Etching method for production of semiconductor devices

Details Etching method for production of semiconductor devices Etching method for production of semiconductor devices

2000-01-07

2002-12-10

Goudreau, George (Department: 1763)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

C438S714000, C438S725000, C438S736000

Reexamination Certificate

active

06492068

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to etching processes for use in the production of semiconductor devices.
2. Description of Related Art
The number of transistors that have been integrated in semiconductor integrated circuits has become higher and higher in recent years, which requires an etching process capable of forming fine patterns with a high etching selectivity. In order to form fine patterns by etching, it is necessary to form fine photoresist patterns that are used as masks in the etching processes. The focus depth in the photolithography process becomes shallower for a finer pattern, and a thinner photoresist (hereinafter simply referred to as “resist”) film should be used.
Further, etching should be performed while controlling the shape of the patterns formed by the etching. In order to control the pattern shape, the sidewalls of the layer subject to etching should be protected. To this end, a process is employed, which comprises etching the resist layer and depositing the etched resist materials on the sidewalls of the layer subjected to etching. In this case, if the initial thickness of the resist layer is not sufficient, the resist film ingredients disappear during the etching, and the shoulders of the etched patterns are faceted. Namely, pattern shape degradation occurs. Because the protection of such sidewalls should be increased for finer patterns, the thickness of the resist film is on a trade-off relationship between the precision in the photolithography process and the pattern-shape maintenance in the etching process. Therefore, in order to achieve a highly precise processing with a thinner resist layer, it is required to provide an etching process capable of sufficiently protecting the sidewalls while mainlining a low etching rate of the resist layer, i.e., a high etching selectivity for the layer to be etched against the resist layer (resist selectivity).
In the etching process, the achievement of a high selectivity for the layer to be etched against the underlying layer (underlying layer selectivity) is essential in order to enhance the performance and reliability of the fabricated semiconductor devices. For example, in the etching to form gate electrodes, a high selectivity for the gate conductive layer against a gate oxide layer should be secured. Similarly, in silicon oxide etching to form contact holes, the selectivity against, for example, a Si substrate and a suicide layer formed on the Si substrate should be sufficiently high. Also, the selectivity against a silicon nitride etch-stop layer used in a self-alignment contact (SAC) process should be sufficiently high. Further, in via hole etching, the selectivity should be high against an underlying metal such as TiN used as an antireflection layer.
In addition, in the etching process, vertical sidewalls are not always most desirable. For example, a wiring pattern should preferably be etched in a normal taper manner in order to improve the coverage of an interlayer insulating film. Also, a contact hole or via hole should preferably be etched in a normal taper manner in order to improve the coverage of a metal wiring layer in the hole.
To achieve the satisfactory sidewall protection effect, the high resist selectivity, the high underlying layer selectivity and to control the angle of the sidewall, various etching gas atmospheres including a primary etching gas for producing a primary etchant species and a variety of additional gases have been investigated. For example, in oxide film etching using C
x
F
y
gas as a primary etching gas, it was proposed to increase the C/F ratio in the plasma. Because F radicals serving as a primary etchant species are also capable of etching an underlying layer, such as a Si substrate, the surface of the underlying layer should preferably be covered with a protective film after the etch-off of the layer to be etched. By raising the C/F ratio, a number of CF
2
radicals and CF radicals, which act as precursors to form a polymer film, can be increased. A fluorocarbon protective film can be thereby formed on the exposed surface of the underlying layer. For this reason, as the primary etching gas for oxide etching, C
2
F
6
, C
3
F
8
and other straight-chain fluorocarbons, and C
4
F
8
and other unsaturated fluorocarbons having a large number of carbon atoms, have been used. As the additional gas, H
2
, CHF
3
, CH
2
F
2
, CH
3
F and other hydrogen-containing gases and CO have been used to scavenge excess fluorine in the plasma and thereby increase the C/F ratio in the plasma.
Meanwhile, in the fabrication of semiconductor devices with a design rule of 0.25 &mgr;m or less, a bottom antireflection coating (BARC) is widely used to enhance the precision of photolithography process. According to the process, a BARC layer composed of an organic substance is coated under a resist layer. The BARC layer serves to planarize the substrate surface, as well as to suppress the reflection of the exposing light from the underlying layer so as to improve the precision in the photolithography process. The BARC layer is etched using oxygen radicals as a primary etchant species after the development of the resist pattern. A layer to be etched is then etched using the resist pattern and BARC pattern as a mask. A dry etching technique may also be applied in developing the resist layer in the future. For instance, the surface of the resist is treated with a silylating agent such as hexamethyldisilazane (HMDS) before or after the photoexposure. A patterned silylated surface layer, which is resistant to oxygen plasma, is formed. And the resist in un-silylated regions are removed by reactive ion etching (RIE) using oxygen as a primary etching gas.
The above-mentioned conventional etching processes for use in the production of semiconductor devices have, however, the following disadvantages.
Etching is performed in various discharging types, such as reactive ion etching (RIE), magnetron enhanced reactive ion etching (MERIE), electron cyclotron resonance (ECR), and helicon-wave etching, and electron energy in one discharging type is different from that in another type. Further, a compound molecule has a specific dissociation energy. Accordingly, the etchant gas must be selected in accordance with the type of the etching system. Dissociation of a molecule cannot be significantly controlled with a high precision even if a suitable gas species for each etching system is selected. Therefore, in silicon oxide etching, a C
x
F
y
gas cannot be controllably decomposed to obtain a sufficient amount of CF
2
radicals and/or CF radicals. The use of a gas having a high C/F ratio is preferred to improve the selectivity. However, the use of a gas having a high C/F ratio invites, for example, a decreased etching rate, difficulty in removal of the resist pattern after etching, and an increased contact resistance due to carbon implantation into the substrate surface.
Specifically, when a gas having a high C/F ratio is used, the etching rate is reduced due to a polymer film formed on the surface of the layer to be etched. If the discharging power is increased to compensate for such reduction in etching rate, CF
2
radicals and/or CF radicals are further dissociated to increase the amount of fluorine atoms. As a result, the underlying layer selectivity is sacrificed although the etching rate can be raised. As thus described, conventional gas systems, which are mainly directed to forming CF
2
radicals and/or CF radicals and to polymerizing the same, cannot yield a high resist and/or underlying layer selectivity while concurrently fulfilling other requirements.
In the BARC etching process, oxygen radicals serving as a primary etchant species etch the resist and BARC in a relatively isotropic manner, resulting in a loss of critical dimension (CD). A possible solution to the CD loss is to increase the substrate bias voltage. Ions accelerated by the substrate bias bombard the resist layer and organic species sputtered from the resist layer adhere on the sidewalls, thereby formi

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