Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device, includes: a preparation step, a flow step, and a processing step. The preparation step prepares an etching solution by dissolving titanium in an ammonia-hydrogen peroxide solution in advance before use of the ammonia-hydrogen peroxide solution for etching. The flow step flows the etching solution after the preparation step so that a concentration of the etching solution in a processing bath is constant. The processing step etches a metal film on a semiconductor wafer with the etching solution by putting in the processing bath the semiconductor wafer having a resist film and the metal film after the flow step is started. The metal film is preferably formed of titanium, and a temperature of the etching solution is preferably adjusted by flowing the etching solution so that the etching solution flows via a temperature controller.

FIELD

The present invention relates to a method for manufacturing a semiconductor device.

BACKGROUND

Various techniques of inhibiting changes in etching rate are known with respect to wet etching on metal film, as described in Patent Literatures 1 and 2 shown below.

CITATION LIST

Patent Literature

SUMMARY

Technical Problem

In a semiconductor manufacturing process, wet etching on titanium film is performed by using an ammonia-hydrogen peroxide solution. A technique of circulating an etching solution for concentration uniformization or temperature stabilization is also known. However, if the ammonia-hydrogen peroxide solution is circulated, the decomposition of hydrogen peroxide is promoted and, therefore, the concentration of hydrogen peroxide lowers with passage of time. The etching rate is strongly influenced by the concentration of hydrogen peroxide. Therefore, the etching rate is reduced as time elapses after the preparation of the ammonia-hydrogen peroxide solution, and the etching rate cannot be constantly maintained.

The present invention has been achieved to solve the above-described problem, and an object of the present invention is to provide a semiconductor device manufacturing method capable of constantly maintaining the etching rate for a long time period.

Solution to Problem

A method for manufacturing a semiconductor device according to the present invention, includes: a preparation step of preparing an etching solution by dissolving titanium in an ammonia-hydrogen peroxide solution in advance before use of the ammonia-hydrogen peroxide solution for etching; a flow step of flowing the etching solution after the preparation step so that a concentration of the etching solution in a processing bath is constant; and a processing step of etching a metal film on a semiconductor wafer with the etching solution by putting in the processing bath the semiconductor wafer having a resist film and the metal film after the flow step is started.

Advantageous Effect of Invention

According to the present invention, the etching rate can be constantly maintained for a long time period by inhibiting the decomposition of hydrogen peroxide in an ammonia-hydrogen peroxide solution.

DESCRIPTION OF EMBODIMENT

FIG. 1is a flowchart showing a method for manufacturing a semiconductor device according to an embodiment of the present invention.FIGS. 2 and 3are diagrams showing a wet etching apparatus50according to the embodiment of the present invention. In the present embodiment, an etching process shown in the flowchart ofFIG. 1is performed by using the wet etching apparatus50to etch a titanium film14on a silicon carbide (SiC) wafer10shown inFIG. 6.

As shown inFIG. 2, the wet etching apparatus50is provided with a processing bath20, a first piping30, a circulation pump32, a temperature controller34, a second piping36and a concentration meter38. An etching solution29is stored in the processing bath20. One end and the other end of the first piping30are connected to the processing bath20for the purpose of circulating the etching solution29therein. The circulation pump32and the temperature controller34are provided at intermediate positions in the first piping30. The etching solution29can be circulated through the first piping30by operating the circulation pump32. The temperature controller34is provided at an intermediate position in the first piping30. The etching solution29is caused to flow through the temperature controller34to enable control of the temperature of the etching solution29. One end of the second piping36is connected to an outlet of the circulation pump32, while the other end of the second piping36is positioned above the processing bath20. Droplets39of the etching solution29can be fed from the other end of the second piping36. The concentration meter38is provided at an intermediate position in the second piping36. From a value measured with the concentration meter38, the concentration of the circulated etching solution29can be known. For example, the rate of flow in the first piping30may be set to several liters per minute, and the rate of flow in the second piping36may be set to several cubic centimeters per minute.

As a concrete construction of the processing bath20, a processing bath including a plurality of baths, for example, as shown inFIG. 3may be used.FIG. 3shows a sectional view of a processing bath20. The processing bath20includes an inner bath201in which the SiC wafer10is to be put, and an outer bath202provided so as to surround the inner bath201. The etching solution29overflowing from the inner bath201flows into the outer bath202. The circulation pump32draws out the etching solution29in the outer bath202from one end of the first piping30connected to a bottom surface of the outer bath202. The circulation pump32feeds the etching solution29into the inner bath201via the other end of the first piping30connected to a bottom surface of the inner bath201. The etching solution29is thereby circulated in the processing bath20. The present invention is not limited to the processing bath20having the two-bath structure shown inFIG. 3. A one-bath structure may suffice for the present invention. The etching solution29in the processing bath may be agitated by using well-known means other than the means for circulation using the first piping30.

Steps shown in the flowchart ofFIG. 1will subsequently be described. First, in steps S100and S102, the etching solution29is prepared. More specifically, the etching solution29is prepared by dissolving titanium in an ammonia-hydrogen peroxide solution22before etching use.

First, in step S100, the ammonia-hydrogen peroxide solution22is compounded.FIG. 4is a diagram showing a step of preparing the etching solution29according to the embodiment of the present invention. Ammonia water24having no titanium dissolved therein, a hydrogen peroxide solution26and pure water28are successively put in the processing bath20, thereby preparing the ammonia-hydrogen peroxide solution22. It is preferable to put each liquid at the time of preparation of the solution because the concentration changes if the solution is compounded in advance. The ammonia-hydrogen peroxide solution22is in a state of being used before etching, i.e., in an unused state, and therefore contains no metal such as titanium.

Subsequently, the process advances to step S102and titanium is dissolved in the ammonia-hydrogen peroxide solution22.FIG. 5is a diagram showing the step of preparing the etching solution29according to the embodiment of the present invention. Referring toFIG. 5, the titanium film42is dissolved in the ammonia-hydrogen peroxide solution22, for example, by putting in the processing bath20a semiconductor wafer44having a titanium film42formed on a semiconductor substrate40. e.g., a silicon (Si) substrate. The semiconductor wafer44is for dissolving titanium in the ammonia-hydrogen peroxide solution22in advance. Therefore, the semiconductor wafer44, unlike the wafer on etching is to be performed, has no resist provided for patterning of the titanium film42. The present invention is not limited to the embodiment using the semiconductor wafer44. For example, fine titanium particles may be put in the processing bath20. The etching solution29is completed by step S102.

Subsequently, in step S104, the circulation pump32is operated in the apparatus arrangement shown inFIG. 2to circulate the etching solution29. In a preferable mode according to the present embodiment, the etching solution29in the processing bath20is circulated by using the circulation pump32in order to make constant the concentration and temperature of the etching solution29in the processing bath20. The etching uniformity is improved by circulating the etching solution29. That is, the concentration of the etching solution29in the processing bath20can be constantly maintained by circulating the etching solution29in the processing bath20with the circulation pump32. Since the temperature controller34is attached to the processing bath20, and since the etching solution29is circulated so that the temperature of the etching solution29is constant, changes in etching speed with changes in temperature can also be inhibited.

When the ammonia-hydrogen peroxide solution is circulated, the decomposition of hydrogen peroxide is promoted and, therefore, the concentration of hydrogen peroxide becomes lower with passage of time. The etching rate is strongly influenced by the concentration of hydrogen peroxide. Therefore, the etching rate is reduced as time elapses after the preparation of the ammonia-hydrogen peroxide solution22, and the etching rate cannot be constantly maintained. In particular, for two reasons described below, the decomposition of hydrogen peroxide tends to be promoted when the ammonia-hydrogen peroxide solution22is circulated. The first reason is that in the case where the two-bath-type processing bath20is used as shown inFIG. 3, the area of contact between the ammonia-hydrogen peroxide solution22and atmospheric air is increased because the structure is such that the ammonia-hydrogen peroxide solution22overflows out of the inner bath201into the outer bath202when the ammonia-hydrogen peroxide solution22is circulated. The second reason is that oxygen dissolved in the ammonia-hydrogen peroxide solution22escapes from the solution by a cavitation effect due to changes in pressure in the circulation pump32.

The inventor of the present invention earnestly made studies and found that the decomposition of hydrogen peroxide in the ammonia-hydrogen peroxide solution can be inhibited by dissolving titanium in advance. This enables constantly maintaining the etching rate for a long time period. The reason that the decomposition of hydrogen peroxide is inhibited if titanium is dissolved immediately after the preparation of the ammonia-hydrogen peroxide solution will be described below. A reaction of hydrogen peroxide shown by formula 1 below occurs in an alkaline solution to produce a hydroperoxy radical. i.e., OOH.
H2O2+OHH2O+OOH  (formula 1)

It is thought that the hydroperoxy radical acts to promote the decomposition of hydrogen peroxide by reacting with hydrogen peroxide in an alkaline solution. It is thought that the reaction shown by formula 2 below occurs and the decomposition of H2O2progresses at an increasingly fast rate.
H2O2+OOH→O2+H2O+OH  (formula 2)

By dissolving titanium in the ammonia-hydrogen peroxide solution, a reaction shown by formula 3 below is caused.
TiOOH+NH3=NH2OH/TiOH  (formula 3)

The hydroperoxy radical is consumed with priority to produce hydroxylamine. Hydroxylamine, i.e., NH2OH, is a salt of titanium. Because the reaction shown by formula 3 is caused, the reaction of the hydroperoxy radical causing decomposition of hydrogen peroxide as shown by formula 2 can be inhibited. The amount of titanium to be dissolved in the ammonia-hydrogen peroxide solution22may be experimentally determined so that the reaction shown by formula 2 above is sufficiently inhibited.

Subsequently, in step S106, the SiC wafer10is put in the processing bath20and immersed in the etching solution29.FIG. 6is a diagram showing an etching step according to the embodiment of the present invention. The SiC wafer10to be etched has a titanium film14laid on an SiC substrate12and a resist film16laid on the titanium film14and has the resist film16patterned into a desired shape.FIG. 6shows a state where an etched groove15is formed in the titanium film14.

In the present embodiment, the titanium film14on the SiC wafer10is etched in step S106. The present invention, however, is not limited to this. A film of a metal other than titanium may be etched by using the etching solution29. For example, nickel film may be laid instead of the titanium film14on the SiC wafer10and etched in step S106. From the viewpoint of prevention of contamination, however, it is preferable that the metal dissolved in advance and the metal to be etched be the same. The manufacturing method according to the present embodiment is therefore suitable for etching on the titanium film14. Since each of titanium and nickel is Schottky-junctioned to silicon carbide, the etching method according to the present embodiment is preferably used for forming a Schottky barrier electrode on the SiC substrate12.

The results of experiments on the embodiment of the present invention will be described below with reference toFIGS. 7 to 9.FIG. 7is a diagram showing the results of an experiment on a comparative example compared with the embodiment.FIGS. 8 and 9are diagrams showing the results of experiments on the embodiment of the present invention. InFIGS. 7 and 9, the NH3concentration is plotted with a rhombus; the H2O2concentration, with a square; and the amount of removal of titanium in a case where the SiC wafer10is immersed in the etching solution29for 4 minutes, with a triangle. InFIGS. 7 and 9, the scale on the left-hand side indicates the concentration [%] while the scale on the right-hand side indicates the amount of removal of titanium film [nm].

FIG. 7is a diagram showing the results of an experiment using, as a comparative example, an ammonia-hydrogen peroxide solution in which titanium is not dissolved. The results are changes in concentration with respect to elapsed time and an amount of removal of titanium. The elapsed time is a time elapsed after the preparation of the ammonia-hydrogen peroxide solution. In a region where the elapsed time was 0 to about 10 hours immediately after the start of the experiment, the H2O2concentration lowered rapidly to 10% or less. When the elapsed time became about 100 hours, the H2O2concentration lowered to about 1 to 0%. The amount of removal of titanium was measured at the stage at which the elapsed time was 100 hours and found to be substantially zero nm.

FIG. 8shows the relationship between the H2O2concentration and the titanium etching rate. The amount of removal of titanium in the case where the SiC wafer10was immersed in the etching solution29for 4 minutes was plotted with respect to the H2O2concentration changed. If the H2O2concentration is reduced, the titanium etching rate lowers, as shown inFIG. 8.

FIG. 9shows the results of an experiment using the etching solution29according to the present embodiment. In contact to the results shown inFIG. 7, the reduction in H2O2concentration was extremely small even when the elapsed time became 100 hours or longer. The reduction in H2O2concentration was stable at about 12%. Etching of the titanium film14was performed at a stage at which the elapsed time was about 160 hours in the experiment on the present embodiment. The amount of removal by this etching was about 300 nm. Thus, in the present embodiment, the etching rate can be constantly maintained for a long time period by inhibiting the decomposition of hydrogen peroxide in the ammonia-hydrogen peroxide solution.

REFERENCE SIGNS LIST