the silicon substrate, such as a knock-on effect, and the interference phenomenon continued from the surface to a certain depth. appear.

As a result, an analysis result such that phosphorus is always present on the surface, similar to that shown by the curve A in FIG. 1, appears, so that an accurate measurement and an accurate determination of the photoresist coating thickness can be made. Was difficult.

Therefore, a lot of experiments had to be carried out to determine the appropriate thickness value, and it was not possible to determine the exact value. Then, there was a tendency to perform an ion implantation step. In addition, the process failure rate due to the photoresist slop problem and the residue problem described above was high.

The present invention has been made in view of such a conventional problem, and an object of the present invention is to provide an ion implantation process capable of determining an appropriate photoresist coating thickness with a minimum number of test measurements. To provide a method for determining the thickness of a masking photoresist.

[0015]

According to a first aspect of the present invention, there is provided a method for determining a thickness of a masking photoresist for an ion implantation step. Preparing a plurality of coated wafers of photoresist having different coating thicknesses known, performing an ion implantation process on the plurality of wafers at a constant voltage and a constant amount of ion current; Performing a SIMS analysis on the wafer after the above process, observing a change in impurity concentration according to the depth at the surface, and determining a penetration depth from the surface of the sample corresponding to a certain detection level of the analysis equipment; Thirdly, determining a functional relationship between the thickness of each photoresist and the penetration depth corresponding to a certain detection level of the analysis equipment, and fourthly, using the functional relationship,
Determining a photoresist thickness corresponding to a transmission thickness of zero. Therefore, an appropriate photoresist coating thickness can be determined with a minimum number of test measurements.

A second aspect of the present invention is that the step of removing the photoresist is performed before the SIMS analysis in order to eliminate the charging problem.

According to a third aspect of the present invention, the depth of penetration in the present invention may be defined in different ways depending on the analytical equipment, but they can be substituted for each other as long as there is no fundamental difference. Things. Preferably, the fixed detection level of each analysis equipment is set to the minimum effective detection sensitivity level.

According to a fourth aspect of the present invention, the detection level of the impurity based on the penetration depth on the surface of the wafer is obtained by: Can be easily determined by drawing an auxiliary line perpendicular to the axis and reading the penetration depth at the point of intersection. At this time, in the range where the detection level is low, it is common that noise appears on the curve. Therefore, the value of the part where noise appears is determined by observing the intermediate value as a slow single line. Further, it is considered that the functional relationship between the coating thickness of the photoresist and the transmission depth can be calculated more strictly in a more complicated relationship. However, in general, according to an experiment such as the following embodiment, the coating thickness of the photoresist and the penetration depth can be regarded as having a shape that is roughly proportional to the photoresist.
It is determined that it is sufficient to derive a quadratic relation. Therefore, a method using a graph showing a simple straight line is possible.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described in detail with reference to the accompanying drawings.

In the following embodiments, CMEC is commonly used.
A ims4f SIMS was used, a cesium ion beam was used as a primary ion beam, the current by the ion beam was 5E-8A, the acceleration voltage was 10 KV, and the size of the ion beam was 250 μm * 250 μm. At a certain level of detection of impurities, 1E15 atom, which is the minimum effective detection sensitivity of phosphorus (P) on this equipment
An s / cc level was assumed.

[Embodiment 1] In Embodiment 1, five types of wafers having different photoresist coating thicknesses were used on a 64M DRAM wafer as samples, and the conditions for forming an N-well by ion implantation were as shown in Table 1. It is as shown in 1.

[0022]

[Table 1] Table 1 shows a sample history under the condition of an ion implantation acceleration voltage of 800 KeV. FIG. 1 is a graph for each sample obtained by SIMS analysis, showing the impurity detection level with respect to the penetration depth from the surface.

In FIG. 1, the samples corresponding to the A curve are those shown in Table 1 with a photoresist thickness of 1.6,
The sample corresponding to the curve is one with a photoresist thickness of zero (0). In each curve, FIG. 2 is a graph in which the transmission depth (transmission thickness) corresponding to the minimum effective detection sensitivity level of impurities is read, and the relationship with the photoresist thickness is indicated by dots.

The three points corresponding to B, C, and D in FIG. 1 indicate a linear proportional relationship. The photoresist thickness at the point where the straight line extending these three points intersects the transmission depth (transmission thickness) of zero (0) corresponds to 1.5. Therefore, the photoresist coating thickness suitable for the ion implantation conditions of the sample is 1.5 μm.

[Second Embodiment] In the second embodiment, as a sample, five kinds of wafers having different coating thicknesses of a photoresist on a 4M SRAM wafer are used, and N-well forming conditions by ion implantation are shown in Table 2. It is as follows.

[0026]

[Table 2] Table 2 shows the sample history under the condition of the ion implantation acceleration voltage of 1.2 MeV, and FIG. 3 is a graph for each sample by SIMS analysis, showing the impurity detection level with respect to the penetration depth from the surface.

In FIG. 3, the A, B, and C curves are shown as a single curve.

At this time, the impurity detection sensitivity level is:
Since this is due to interference by hydrated silicon and the knock-on effect, the permeation depth cannot be accurately determined only from this curve, and the permeation depth (permeation thickness) is regarded as zero (0).

Further, by drawing a graph as shown in FIG. 4, when determining the coating thickness of the photoresist, it is not used, or only the minimum one, that is, the one with the smallest coating thickness is used. This is because, when the two points (A2, B2) in FIG. 4 derived from the A and B curves are expressed by a straight line indicating a linear function, the transmission depth (transmission thickness) becomes zero (0) or more. This is because it does not exist in a meaningful section (predetermined section).

The samples corresponding to the curve A are those shown in Table 2 with a photoresist thickness of 4.5, and the samples corresponding to the E curve are sequentially those having a photoresist thickness of 1.19. It is. FIG. 4 is a graph in which, in each curve, the transmission depth (transmission thickness) value corresponding to the minimum effective detection sensitivity level of impurities is read, and the relationship with the photoresist thickness is indicated by dots. Three points corresponding to C, D, and E in FIG. 3 roughly indicate a linear proportional relationship. The thickness of the photoresist at the point where the straight line connecting these three points intersects the transmission depth (transmission thickness) of zero (0), that is, the point of FIG. 4 extracted by the C curve, corresponds to 2.5. . Therefore, the photoresist coating thickness suitable for the ion implantation conditions shown in Table 2 of the sample is 2.5 μm.

[Third Embodiment] In the third embodiment, four kinds of wafers having different photoresist coating thicknesses are used for a 4M SRAM wafer as a sample, and N-well forming conditions by ion implantation are shown in Tables. 3

[0032]

[Table 3] Table 3 shows the sample history under the condition of the ion implantation acceleration voltage of 2 MeV. FIG. 5 is a graph for each sample by SIMS analysis, showing the impurity detection level with respect to the penetration depth from the surface. In FIG. 5, the A and B curves are shown overlapping. Samples corresponding to the A curve are those shown in Table 3 with a photoresist thickness of 3.5, and samples corresponding to the D curve are those sequentially shown with a photoresist thickness of 1.5. .

FIG. 6 is a graph in which the value corresponding to the minimum effective detection sensitivity level of the impurity is read from each curve, and the relationship with the photoresist thickness is indicated by dots. C and D in FIG.
The two points (C3, D3) corresponding to are regarded as having a linear proportional relationship. The photoresist thickness at the point where a straight line extending these two points intersects the transmission depth (transmission thickness) of zero (0) corresponds to 2.7. Accordingly, the photoresist coating thickness suitable for the ion implantation conditions of the sample is 2.7 μm.

Under the ion implantation conditions corresponding to the above three embodiments, in the conventional process, the thickness of the applied photoresist was 3 μm, 4 μm, and 4.5 μm, respectively.
However, in the method of determining the thickness of the masking photoresist for the ion implantation process according to the embodiment of the present invention, the thickness of the applied photoresist is 1.5 μm.
m, 2.5 μm and 2.7 μm. To obtain such values, only four to five samples each were used.

Although the present invention has been described in detail only with respect to the specific examples described, it will be apparent to those skilled in the art that various changes and modifications can be made within the technical idea of the present invention. It is to be understood that such changes and modifications are intended to fall within the scope of the appended claims.

[0036]

As described above, the first aspect of the present invention relates to a method of determining the thickness of a masking photoresist for an ion implantation step, in which a first method for determining a thickness of a masking photoresist having different coating thicknesses is known. Prepare a plurality of coated wafers of resist, a constant voltage on the plurality of wafers,
Secondly, a step of performing an ion implantation step with a constant amount of ion current, and secondly, performing a SIMS analysis on the wafer after the above step, observing a change in impurity concentration depending on a depth on the surface, and setting a constant amount of analysis equipment. Determining a penetration depth from the surface of the sample corresponding to the detection level; and, third, determining a functional relationship between the thickness of each photoresist and the penetration depth corresponding to a certain detection level of the analytical equipment. And fourthly, a step of determining a photoresist thickness corresponding to a transmission thickness of zero by using the functional relationship. Accordingly, there is an advantage that an accurate photoresist coating thickness can be determined. Also, as a result, in the conventional process,
The photoresist coating thickness can be significantly reduced without the risk of uncertainty compared to that under actual conditions, thus reducing slop and residue problems when applying thick photoresist Will be able to

In the second invention, the step of removing the photoresist is performed before performing the SIMS analysis, so that the charging problem can be eliminated.

In the third invention, the depth of penetration in the present invention may be defined in different ways depending on the analytical equipment, but they can be interchanged as long as there is no fundamental difference. Preferably, the fixed detection level of each analysis equipment is set to the minimum effective detection sensitivity level.

According to a fourth aspect of the present invention, the detection level of the impurity based on the penetration depth on the wafer surface is represented by a curve represented through a graph by equipment for each of the photoresist coating thickness of the sample,
It can be easily determined by drawing an auxiliary line perpendicular to the axis of the impurity detection level and reading the penetration depth at the point of intersection.

[Brief description of the drawings]

FIG. 1 is a graph according to the present invention showing the impurity detection level according to the penetration depth from the surface for each sample by SIMS analysis after ion implantation under each ion implantation condition.

FIG. 2 shows a penetration depth value for each sample obtained from FIG.
5 is a graph according to the present invention showing a relationship with a photoresist thickness.

FIG. 3 is a graph according to the present invention showing the impurity detection level according to the penetration depth from the surface for each sample by SIMS analysis after ion implantation under each ion implantation condition.

FIG. 4 shows penetration depth values for each sample obtained from FIG.
5 is a graph according to the present invention showing a relationship with a photoresist thickness.

FIG. 5 is a graph showing impurity detection levels according to penetration depth from a surface for each sample by SIMS analysis after ion implantation under each ion implantation condition according to the present invention.

FIG. 6 shows a penetration depth value for each sample obtained from FIG.
5 is a graph according to the present invention showing a relationship with a photoresist thickness.

Claims (4)

[Claims] 1. A method of determining a thickness of a masking photoresist for an ion implantation step, comprising: first preparing a plurality of photoresist coated wafers having known coating thicknesses and different coating thicknesses; A step of performing an ion implantation step at a constant voltage and a constant ion current amount on each of the wafers; and, second, performing a SIMS analysis on the wafer after the above step to determine a change in impurity concentration due to a depth at the surface. Observing and determining the penetration depth from the surface of the sample corresponding to a certain detection level of the analysis equipment; and thirdly, the thickness of each photoresist and the transmission depth corresponding to the certain detection level of the analysis equipment. And a fourth step of determining a photoresist thickness corresponding to a transmission thickness of zero using the functional relationship. Thickness determining method of the masking photoresist conductor ion implantation process. 2. The method of claim 1, further comprising removing photoresist from the wafer after the ion implantation and before SIMS analysis. Thickness determination method. 3. The method according to claim 1, wherein said constant detection means sets a minimum effective detection sensitivity level of the analysis equipment. 4. The thickness of a masking photoresist in a semiconductor ion implantation process according to claim 1, wherein the functional relationship is a linear functional relationship in a predetermined section. Decision method.