Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2000-08-31
2003-05-27
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S710000, C438S712000, C438S713000
Reexamination Certificate
active
06569774
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of semiconductor device fabrication, and more particularly to improved methods for etching high aspect ratio contact openings in oxide layers.
BACKGROUND OF THE INVENTION
In the fabrication of semiconductor devices, numerous conductive device regions and layers are formed in or on a semiconductor substrate. The conductive regions and layers of the device are isolated from one another by a dielectric or insulating layer, for example, silicon dioxide or a doped oxide such as phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG). These dielectric layers typically overlay a silicon-comprising surface such as single crystal silicon, epitaxial silicon, polysilicon, or suicides such as titanium silicide.
At several stages during wafer fabrication, it is necessary to form contact openings through insulative material to establish electrical communication with the integrated circuitry. Such contact openings, when filled with a conducting material such as a metal or polysilicon, electrically connect devices with the integrated circuitry. To ensure formation of desired dimensions and profile for contact openings, the etchant must be highly selective to promote removal of the insulation layer and not the underlying layer.
Contact openings with high aspect ratios, i.e., a high height-to-width ratio, are formed to be fairly narrow, typically with vertical sidewalls to ensure that a sufficiently large contact area is provided at the bottom of the contact opening for conductive material that is subsequently formed in the opening. To form the openings, a masking layer such as a photoresist is formed over the insulative layer, i.e., silicon oxide layer, and is subsequently patterned to define the contact openings. The contact opening is etched using an etch that is highly selective relative to masking layer. Conventional processes used to form a contact opening involve etching through the insulative layer by exposure to a plasma formed in a plasma reactor. Reactive ion etching (RIE) and plasma etching (PE) are common dry-etch plasma methods used to open contact openings anisotropically through a dielectric.
A fluorocarbon plasma is typically used to etch silicon dioxide. Such a plasma typically includes one or more fluorocarbons as the primary active constituents, for example CF
4
, CHF
3
, and C
3
F
8
, CH
2
F
2
, CH
3
F, C
2
F
6
, C
4
F
6
, C
n
F
n+4
, and mixtures thereof. The fluorinated gas dissociates and reacts with the silicon oxide to form volatile silicon difluoride (SiF
2
) or silicon tetrafluoride (SiF
4
) and carbon monoxide or carbon dioxide.
Although the plasma etch rate of the oxide is generally faster than the resist erosion rate, when dry etching an opening having a high aspect ratio, the etch chemistry causes the resist layer to gradually erode away, often before the desired depth of the opening is achieved. Another problem is related to faceting or chamfering of the photoresist mask at the edge of the opening, caused by ion bombardment. This can wear away the underlying oxide resulting in surface roughness and striations in the etch features, and the loss of critical dimensions of the opening being etched. In an array such as a memory cell, contacts are positioned in close proximity to each other, and the erosion and localized breakdown of the photoresist can result in the development of notches and other blemishes in the surface of the contact, which can extend to and short an adjacent contact.
The plasma etching processes generates very reactive ionized species, atomic fluorine, and C
x
F
y
radicals that combine to form polymeric residues. A drawback with plasma etching and RIE of silicon oxide using some fluorinated etch gases is the buildup of carbon-fluorine based polymer material on the sidewalls of vias and other openings that can deposit during the etch.
FIG. 1
illustrates a wafer fragment
10
, a contact opening
12
etched through an opening in a mask
14
downwardly through a silicon oxide layer
16
deposited on a substrate
18
, and the effect of the buildup of polymeric residues
20
on the sidewalls
22
and the bottom surface
24
of the contact opening
12
formed during a typical prior art etch process.
The continuous buildup of polymeric etch residues on sidewalls
22
of the oxide opening
12
tends to constrict the opening, inhibiting the etch and resulting in the profile of the sidewalls becoming tapered, as depicted in FIG.
1
. Polymer material can also build up in the bottom
24
of the opening
12
. Since an anistropic etch of the oxide layer
16
relies on ionic bombardment of the bottom
24
of the opening being etched, if too much polymeric layer
20
deposits on the bottom surface
24
of the opening during etching, etching of the contact opening will cease if the layer
24
is not removed.
Therefore, a need exists for a method of etching silicon oxide layers to provide high aspect ratio openings that overcomes these problems. It would be desirable to provide an etching process for the formation of deep contact openings through an oxide layer that inhibits or regulates the deposition of polymeric etch residues on the sidewalls of the openings and improves resist selectivity and eliminates striations and notching.
SUMMARY OF THE INVENTION
The invention provides an improved process for plasma etching of a silicon oxide layer to form a via or other contact opening while controlling the deposition of polymeric residues on the surface of a mask layer and the sidewalls and bottom surface of the contact opening. In particular, the invention improves resist selectivity and reduces striations by the addition of nitrogen-comprising gases such as NH
3
to fluorocarbon (C
x
F
y
) and fluorohydrocarbon (C
x
F
y
H
z
) etch chemistries.
The etching is performed by exposing the silicon oxide layer through a mask opening to an etch gas in an ionized state in a reaction chamber of a plasma-generating device. The etch gas includes one or more organic fluorine-comprising gases such as CF
4
, CHF
3
, CH
2
F
2
, among others, and can include one or more nitrogen-comprising gases. Suitable nitrogen-comprising gases are those that do not substantially etch the resist layer and/or deposit or build-up polymer on the mask layer. Exemplary nitrogen-comprising gases include N
2
O, NH
3
, N
2
H
4
, and RNH
2
where R is a C
1
-C
3
hydrocarbon or fluorohydrocarbon, among others. The etch gas can optionally include one or more inert carrier gases such as argon or helium. The etching is preferably performed by reactive ion etching or plasma etching. The etch gas can be exposed to a microwave electric field and/or a magnetic field during the etching step.
In one embodiment of the invention, the method involves etching the layer of silicon oxide to provide an opening extending therethrough by exposing the silicon oxide layer through a mask opening to a first etch gas and then a second etch gas, in an ionized state in a reaction chamber of a plasma generating device. A first etching is performed by exposing the silicon oxide layer to etch a contact opening through the silicon oxide layer, preferably to a depth of at least about 0.5 micron, and an aspect ratio of at least about 2:1. The first etch gas includes at least one organic fluorocarbon and, optionally, one or more nitrogen-comprising gases in a minor amount such that there is essentially little or no polymeric material formed on the mask layer and the silicon oxide layers during the etching step.
A second etching is then performed by exposing the silicon oxide layer to a second etch gas to increase the opening downwardly through the silicon oxide layer while a polymeric material is formed on the mask layer during the etching step. The second etch gas includes at least one organic fluorocarbon and an effective amount of at least one nitrogen-comprising gas to reduce the resist etch rate and/or provide formation of polymeric material on the mask layer during the etching step. The second etch gas can be provided as a separate gas or by increa
Micro)n Technology, Inc.
Utech Benjamin L.
Vinh Lan
Whyte Hirschboeck Dudek SC
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