JPH03188642A - Etching method for ultramicro element analysis of thin film semiconductor - Google Patents

Abstract

PURPOSE:To reduce etching time by using only the reaction decomposition liquid of the surface side thin film of a substrate as sample liquid, by keeping droplets of the reaction decomposition liquid on the surface side of a semiconductor substrate, and gathering them as sample liquid. CONSTITUTION:Mixed acid gas (b) ascending from a Teflon beaker 2 for evaporation is rapidly introduced together with nitrogen gas flow (a) to a wafer 5 on a table. An oxide film formed on the surface side of the wafer 5 is decomposed by the chemical reaction with the gas (b). Reaction decomposition liquid 6 becomes droplets to exist on the surface. After that, a closed vessel 1 is opened; each of the wafers 5 is taken out from the vessel 1 while the rear of the wafer 5 is fixed; nitric acid is dripped on the wafer surface; the wafer 5 is inclined; all droplets of the decomposed liquid are gathered while the droplets are rolled on the surface. The gathered decomposition liquid 6 is turned into sample liquid after metering. This liquid is used to analyze a trace of metallic impurity in the oxide film on the surface of the wafer 5.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is a method for etching a semiconductor thin film as a pre-process for analyzing ultratrace elements contained in a thin film of a semiconductor substrate such as an ultra-large scale integrated circuit. Concerning an etching method in ultratrace element analysis.

<Prior Art> Thin films of Sin, Si, N4, etc. formed on Si wafers are used for partial diffusion masking of dopants in Si semiconductor devices and as protective films for metal vapor deposition films. Trace metal impurities such as Na and Fe contained in these films degrade the electrical insulation properties of the device and shorten its life. Therefore, techniques for analyzing these ultra-trace components are essential for improving the performance of ultra-large scale integrated circuits.

Conventional etching methods used in ultratrace elemental analysis of semiconductor thin films include the following.

Inside the Teflon sealed container 1, an evaporation beaker 9, a Teflon wafer carrier 12, and a decomposition liquid receiver 11 are provided. First, several wafers 5, which are semiconductor substrates, are placed against the wafer carrier 12. Add hydrofluoric acid 10 placed in an evaporation beaker 9. The hydrofluoric acid 10 is then vaporized by the ambient temperature and left for about 2 hours. Then, the hydrofluoric acid gas reacts with the oxide film on the wafer 5 and is decomposed, and the reaction and decomposition droplets 6 are stopped in the decomposition liquid receiving tray 11.

A method is adopted in which all of the reaction decomposition droplets 6 are collected using a micropipette, and then diluted to a certain amount with pure water, and this is used as a sample solution.

Note that trace metal impurities such as Na and Fe contained in the sample solution are analyzed by gas phase decomposition/graphite furnace atomic absorption spectrometry.

<Problems to be Solved by the Invention> However, in the case of the above-mentioned conventional method, the following drawbacks have been pointed out.

The first drawback is that even though the item to be analyzed is trace metal impurities in the oxide film on the front side of the wafer 5, the back side of the wafer 5 is also etched by the hydrofluoric acid gas, resulting in some metal impurities in the sample solution. This also includes the reaction decomposition liquid on the back side of the wafer 5, making it impossible to accurately analyze trace metal impurities in the oxide film on the front side of the wafer 5.

The second drawback is that since the method of vaporizing the hydrofluoric acid 10 placed in the evaporation beaker 9 at ambient atmospheric temperature takes a long time to etch the oxide film on the wafer 5, the semiconductor thin film is This is a major hindrance to efficiently performing ultratrace element analysis.

The third drawback is that some trace metal impurities that have an adverse effect on integrated circuits cannot be decomposed by hydrofluoric acid gas, such as C.
u, etc., and this type of element cannot be analyzed.

The present invention was devised in view of the above circumstances, and its purpose is to use only the reaction decomposition liquid of the thin film on the surface side of a semiconductor substrate as a sample solution, or to shorten the etching time. An object of the present invention is to provide an etching method for ultratrace elemental analysis of semiconductor thin films.

Means for Solving the Problems> The etching method for ultratrace elemental analysis of semiconductor thin films according to the first aspect of the present invention includes horizontally disposing a semiconductor substrate inside a closed container containing a beaker for evaporating a decomposition liquid. , the thin film is decomposed by a chemical reaction between the decomposition gas evaporated from the decomposition liquid evaporation beaker and the thin film on the surface side of the semiconductor substrate, and droplets of the reaction decomposition liquid accumulated on the surface of the semiconductor substrate are collected.

The etching method for ultratrace elemental analysis of a semiconductor thin film according to the second aspect of the present invention is such that a semiconductor substrate is placed inside a closed container containing a beaker for evaporating a decomposition liquid, and the decomposition liquid in the beaker for evaporating a decomposition liquid is is heated to generate a decomposed gas, a carrier gas is flowed into the sealed container to guide the decomposed gas toward the semiconductor substrate, and the thin film is decomposed by a chemical reaction between the decomposed gas and the thin film on the semiconductor substrate. .

<Example> Hereinafter, an example of the etching method for ultratrace elemental analysis of a semiconductor thin film according to the present invention will be described.

FIG. 1 is a sectional view of the etching apparatus.

In the figure, 1 is a Teflon airtight container, and the - side length is all 3.
It is a 0cm cube. A Teflon evaporating beaker 2 and a Teflon table 4 are provided on the bottom of the Teflon sealed container 1, respectively.

A mixed acid 3 (corresponding to a decomposition liquid) is placed in the recess of the Teflon evaporation beaker 2. Further, although not shown in the drawings, the Teflon evaporation beaker 2 has a built-in heater, etc., so that heat of vaporization can be given to the mixed acid 3. There are multiple Teflon tables 4 (four in the illustrated example).
Each shelf of the Teflon table 4 is provided with a convex portion 41 that supports the back surface of the wafer 5 horizontally.

Furthermore, a nitrogen gas suction lower is provided at the upper right side of the Teflon sealed container 1 in the figure to introduce nitrogen gas a as a carrier gas, while a nitrogen gas suction lower is provided at the lower left side in the figure. A nitrogen gas outlet 8 is provided for discharging nitrogen gas. Although not shown, both the nitrogen gas suction lower and the nitrogen gas discharge port 8 are connected to a nitrogen gas piping system.

Next, the wafer 5 is etched using the etching apparatus as described above.
A method for etching the thin film formed in the above will be explained in detail.

First, four wafers 5 are placed on each shelf of the Teflon table 4. Then, mixed acid 3, which is a mixture of 20% hydrofluoric acid and 30% nitric acid, is put into the recess of the Teflon beaker 2, and the Teflon airtight container 1 is sealed.

After that, in order to increase the evaporation efficiency of the mixed acid 3, the built-in heater etc. in the Teflon evaporation beaker 2 is heated to keep the mixed acid 3 at about 40°C, and the mixed acid gas b (corresponding to decomposed gas) is continuously supplied. and generate it.

After a short period of time, high-purity nitrogen gas a is passed through the Teflon sealed container 1. That is, Teflon sealed container 1
High-purity nitrogen gas a is fed into the container via a nitrogen gas suction lower, while the fed nitrogen gas a is discharged to the outside of the Teflon sealed container 1 through a nitrogen gas outlet 8. Note that the rate at which the nitrogen gas a is fed into the Teflon sealed container 1 is set here to approximately 111/min so that the atmosphere in the Teflon sealed container l is replaced in 30 minutes.

Then, the mixed acid gas b rising from the Teflon evaporation beaker 2 rides on the flow of nitrogen gas a and reaches the Teflon table 4.
It is quickly guided to the upper wafer 5. At this time, the mixed acid gas b sufficiently contacts the front side of the wafer 5, but the contact with the back side is extremely small. This is because the gap between the front side of the wafer 5 and the Teflon table 4 is sufficiently secured, but the gap between the back side of the wafer 5 and the Teflon table 4 is extremely small, so the mixed acid gas b is This is because it is difficult for the mixed acid gas b to flow toward the back surface side, and furthermore, the mixed acid gas b is efficiently guided toward the front surface side of the wafer 5 by riding on the downward current of the nitrogen gas a.

Then, the oxide film (not shown) formed on the surface side of the wafer 5 is decomposed by a chemical reaction with the mixed acid gas b, and as a result, the surface of the wafer 5 becomes a flowing water surface, and the reaction decomposition liquid 6 becomes droplets. Scattered on this surface. However, since the wafer 5 is arranged horizontally, droplets of the reaction decomposition liquid 6 will not spill.

Thereafter, the Teflon sealed container 1 was opened, and while fixing the back side of the wafer 5 with a vacuum bottle set, the wafers 5 were taken out individually from the Teflon sealed container 1, and approximately 50 μm was applied to the surface of the taken out wafer 5. of nitric acid (approximately 2%) is added dropwise.

Then, the wafer 5 is tilted and all the droplets of the reaction decomposition liquid 6 are collected into a micropipette while being rolled on the surface thereof.

The reaction decomposition liquid 6 collected into a micropipette for each wafer 5 is measured and then used as a sample solution. Using this sample solution, trace metal impurities such as Cu in the oxide film on the front side of the wafer 5 are analyzed.

In the method according to this embodiment, since the reaction decomposition liquid 6 collected by the micropipette is only from the surface side of the wafer 5, trace metal impurities in the thin film on the surface side of the wafer 5 can be accurately analyzed. In addition, the decomposition solution contains nitric acid in addition to hydrofluoric acid, and a method is adopted in which a diluted nitric acid solution is further dropped onto the surface of the wafer 5 when the reaction decomposition solution 5 is recovered, so trace metals can be removed. It is also possible to analyze impurities. In our laboratory example, trace metal impurities were reduced to 10”atms/cm2~109at, which is an order of magnitude higher than conventional methods.
It has become possible to detect up to about ms/cm2.

Furthermore, a method is adopted in which the mixed acid gas 3 in the Teflon evaporation beaker 2 is heated to generate the mixed acid gas b, and the mixed acid gas b is quickly guided to the wafer 5 side by the nitrogen gas b. The time required for this can be extremely shortened, and in this example, it was possible to shorten the time from 120 minutes to 30 minutes.

<Effects of the Invention> As described above, in the case of the etching method for ultratrace elemental analysis of semiconductor thin films according to the first aspect of the present invention, the surface of the semiconductor substrate is When the side thin film decomposes, droplets of the reaction decomposition liquid accumulate on the front side of the semiconductor substrate, and the collected droplets are collected as a sample solution. Only the liquid can be used as the sample solution. Therefore, it is possible to accurately analyze trace metal impurities in the thin film on the surface side of a semiconductor substrate, which is a great advantage in promoting higher performance of ultra-large scale integrated circuits and the like.

In the case of the etching method for ultratrace elemental analysis of semiconductor thin films according to the second aspect of the present invention, the decomposition liquid in the beaker for evaporating the decomposition liquid is heated to generate decomposition gas, and the decomposition gas is placed in a closed container. Since a method is adopted in which the carrier gas is introduced to the semiconductor substrate side, the time required for etching can be extremely shortened.

Therefore, there is a great advantage in efficiently analyzing trace metal impurities in thin films of semiconductor substrates.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining an embodiment of the etching method for ultratrace elemental analysis of semiconductor thin films according to the present invention, and is a sectional view of an etching apparatus. FIG. 2 is a diagram for explaining the conventional method and corresponds to FIG. 1.・Mixed acid・Wafer・Reaction decomposition liquid・Nitrogen gas・Mixed acid gas

Claims (2)

[Claims] (1) A semiconductor substrate is placed horizontally inside a closed container containing a beaker for evaporating decomposition liquid, and the thin film is formed by a chemical reaction between the decomposition gas evaporated from the beaker for evaporating decomposition liquid and the thin film on the surface side of the semiconductor substrate. 1. An etching method for ultratrace elemental analysis of a semiconductor thin film, characterized in that droplets of the reaction decomposition liquid collected on the surface of the semiconductor substrate are collected. (2) A semiconductor substrate is placed inside a closed container containing a beaker for evaporating a decomposition liquid, the decomposition liquid in the beaker for evaporating a decomposition liquid is heated to generate a decomposition gas, and a carrier gas is introduced into the closed container. 1. An etching method for ultratrace elemental analysis of a semiconductor thin film, characterized in that the decomposed gas is introduced to the semiconductor substrate side, and the thin film is decomposed by a chemical reaction between the decomposed gas and the thin film on the semiconductor substrate.