JPH01136340A - Pressure reduced wet etching method - Google Patents

Abstract

PURPOSE:To reduce dispersion in widths of metal wirings, which are formed by etching, by changing pressure reducing conditions during one trial of etching. CONSTITUTION:When etching liquid 2 becomes a specified operating temperature, a wafer carrier 5 and wafers 6 are immersed in the etching liquid 2 together. Then the surface of part of the Al/Si (1%) film is dissolved and reacted, and etching is made to progress. The pressure reducing conditions in the etching are as follows: initial pressure is set at P1; the pressure is decreased to P2 during a specified period theta1; the pressure is kept at P2 for a specified interval theta2; and the pressure is returned to P1 again during a period theta3. The change of the pressure is repeated until the finish of the etching several times periodically. The pressure reduction is performed by operating a rotary vacuum pump 10 in controlling the operation. The operation of the rotary vacuum pump 10 is controlled with a time controller 11 so that boost etching is performed during the period of about 10% of the required etching time after the finish signal is received from an etching-finish detecting sensor 12.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an etching method for etching, for example, a metal film of a semiconductor device into a predetermined pattern. This invention relates to a method of etching with high precision through control.

[Prior art]

As a conventional reduced pressure etching method, there is a method using an apparatus as shown in FIG. 4, for example.

In FIG. 4, an etching bath 1 is filled with an etching solution 2, and a semiconductor wafer 6 mounted on a wafer carrier 5 is immersed in the etching solution 2 whose temperature is controlled to be constant.

On the surface of the semiconductor wafer 6, a part to be etched is usually formed, such as a metal film, which is partially formed with an insulating film interposed therebetween, and a photo process is used to form a specific wiring thereon. A resist pattern is provided.

Further, a space (above the etching liquid 2) of the etching tank 1, which is configured to be airtight, is drawn by a vacuum pump 10 and is depressurized.

The above-mentioned reduced pressure state is maintained at a desired constant degree of vacuum during one etching trial, which is set by a time controller 11 which is activated in response to a signal from an etching end detection sensor 12. In order to maintain the above-mentioned constant degree of vacuum, for example, information on the degree of vacuum is detected by the vacuum gauge 4,
The slow leak valve 7 is controlled by the controller 3 that receives it.
By opening and closing, the degree of vacuum can be easily controlled to a constant value.

Wet etching performed under reduced pressure as described above has the following advantages over etching under normal pressure without reduced pressure.

That is, in wet etching of a metal film such as a shipboard formed on a semiconductor substrate, the etching solution is usually
An acid solution or an alkaline solution is used, and a gas such as H2 is produced as a result of etching, for example, when etching an M film with an aqueous phosphoric acid solution. There are many examples of this, not only when etching metal films, but also when electrolytically etching Si with hydrazine.
2 gases are generated.

However, gas bubbles generated from the surface of the object to be etched during etching cannot be promptly removed.
If it remains attached to certain surface areas for a long period of time, etching will not progress on those areas and etching remains, making it impossible to obtain the desired wiring pattern and causing short circuits (bridges) between wiring. Trouble arises.

To deal with such troubles, vacuum etching has the advantage of rapidly expanding the bubbles generated during etching and quickly removing them from the wafer surface, thereby preventing etching residue. Since the pressure is not distributed in the etching tank, bubbles are released uniformly over the entire surface of the wafer, which refreshes the etching surface.At the same time, the etching solution is stirred, reducing density and temperature unevenness. , it has the effect of improving the uniformity within and between wafers (for example, Takano "Electronic Materials" p. 91. March 1983 issue and Journal of Electrochemical Society Ha
ra, T et al, “Journal of
Electrochemical 5ociety, v
ol, 132. p. 29731985).

The vacuum etching described above is equipped with a mechanism for detecting the end point of etching using an optical method or electrical sensing, and has been put to practical use as an etching device for fine dimensions of one order of magnitude in both thickness and width formed on a wafer. (For example, as described in the above-mentioned document and "Pametal Surface Technology" vol. 36. p. 6931985).

[Problem that the invention seeks to solve]

However, in the conventional low-pressure wet etching as described above, there are cases where it is difficult to uniformly and accurately etch wiring patterns with minute dimensions, as in some places being etched thinner than the required wiring width. be. If a portion is etched to be thinner than the required wiring width, there is a problem in that the reliability of the semiconductor device decreases and quality deteriorates.

Possible causes of the above problem are as follows.

In other words, wiring on a semiconductor substrate made of a locust film or A-gold film is usually made of an insulating film such as an oxide film provided on a semiconductor wafer containing a certain concentration of boron, phosphorus, etc. ions as carriers. and is electrically connected to the semiconductor wafer at its distal end or at a suitable intermediate location. And the connection consists of p near the surface of the wafer.
It is electrically insulated by an n-junction or the like, and is electrically connected to almost the entire wafer.

Therefore, one section of the wiring section is electrically connected to the wafer, and the other section is insulated from the wafer.

FIG. 5 is a diagram showing the above state.

In FIG. 5, for example, when etching the shell film formed on the S1 wafer 40 with phosphoric acid, most of the At film is electrically connected to the wafer because each wiring is not separated in the initial stage. , A corrosion reaction occurs between SL and SL due to contact with different metals, and M becomes the anode.
, S, and t become negative 4@ (cathode), promoting the dissolution of At. Of course, this is due to the non-uniformity of the At film (segregation of impurities, strain, surface condition, etc.).

The hemlock is also dissolved by local battery action. Then, the 7th node side will melt, and H2 will be generated on the cathode side.

Next, when the etching progresses and each wiring is separated, the side wiring portion 30 shown in FIGS. 5(A) and 5(B) is formed. In (A), at contact points X and Y between the head and Si, the pn junction is reverse biased and All and Si are insulated, but at the contact point Z, the head and Si are electrically connected. . As a result, the wiring section 30 in (A) is electrically connected to the SL. On the other hand, in (B), the only contact points are X and Y, and 1M and Si are insulated and □.

Therefore, when the wiring portion 30 is etched in (A), the wiring portion 30 made of Aa serves as an anode, and the Si wafer 40 serves as a cathode, thereby promoting the etching of Aa. On the other hand, in (B), since the oxide is insulated and separated from the Si, the dissolution of the oxide occurs only by the local battery action. Therefore, there is a difference in etching speed between the etching shown in (A) and the etching shown in (B), and the undercut formed under the resist formed in the photo process after etching for a certain period of time is less in (B) and less in (A). Since the width of (A) is extremely narrower than that of (, B), variations in wiring width will occur.

The above phenomenon also occurs in etching under normal pressure rather than reduced pressure, but in etching under normal pressure, the difference between (A) and (B) is small. For example, normal pressure (atmospheric pressure), 50°C, etching solution 85wt% phosphoric acid,
A of 1#lI thickness on a 3 inch wafer under the condition of manual stirring.
When etching a/S i' (1%) film (Afi alloy film containing 1% SL), the difference in width between the (A) type and (B) type interconnects shown in Figure 5 is within about 4%. be. On the other hand, when the above-mentioned etching is performed under a reduced pressure of 30 torr, the difference between type (A) and type (B) is that it is affected by the properties of the Aa film and the degree of fatigue of the etching solution.
A maximum difference of about 40% occurs in the wiring width.

The present invention has been made in order to solve the problems of the prior art as described above, and it is an object of the present invention to provide a reduced-pressure wet etching method that eliminates the above-mentioned problems while taking advantage of the advantages of reduced-pressure wet etching as described above. With the goal.

[Means to solve the problem]

In order to achieve the above object, one aspect of the present invention is as follows.

In wet etching performed under reduced pressure, the reduced pressure conditions are changed during one etching trial.

As described above, in the present invention, by changing the reduced pressure conditions during etching, the bubble film covering the surface of the metal to be etched is vibrated, thereby causing the bubbles to separate from the metal surface, so that the bubbles covering the metal surface can be etched. It is believed that the above-mentioned problems can be solved by this method. Note that the details of the action will be described later.

[Embodiments of the invention]

FIG. 1 is a diagram of an embodiment of an etching apparatus used in the present invention.

The apparatus shown in FIG. 1 is the same as the apparatus shown in FIG. 4, with the following means added for changing the reduced pressure conditions in the etching bath 1 in a undulating manner during one trial of wet etching.

That is, in FIG. 1, 21 is a leak solenoid valve that releases the reduced pressure to the atmosphere, and 20 is a controller with a timer that temporally controls the opening and closing of the leak solenoid valve 21 while detecting the pressure in the etching tank 1. , and the same reference numerals as in FIG. 4 indicate the same parts.

Further, on the Si wafer 6, for example, A to be etched is etched.
jL/5i (1%) film is formed, and a resist for etching the film into a predetermined pattern is further applied to the M/5i (1%) film.
1%) is formed on the film by a photo process. Further, in the etching bath 1, an 85 wt % phosphoric acid aqueous solution, for example, is placed as an etching solution 2, and the temperature of the solution is controlled to be constant.

An example of the operation method is as follows.

When the etching solution 2 reaches a constant operating temperature, for example 50° C., and the wafer 6 is immersed together with the wafer carrier 5 in the etching solution 2, the surface portion of the AM/5L (1%) film in contact with the etching solution 2 is removed. Etching progresses as a result of the dissolution reaction.

The reduced pressure conditions during this etching are varied as shown in FIG. 2, for example.

That is, the initial pressure is set to pt (it is practical to use atmospheric pressure, and the following explanation will be based on atmospheric pressure), the pressure is lowered to P2 for a certain period θ, and then the pressure is lowered to P2 for a certain period θ8. The pressure is maintained at P2 for a period of time θ3, and then returned to Pl again during a period θ3. The above pressure change is periodically repeated several times until etching is completed.

In the above control, pressure reduction is achieved by constantly operating the rotary vacuum pump 10. Further, the operation of the rotary vacuum pump IO is controlled by the time controller 11 so that after receiving the end signal from the etching end detection sensor 12, back-up etching is performed for about 10% of the etching time required up to that point.

In addition, the operation to maintain the pressure at a constant value P for a certain period of time is carried out by detecting the pressure in the etching tank 1 with the vacuum gauge 4 against the reduced pressure that is constantly drawn by the rotary vacuum pump 10. This is realized by the controller 3 controlling the slow leak valve 7 according to the result.

That is, when it is detected from the detection value of the vacuum gauge 4 that the pressure in the etching tank 1 has become higher than P2, the controller 3 closes the slow leak valve 7 to increase the degree of pressure reduction, and conversely, the pressure is increased. When the pressure becomes lower than P3, the slow leak valve 7 is opened to release the reduced pressure.

Further, the fixed period θ3 during which the pressure is maintained at P2 is controlled by the timer controller 20.

That is, the leak electromagnetic valve 21 is opened when a certain period of time θ2 has elapsed since the vacuum gauge 4 detected that the pressure reached P3. Then, the leak solenoid valve 21 is opened to increase the pressure, and when the vacuum gauge 4 detects that the inside of the etching tank 1 reaches P, the leak solenoid valve 21 is closed by the timer controller 20. .

The period θ for returning the pressure to Pl is determined by the diameter of the leakage solenoid valve 21 and the pressure P2, but is usually several seconds or less if the leakage solenoid valve 21 is made sufficiently large.

By repeating the above operations until the end of back etching, one time of etching is completed.

For example, P1 = atmospheric pressure, P = 17 torr, θ
□ = 10 seconds, θ8 = 60 seconds, θ, = 3 seconds, Aa/5i (1%) with a thickness of 1- formed by sputtering
In the result of pattern etching of the wiring width of an IC designed according to S's five-prison rule, (A) in FIG.
, (B), there was almost no relative difference in the wiring width between the states. By the way, Aa/5t (1%
) For WA, the conditions other than the pressure are the same, and the pressure is 1.
The relative difference in wiring in the case of normal vacuum etching performed at a constant pressure of 7 torr was about 0.84 Il, which was significantly larger than in the example of the present invention described above.

In addition, in the method of the present invention, the characteristics of the wiring other than the line width (uniformity, etc.) were also superior to the case of ordinary vacuum etching.

Next, the effect will be explained.

The reason why there is a large difference in the patterned wiring width by etching under reduced pressure is thought to be because the phenomenon in which gas bubbles generated by the etching reaction cover the surface of the metal to be etched as a gas film is greatly involved in the etching reaction. It will be done.

That is, the etching progresses to (A) in FIG.
When separated into the two pattern wiring parts shown in (B), the wiring in the pattern (A) covered with bubbles causes an etching reaction due to the contact of different metals between the ^fi-5i wafers, for example, and the wafer H3 bubbles are generated from the back side, and at the same time, a small amount of A is present in the solution at the interface between the bubbles and the metal surface to be etched, even though it is covered with bubbles.
Metals such as A can be eluted in the form of ions. These cations diffuse into the bulk of the solution through the solution forming the interface between the gas bubbles forming the gas film, and thus the etching reaction progresses in pattern (A). Of course, at the same time, if a cation such as H+ that contributes to gas production at the cathode is present in the solution between the bubble and the metal surface, the etching reaction will proceed even if a local cell is formed.

Before the wiring is separated into (A) and (B), the wiring part is etched by an etching reaction caused by contact with dissimilar metals accompanied by foaming from the backside of the wafer and an etching reaction caused by the local battery. As the etching progresses and the wiring is separated into (A) and (B), and only (A) is etched, the etching of (A) is rather accelerated than before. This is consistent with what has been actually measured.

On the other hand, in the case of type (B) wiring, the reaction hardly progresses when there are no gas-generating cations in the solution between the bubbles and the metal surface while the wiring is covered with bubbles.

In other words, it is considered that the bubble film acts passively against etching and inhibits etching.

The gas film covering the metal surface in the above-mentioned phenomenon is thought to cover the metal surface uniformly and thinly (that is, the bubble film is poor) when the degree of pressure reduction increases (becomes low pressure).

The above is consistent with the actual measurement results, and the example shown in FIG. 3 is obtained as the relationship between the operating pressure and the relative difference in wiring width. That is, 1. In normal reduced pressure etching, as the pressure P becomes smaller (the reduced pressure becomes larger), the interconnect width relative difference ΔL becomes larger.

In response to the above phenomenon, as in the present invention, the pressure conditions are
It is thought that the change as shown in the figure has the effect of causing the bubble film covering the surface of the metal to be etched to disappear during the cycle formed during etching. That is, reduced pressure P
2 to atmospheric pressure P, the bubble film is vibrated and thereby peeled off from the metal surface. At that time, it is quite conceivable that a large number of bubbles coalesce, etc., which is further effective in peeling off.

Note that the characteristics shown in FIG. 2 are an example of the pressure reduction conditions in the present invention, and as long as the characteristics can promote the separation of bubbles by changing the pressure during etching, as in the above principle, the second It goes without saying that the characteristics are not limited to the characteristics shown in the figure.

Furthermore, it goes without saying that the mechanism for changing the pressure reduction conditions is not limited to the above example.

〔Effect of the invention〕

As explained above, according to the present invention, in wet etching performed under reduced pressure, the reduced pressure conditions are changed during one trial of etching, thereby reducing the variation in line width between metal wirings formed by etching. The advantageous effect of reducing the size of the etching device is that it can be achieved by relatively simple improvement of existing equipment without sacrificing the advantages of reduced pressure etching.

In addition, the method of the present invention has the effect of stirring the vicinity of the interface where the etching reaction is occurring even more than normal vacuum etching, thereby eliminating density unevenness and temperature unevenness and promoting mass transfer. It can also contribute to improving surface uniformity and etching speed.

[Brief explanation of the drawing]

Figure 1 is an embodiment of the apparatus used in the method of the present invention;
The figure shows an example of the reduced pressure conditions of the present invention, Fig. 3 is a characteristic diagram of the relative difference between the pressure and the width of formed wiring, Fig. 4 is an example of a conventional device, and Fig. 5 is an A for explaining problems. (This is a wiring pattern diagram. Explanation of symbols) 1...Etching tank 2...Etching liquid 3...Slow leak valve opening/closing controller 4...Vacuum gauge 5...Wafer carrier 6...・To the sea urchin 7...Slow leak valve 8...Piping 9...Etching liquid gas trap 10...Rotary vacuum pump 11...Time controller (with etching completion detection receiver) 12...Etching completion Detection sensor 20...Controller with timer 21...Leakage solenoid valve

Claims (1)

[Claims] A reduced pressure wet etching method characterized by changing reduced pressure conditions during one trial of etching in wet etching performed under reduced pressure.