JP2000012528A - Method for etching film covering stepped parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent etch residue by selectively implanting ions into regions of a film which is directly above the lower sides of steps covered with this film prior to etching and then etching the film. SOLUTION: Damage steps 24a of a polysilicon film 24 is subjected to ion implantation, the crystal structure of the steps 24a is damaged, resulting in a slightly brittle structure and hence high-etching rate parts 24b. Then the etching is made with a resist pattern 26 used as a mask, rapidly advanced at the high-etching rate parts 24b and ends earlier at the steps 24a easy to produce etch residue. When the overall etching ends, no etch residue is generated, a second layer polysilicon electrode 27 can be satisfactorily formed, and thereby the nonconformities caused by electrode short-circuits, etc., ascribed to the etch residue can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for etching a film for covering a step formed by a pattern such as a wiring or an electrode so as to eliminate an etching residue.

[0002]

2. Description of the Related Art As a process for forming a fine structure, for example, in the manufacture of a solid-state imaging device 1 as shown in FIG. 3, semiconductor manufacturing techniques such as photolithography and etching are often used.

Here, the solid-state imaging device 1 shown in FIG.
An imaging area 4 in which a photosensor unit 2 that performs photoelectric conversion and a vertical transfer register 3 that transfers signal charges is formed, and a horizontal transfer register 6 that transfers the signal charges transferred by the vertical transfer register 3 to an output unit 5 It is provided with. The transfer of the signal charges in the vertical register 3 is performed by a transfer electrode (not shown) formed on the vertical register 3.

In manufacturing the solid-state imaging device 1 having such a configuration, in order to form a transfer electrode, particularly in the imaging region 4, a first-layer polysilicon electrode 8 is formed on a silicon substrate 7 as shown in FIG. After that, the second line shown by the broken line in FIG.
A polysilicon film is formed for forming the layer polysilicon electrode 9 and is etched to form a predetermined pattern. In FIG. 4, the description of the configuration of the second-layer polysilicon electrode, the light-shielding film, the color filter, the lens, and the like is omitted.

[0005] At this time, in the vicinity of the step portion indicated by A in FIG. The process in which this etching residue occurs will be described with reference to FIGS. 5A to 5C which are sectional views taken along line BB of FIG. 4. First, as shown in FIG. Electrode 8
A polysilicon film 11 is formed thereon via an insulating film 10 formed so as to cover the polysilicon film. When the polysilicon film 11 is formed in this manner, it is needless to say that the polysilicon film 11 is also provided on the step formed by the presence or absence of the first-layer polysilicon electrode 8, that is, the first-layer polysilicon electrode 8. From just above the edge to the outside, the step 1
1a is formed.

When the step portion 11a is formed in this manner, the polysilicon film 11 has a difference between the film thickness of the flat portion indicated by C and the film thickness of the step portion 11a indicated by D in FIG. Will occur. That is, it is located immediately above the lower portion side of the step formed by the presence or absence of the first layer polysilicon electrode 8.
The thickness of the step portion 11a of the polysilicon film 11 is larger than that of the flat portion due to polysilicon deposited on the side surface of the first layer polysilicon electrode 8.

Next, in order to pattern the polysilicon film 11, a resist pattern 12 is formed as shown in FIG.
Is formed, and the polysilicon film 11 is etched using this as a mask. Since this etching method is microfabrication, it is usually performed by reactive ion etching (RIE).
Anisotropic dry etching such as is employed.

However, when the polysilicon film 11 is etched by the resist pattern 12 as shown in FIG. 5B, due to the difference in film thickness between the flat portion and the step portion 11a, as shown in FIG. That is, the etching residue 13 is generated at the position that was the step portion 11a.

As shown in FIGS. 5A to 5C, a natural oxide film 14 is formed on the surface of the polysilicon film 11, and this natural oxide film 14 is also perpendicular to the silicon substrate 7. The thickness in the direction is larger in the step portion 11a than in the flat portion. Therefore, since the natural oxide film 14 has a lower etching rate than the polysilicon film 10, the natural oxide film 14 has a vertical direction (perpendicular to the silicon substrate 7).
Is slowed down, so that the etching residue 13 is more likely to occur.

[0010]

However, if such an etching residue 13 occurs, there is a disadvantage that an electrode short circuit is likely to occur. Further, in the case of the solid-state imaging device 1, the location where the etching residue 13 is formed becomes the photosensor portion 2, so that impurities contained in the etching residue 13 in the process after this etching process are removed. Is contaminated, resulting in a defect.

As a measure to suppress the generation of such an etching residue 13, it is conceivable to increase the etching amount. In this case, however, the insulating film 15 on the silicon substrate 7 in the flat portion is thinned by over-etching. As a result, the insulating film 15 itself and the underlying layer are damaged, which causes inconveniences such as the occurrence of many defects called white spots and a decrease in yield.

The present invention has been made in view of the above circumstances. It is an object of the present invention to remove an etching residue when etching a film covering a step portion on a substrate formed by a pattern of wiring, electrodes, and the like. An object of the present invention is to provide an etching method in which the remaining is prevented.

[0013]

In the method of etching a film covering a step portion according to the present invention, a film covering a step portion on a substrate formed by a pattern of wirings, electrodes, etc. is etched prior to the etching. In the membrane,
The ion-implantation process is selectively performed immediately above the lower portion of the step portion, and thereafter, the film is etched.

According to this etching method, in the film covering the step portion, the ion implantation process is selectively performed in advance on the portion immediately above the lower portion of the step portion where the film thickness is larger than that of the flat portion. Immediately above the stepped portion on the lower side where the etching residue tends to occur is damaged by the ion implantation process, and the etching rate increases, so that when the etching process is performed thereafter, the etching residue does not occur.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on an embodiment. In the present example, the case where the method for etching a film covering a step portion according to the present invention is applied to the step of forming a transfer electrode in the imaging region 4 shown in FIG. 4 in the solid-state imaging device 1 shown in FIG. This will be described with reference to FIGS. 1A to 1F which are sectional views taken along line BB of FIG.

In this embodiment, as shown in FIG. 1A, a first-layer polysilicon electrode 22 formed on a silicon substrate 20 with an insulating film 21 interposed is formed on a first-layer polysilicon electrode 22 with an insulating film 23 covering the same. A silicon film 24 is formed. Note that a natural oxide film (not shown) is formed on the surface of the polysilicon film 24. When the polysilicon film 24 is formed in this manner, as described above, this polysilicon film 24 is also formed on the step formed by the presence or absence of the first-layer polysilicon electrode 22, that is, the first-layer polysilicon film. A step 24a is formed from just above the edge of the electrode 22 to the outside.

Next, as a mask for ion implantation, FIG.
A resist pattern 25 is formed as shown in FIG.
Here, the resist pattern 25 exposes a step portion 24 a of the polysilicon film 24, that is, a portion immediately above the lower portion side of the step portion on the silicon substrate 20 formed by the presence or absence of the first-layer polysilicon electrode 22. The opening 25a is formed by a known photolithography technique. Then, using this resist pattern 25 as a mask, ion implantation is performed at a low energy and a high dose to damage the step portion 24a of the polysilicon film 24 facing the outside in the opening 25a.

Here, conditions for this ion implantation are as follows.
It is assumed that the ion species is left only in the polysilicon film 24 for performing the etching later. That is, if the ion species penetrates through the polysilicon film 24 and reaches the lower layer, the damage to the lower layer by etching increases, which affects the product.

The ionic species is not particularly limited, and any ionic species can be used. In general, when the ionic species has a large atom, the ionic species hardly reach the lower layer portion. Since (As) is more difficult to penetrate, it is preferable to use arsenic to satisfy the above condition.

Regarding the dose, the polysilicon film 24
The polysilicon film 24 needs to have an amount capable of damaging the substrate and increasing the etching rate.
It is necessary that the ions do not reach the lower layer, so that there is no adverse effect on the performance of the obtained product. Specifically, it is preferably 1.0 × 10 ¹⁴ / cm ² or more. Is appropriately determined by the thickness of the polysilicon film 24.

The acceleration energy is appropriately determined depending on the type of the film to be etched (polysilicon in this example), its thickness, and the type of ion used. Condition that does not reach the lower layer.

After the ion implantation is performed as described above, the resist pattern 25 is peeled and removed as shown in FIG. The step portion 24a of the polysilicon film 24 is damaged by the ion implantation, and its crystal structure is damaged, and the structure becomes slightly fragile, thereby forming a high etching rate portion 24b.

Next, in order to pattern the polysilicon film 24, a resist pattern 26 is formed as in the prior art as shown in FIG. 1D, and etching is performed using this as a mask. In addition, since this etching method is fine processing, reactive ion etching (R
Anisotropic dry etching such as IE) is employed.

When etching is performed under such conditions, the etching of the high etching rate portion 24b formed in the step portion 24a proceeds rapidly as shown in FIG. 1 (e). Etching ends first at the step portion 24a where the remainder is likely to occur.
Therefore, when the entire etching is completed, no etching residue is generated as shown in FIG. 1F, whereby the second-layer polysilicon electrode 27 can be favorably formed.

As described above, in the etching method according to the present embodiment, by selectively performing ion implantation in advance on the step portion 24a, the etching rate of the step portion 24a where this etching residue easily occurs is increased, Since an etching residue is not generated when an etching process is performed thereafter, it is possible to prevent inconveniences such as an electrode short-circuit caused by the etching residue.
In addition, since the etching residue can be eliminated as described above, the etching amount in the forming etching can be reduced, and the etching damage of the insulating layer 21 which is the base of the polysilicon film 24 can be reduced. And a stable yield can be obtained.

In the above embodiment, the resist pattern 25 used as a mask when performing ion implantation is a pattern in which an opening 25a for exposing the step portion 24a of the polysilicon film 24 is used. The structure is not limited to this. For example, as shown in FIG.
A resist pattern (resist layer) 28 having a concave portion 28a immediately above a may be used as a mask when performing ion implantation.

If such a resist pattern 28 is used, even if ion implantation is performed with an increased acceleration energy,
Since the concave portion 28a is formed in the step portion 24a instead of the opening 25a shown in FIG. 1B, ions penetrate the polysilicon film 24 by absorbing energy by the resist forming the concave portion 28a. Instead, it stays in the polysilicon film 24 and the etching rate of the step portion 24 is increased. Therefore, even if an ion implantation apparatus that does not allow extraction of ion species when the acceleration energy is low is used, ion implantation can be performed with a high acceleration energy, so that such an apparatus can be used. .

Here, as a method of forming the resist pattern 28, the resist pattern 2 shown in FIG.
Exposure is stopped in the middle without exposing the resist to the lower surface during patterning as shown in 5 so that a concave portion 28a is formed instead of an opening for exposing the base after development.

As for the resist pattern having a concave portion, for example, as shown in FIG. 2B, a first resist layer 29 covering the polysilicon film 24 and the first resist layer 29 formed on the first resist layer 29 are formed. It may be formed by a laminated film with the second resist layer 30 having the opening 30a for exposing the resist layer 29. That is, in the formation of a resist pattern using this laminated film, the first resist layer 29 is formed by performing a resist coating once and developing the entire surface, and then the resist is coated again and the opening 30a is formed.
Is performed to form the second resist layer 30.

According to such a forming method, the step in the resist pattern formed of the obtained laminated film is reduced, so that the in-plane uniformity of the patterning can be improved. Also, in the case of etching a thin film, the ion implantation can be controlled by increasing the thickness of the first resist layer 29.

In the above embodiment, the case where the present invention is applied to the formation of a transfer electrode of a solid-state imaging device has been described. However, the present invention is not limited to this, and for example, a wiring portion having a complicated structure, The present invention can also be applied to a place where etching residue easily occurs, such as a place where a height difference of a step portion is large. In addition to suppressing etching residue, it is also applicable to the process of forming two or more types of film thickness by one etching by combining ion implantation, etching and patterning when forming electrode wiring etc. It is possible.

[0032]

As described above, according to the method of etching a film covering a step portion according to the present invention, in a film covering a step portion, a film is formed at a position immediately above a lower portion of the step portion where the film thickness is larger than that of a flat portion. Since this is a method in which the ion implantation process is selectively performed, the portion immediately above the lower portion of the step portion where this etching residue is apt to occur is damaged by the ion implantation process, and the etching rate can be increased, thereby increasing the etching rate. It is possible to prevent the occurrence of a residue such as an electrode short-circuit caused by the residue of the etching by making the residue of the etching no longer occur when the etching process is performed.

Further, since the film to be etched is damaged by the ion implantation process and the etching rate can be locally increased, the shape after the etching can be controlled. Further, since the etching residue can be eliminated as described above, the etching amount in the forming etching can be reduced, and the etching damage of the underlying layer of the film to be etched can be reduced, whereby stable quality can be obtained.

Further, if the ion implantation process is performed using a resist layer having a concave portion at a position where the etching rate is to be locally increased, the ion implantation apparatus can be implemented as
Even if a device that cannot extract ion species when the acceleration energy is low is used, ion implantation can be performed with a high acceleration energy, and thus the range of available devices can be widened. Further, if the resist layer having the concave portion is formed by a laminated film of the first resist layer covering the film to be etched and the second resist layer having the concave portion, the resist coating is performed twice. This makes it possible to improve the in-plane uniformity and control the ion implantation by controlling the resist film thickness.

[Brief description of the drawings]

FIGS. 1A to 1F are views showing an embodiment in which the method of etching a film covering a step portion according to the present invention is applied to a step of forming a transfer electrode in a solid-state imaging device; FIG. 5 is a cross-sectional side view of a main part showing formation of a transfer electrode in order of steps.

FIGS. 2 (a) and 2 (b) are side sectional views for explaining a modification of the embodiment shown in FIGS. 1 (a) to 1 (f).

FIG. 3 is a plan view showing a schematic configuration of a solid-state imaging device according to the present invention.

4 is a main part plan view showing an imaging area of the solid-state imaging device shown in FIG. 3;

FIGS. 5A to 5C are cross-sectional views of a main part for explaining a process in which an etching residue occurs.

[Explanation of symbols]

20 silicon substrate, 22 first layer polysilicon electrode,
Reference numeral 24: polysilicon film, 24a: stepped portion, 25: resist pattern, 25a: opening, 26: resist pattern, 27: second-layer polysilicon electrode, 28: resist pattern, 28a: concave portion, 29: first resist layer , 30 ...
Second resist layer, 30a ... opening

Claims (3)

[Claims] In etching a film covering a step portion on a substrate formed by a pattern of wirings, electrodes, etc., prior to this etching, a film is selectively formed on a portion of the film immediately above a lower portion of the step portion. A method of etching a film covering a step portion, which comprises performing an ion implantation process and thereafter etching the film. 2. Prior to the ion implantation, a resist layer covering the film and having a concave portion immediately above the lower portion of the step portion is formed on the film, and the ion implantation is performed by the ion implantation. 2. The method for etching a film covering a step portion according to claim 1, wherein the etching is performed from above the resist layer so that the ions penetrate the recess and reach the film. 3. The step portion according to claim 2, wherein the resist layer is formed by a laminated film of a first resist layer covering the film and a second resist layer formed with the concave portion. Method for etching the film covering the surface.