JPH0491433A - Multilayer resist and its etching method - Google Patents

Abstract

PURPOSE:To prevent a lower-layer resist from being undercut and to enhance a fine size further by a method wherein, when the lower-layer resist is etched by making use of an upper resist layer on it as a mask, a mixed gas of oxygen and nitrogen is used as an etching gas. CONSTITUTION:When a lower-layer resist 3 is etched by making use of upper resist layers 4, 5 on it as a mask, a mixed gas of oxygen and nitrogen is used as an etching gas. For example, a conductive layer 2 is coated with a lower- layer resist 3; and this assembly is baked. After that, an SOG 4 is coated; a high-resolution photoresist as an upper-layer resist 5 is coated on it; and this assembly is baked. Then, the upper-layer resist 5 is etched; and the SOG 4 is etched by making use of the upper-layer resist 5 as a mask by a dry etching method using a CF4-based gas. Then, when the lower-layer resist 3 is dry-etched by making use of the SOG 4 as a mask, a mixed gas of oxygen and nitrogen is used as an etching gas. It is proper that the content of nitrogen in the mixed gas at this time is set at about 20 to 70%.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a multilayer resist structure used when forming a circuit on a semiconductor substrate, and an etching method using the multilayer resist structure.

[Conventional technology]

2. Description of the Related Art When manufacturing ICs (integrated circuits) and LSIs (large scale integrated circuits), a resist is deposited on a semiconductor substrate and etched into a desired pattern. As circuits become more highly integrated and densely packed, it becomes necessary to form fine patterns, and for this purpose, it is required to improve the resolution when patterning a resist. A multilayer resist was devised to meet such demands, and is composed of a plurality of resist layers. A typical example is a three-layer resist. This consists of a lower layer resist, an intermediate layer resist, and an upper layer resist, and the intermediate layer resist includes a coated and fired silicon oxide film (
Spin On Glass (hereinafter referred to as rsOGJ) is used. FIG. 2 is a diagram showing the process of etching a conventional multilayer resist. In FIG. 2, 1 is a semiconductor substrate, 2 is a conductive layer, 3 is a lower resist layer, 31 is a side wall, 4 is SOG,
5 is an upper layer resist. Here, a case is taken as an example in which a conductive layer 2 is formed on a semiconductor substrate and is etched to form a desired wiring state. Each step will be explained below. ■ Process in Figure 2 (A) After coating the lower resist 3 on the conductive layer 2 and baking it,
Apply 5OG4. Thereon, a high-resolution photoresist (eg, novolak resist) is applied as an upper resist 5 and baked. (2) Etching the upper resist layer 5 in the process shown in FIG. 2 (b). If an electron beam drawing resist is used as the upper resist 5,
Patterning is performed using an electron beam direct writing device and etching is performed. (2) Process of FIG. 2(C) Using the upper layer resist 5 as a mask, 5OG4 is etched by a dry etching method using CF and a series gas. (2) Step 5 in FIG. 2(d) Using OG4 as a mask, the lower resist layer 3 is etched by a dry etching method using oxygen (plasma). At this time, the upper layer resist 5 remaining on 5OG4
is also removed. The etching at this time extends slightly toward the side walls of the lower resist 3, and the side walls 3-1 have an inwardly cut shape, that is, an undercut shape. (2) Step 5OG4 in FIG. 2 (e) is peeled off and removed using HF solution. ■ Process in Figure 2 (to) Use the lower resist layer 3 as a mask and use the RIE method (Reacti).
The conductive layer 2 is etched by ve ion etching. After etching, the lower resist layer 3 is removed by an ashing method or the like. In this way, the conductive layer 2 is formed into a desired pattern of wiring.

[Problem to be solved by the invention]

(Problems) However, the above-mentioned conventional technology has the following problems. The first problem is that the undercut that occurs on the sidewall 3-1 of the lower resist 3 impedes pattern refinement. The second problem is that the 5
The problem is that OG4 easily peels off. The third problem is that the refractive index of light between 5OG4 and the lower resist 3 is different, so the overall film thickness cannot be controlled accurately, and the refractive index is different = 4. Diffuse reflection of light occurs at the interface. The point is that it happens. (Explanation of the problem) (1) The first problem will be explained. As a result of the side wall 3-1 being undercut, Figure 2 (to)
As shown in FIG. 2, the width of the conductive layer 2 becomes narrower than the desired width, and the width is not as designed. In order to make the width as designed, the second
The width of 5OG4 in Figure (d) must be widened. This hinders pattern miniaturization. By the way, it is not as if there were no methods for preventing undercuts in the past. This can be prevented by increasing the discharge power and lowering the pressure as low as possible when etching the lower resist layer 3. However, in this case, the etching rate of the lower resist 3 decreases, and the etching selectivity for the 5OG4 mask (the ratio indicating how much faster the lower resist 3 is etched than the 5OG4 under the same etching conditions) increases. becomes small. As a result, if there is a convex part on the base to which the multilayer resist is applied, the 5OG4 above it will be explained in detail next.
Another problem arises in that the pattern may disappear during etching, and the pattern may not be formed as desired in the end. FIG. 7 is a diagram illustrating the problems of the conventional undercut prevention method. The symbols correspond to those in FIG. 2, 4-1 is a thin film part of 5OG4, 4-2 is a thick film part, and 6 is a convex part of the base. If there is a protrusion 6 on the base to which the multilayer resist is applied, the thickness of the 5OG4 coated above it will be thinner than in the area without the protrusion 6. FIG. 7 shows the stage of etching 5OG4, with FIG. 7(a) showing a cross section and FIG. 7(b) showing a view from above. As shown in FIG. 7(b), if 5OG4 is now etched into a thin strip, the lower resist 3 is spread around it. A convex portion 6 lies below it. In the next step, the lower resist 3 is applied using 5OG4 as a mask.
However, if the discharge power is increased and the pressure is made as low as possible to prevent undercutting, the selectivity of the lower resist 3 to 5OG4 becomes small. That is, the amount of 5OG4 etched simultaneously with the etching of the lower resist 3 increases. Therefore, by the time the etching of the lower resist layer 3 is finished, the thin film portion 4-1
There are cases where all of the information is etched out. In that case, 5OG4 will be interrupted at the thin film portion 4-1. If there is a break, the final pattern obtained will be incorrect. Under these circumstances, there has been a desire for a method that can prevent undercuts without increasing the discharge power or lowering the pressure. (2) The second problem will be explained. While the lower resist 3 and the upper resist 5 are hydrophilic, 5OG4 is hydrophobic. Therefore, it is difficult to form chemical bonds between the 5OG4 and the lower resist 3 or between the 5OG4 and the upper resist 5. Therefore, the upper resist 5 may peel off from the 5OG4,
5OG4 sometimes peeled off from the lower resist layer 3. (3) The third problem will be explained. As the lower resist 3, for example, a novolac resist (for example, TFR-3 manufactured by Tokyo Ohka Kogyo) is used.
The refractive index of this light is 1.64. As the upper layer resist 5, for example, a high-resolution photoresist (for example, T SMR-8700 manufactured by Tokyo Ohka Kogyo) is used, and the light refractive index of this is 1.64. However, the refractive index of 5OG4 is 1.42 to 1.46, which is different from both. Since the overall film thickness of the multilayer resist is also related to how long the etching time should be, etc., it is desirable to be able to control it as accurately as possible. However, as mentioned above, since the refractive index is different, it has not been possible to accurately control the film thickness even using an optical measuring device. Furthermore, diffuse reflection of light occurs at interfaces with different refractive indexes, impairing pattern transfer accuracy and resolution. This kind of thing is also a factor that hinders miniaturization. An object of the present invention is to solve the above-mentioned problems.

[Means to solve the problem]

In order to solve the above problems, the following measures were taken in the multilayer resist and etching method of the present invention. That is, in the multilayer resist etching method of the present invention, a mixed gas of oxygen and nitrogen is used as the etching gas when etching the lower resist layer using the resist layer above it as a mask. In that case, the range of nitrogen content in the mixed gas is selected to be 20 to 70%. Furthermore, in the multilayer resist of the present invention, a coated and fired silicon oxide film made hydrophobic by containing methyl groups is formed on the lower resist layer. In that case,
The content of methyl group to silicon is 0.2 to
Select 0.33. Furthermore, in the multilayer resist of the present invention, a coated and fired silicon oxide film whose refractive index is made equal to that of the lower resist by containing TiO2 is formed on the lower resist. In that case, the content rate of T, 0□ is 3
0 to 70% by weight is selected.

[For production]

If a mixed gas of oxygen and nitrogen is used as the etching gas when etching the lower resist 3, the lower resist 3 will not be undercut, although the reason is still unclear. Therefore, it becomes possible to perform fine patterning. Furthermore, if the structure of the multilayer resist is such that a coated and fired silicon oxide film containing methyl groups and made hydrophobic is formed on the lower resist layer, chemical bonding will easily occur with the lower resist layer and the upper resist layer, which are also hydrophobic. This makes it possible to prevent the resist layer from peeling off. Furthermore, the structure of the multilayer resist is changed to T on the lower layer resist.
If a coated and fired silicon oxide film is formed that has a refractive index equal to that of the underlying resist by containing iO2, the refractive index will be the same, so the thickness of each resist layer as a whole can be accurately controlled using an optical measuring device. become able to do. At the same time, diffuse reflection due to the difference in refractive index does not occur at the boundary surface of the resist layer, thereby improving pattern transfer darkness and resolution.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. [First Embodiment] This embodiment relates to a multilayer resist etching method that prevents the side wall 3-1 of the lower resist 3 from being undercut. FIG. 1 shows a multilayer resist etching process using the etching method of the present invention. The symbols correspond to those in FIG. ■ The process from Figure 1 (A) to Figure 1 ()X) is the same as the process from Figure 2 (A) to Figure 2 (C), so the explanation is omitted. . ■ Conventionally, oxygen (plasma) was used as the etching gas when dry etching the lower resist layer 3 using Step 5OG4 in FIG. Use. If such a mixed gas is used, even if etching is performed with a small discharge power and a high pressure, the lower resist 3 will not be undercut, and the side wall 3-1 will be vertical as shown. However, it has not yet been determined in detail why mixing nitrogen gas prevents under-force and traction. FIG. 3 is a diagram showing the relationship between the N2 content in the etching gas and the resist etching rate, and the relationship between the selectivity of the resist to SOG. Curve A indicates the etching rate, and curve opening indicates the selectivity. As the nitrogen content increases, the etching rate and selectivity decrease. Experiments have shown that if the nitrogen content is up to about 20%, undercuts are still observed, so it is preferable to increase the nitrogen content higher than that. However, if the content is too high, the etching rate and selectivity will decrease as shown in Figure 3, resulting in problems (e.g., 5OG
4) will appear. Therefore, the content is 20
It is appropriate to set it to about 70%. (2) Step 5OG4 in FIG. 1 (E) is peeled off and removed using HF solution. (2) Process of FIG. 1(f) Using the lower resist layer 3 as a mask, the conductive layer 2 is etched by RIE. Since the sidewall 3-1 of the lower resist 3 is vertical, the conductive layer 2 has a width as designed. Therefore, the width of 5OG4 in FIG. 1(d) does not need to be wide enough to accommodate the undercut. Thereby, miniaturization can be improved. [Second Embodiment] In this example, in order to prevent the upper layer resist 5 or 5OG4 from peeling off, the middle layer resist 5OG4 is made hydrophobic when forming a multilayer resist. 5OG4 is

[Third example]

In this example, 5O
G4 contains TiO2 so that the refractive index of 5OG4 light is approximately equal to the refractive index of the lower resist layer 3 and the upper resist layer 5. FIG. 6 is a diagram showing the relationship between the T and O□ contents in SOG and the refractive index. The content on the horizontal axis is weight %. As the content of T,02 increases, the refractive index also increases. The content at which the refractive index is 1.64 is about 50%. Therefore, by setting the content of T and O□ to about 50%, it is possible to make the refractive indexes of 5OG4, the lower resist 3, and the upper resist 5 almost the same. As a result, the thickness of the entire multilayer resist can be accurately controlled using an optical measuring device, and diffuse reflection of light does not occur at the boundaries of the resist layers, improving pattern transfer accuracy and resolution. Next, a specific example of forming the multilayer resist of this example as shown in FIG. 1(a) will be shown. (1) First, a novolak resist (eg, TFR-3 manufactured by Tokyo Ohka Kogyo Co., Ltd.) is applied and fired to form the lower resist 3. Coating is done using a spin coater, for example 430
Run for 30 seconds at 0 rpm. Firing, for example, at 180'C
Do this for 6 minutes. The thickness of the lower resist 3 formed in this manner is approximately 1 μm. (2) Next, SOG (e.g., Tokyo Ohka Kogyo Type
-6゜Ti-3i Film) with about 50% TiO□
The contained material is coated and fired to form an intermediate layer resist. Application is performed using a spin coater, for example, 300O
RPM for 30 seconds. For example, baking at 180°C
Do this for minutes. The thickness of the 5OG4 film formed in this way is approximately 0.1
It is μm. (3) Finally, a high-resolution photoresist for optical drawing (for example, T SMR-8700 manufactured by Tokyo Ohka Kogyo) is coated and baked to form the upper layer resist 5. Coating is performed using a spin coater, for example, at 11,000 rpm for 30 seconds. Firing is performed, for example, at 120'C for 6 minutes. The thickness of 5OG4 thus formed is about 0.5 μm. The substance to be included in the SOG is not limited to Ti 2 O, but any other substance may be used as long as it can increase the refractive index of the SOG until it becomes equal to the refractive index of the lower resist 3 and the upper resist 5.

【Effect of the invention】

As described above, the multilayer resist and the etching method thereof of the present invention have the following effects. ■Prevents undercutting of the lower resist. If a mixed gas of oxygen and nitrogen is used as the etching gas when etching the lower resist layer 3, the lower resist layer 3 will not be undercut, although the reason is still unclear. Therefore, there is no need to design the etching width in consideration of the undercut, and it is possible to further improve the miniaturization. ■Preventing peeling between resist layers. If the structure of the multilayer resist is such that a coated and fired silicon oxide film that is made hydrophobic by containing methyl groups is formed on the lower resist layer, it is likely to chemically bond with the lower resist layer and the upper resist layer, which are also hydrophobic. This improves adhesion. Therefore, peeling between resist layers is prevented. ■Accurate control of film thickness and prevention of diffused reflection at the resist layer interface. The structure of the multilayer resist is T10□ on top of the lower layer resist.
If a coated and fired silicon oxide film is formed which has a refractive index equal to that of the underlying resist by containing the resist, the film thickness can be accurately measured using an optical measuring device. Therefore, the film thickness of each resist interlayer can be accurately controlled. Further, since diffuse reflection due to a difference in refractive index does not occur at the boundary surface of the resist layer, pattern transfer accuracy and resolution are improved. Figure 7: A diagram explaining the problems of the conventional undercut prevention method. In the diagram, 1 is the semiconductor substrate, 2 is the conductive layer, 3 is the lower resist layer, 3-1 is the side wall, 4 is SOG, and 5 is the upper layer. It is a resist.

[Brief explanation of drawings]

Figure 1: A diagram showing a multilayer resist etching process using the etching method of the present invention. Figure 2: A diagram showing a conventional multilayer resist etching process. Figure 3: N2 content in etching gas. Figure 4 shows the relationship between the resist etching rate and resist etching rate and the selectivity of the resist to SOG. Figure 5 shows the relationship between the methyl group content and the RIB etching rate. Infrared absorption. The 6th part of the same shows the spectrum measurement results.
Figure: Diagram showing the relationship between T, 0□ content and refractive index in SOG Patent applicant Fuji Xerox Co., Ltd. Representative Patent Attorney Tomio Honjo 1 Figure 2 Figure 2 Δ゛, \J- O'ooi'f;,'rgo%sumu ゛be KJ-1 (N + ordinary' hem shouting down)

Claims (6)

[Claims] (1) A multilayer resist etching method characterized in that a mixed gas of oxygen and nitrogen is used as an etching gas when etching a lower resist layer using the resist layer thereon as a mask. (2) The multilayer resist etching method according to claim 1, characterized in that the nitrogen content in the mixed gas is 20 to 70%. (3) A multilayer resist characterized in that a coated and fired silicon oxide film made hydrophobic by containing methyl groups is formed on a lower resist layer. (4) The content of methyl groups in silicon is 0.2 to 0.
．． 3. The multilayer resist according to claim 3, wherein (5) A multilayer resist characterized in that a coated and fired silicon oxide film is formed on the lower resist layer to have a refractive index equal to that of the lower resist layer by containing TiO_2. (6) The multilayer resist according to claim 5, characterized in that the content of TiO_2 is 30 to 70% by weight.