Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1999-01-19
2002-04-02
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S950000, C430S315000, C430S320000, C427S574000, C427S578000, C438S636000, C438S783000, C423S325000
Reexamination Certificate
active
06365320
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the art of microelectronic integrated circuits, and more specifically to a process for forming an anti-reflective film for semiconductor fabrication using extremely short wavelength deep ultraviolet photolithography.
2. Description of the Related Art
Photolithography is a semiconductor fabrication process that is widely used for patterning material layers on a semiconductor wafer or structure. The material layers can be non-metal (e.g. silicon, polysilicon), metal (e.g. aluminum), etc.
A layer of photoresist is formed over the material layer to be patterned, and exposed to light through a mask which has opaque and transparent areas corresponding to the desired pattern. Light passing through the transparent areas in the mask causes a chemical reaction in the underlying areas of the photoresist such that these areas will be dissolved away when the wafer is exposed to a developing solution.
The result is a photoresist layer having openings therethrough which correspond to the transparent areas of the mask. The patterned photoresist layer is then used as an etch mask such that areas of the material layer which are exposed by the openings in the photoresist layer will be selectively removed upon exposure to an appropriate etching solution. This is possible by selecting the etching solution to have a much higher etch rate for the material layer than for the photoresist. Preferably the etch rate for the photoresist will be substantially zero.
With feature sizes of integrated circuits constantly shrinking, photolithographic resolution or definition is becoming increasingly sensitive to reflection during the light exposure step. This is especially problematic at very short wavelengths in the ultraviolet band, including deep ultraviolet exposure which is conventionally performed at a wavelength of 248 nanometers.
As illustrated in
FIG. 1
, a semiconductor structure
10
which is in an intermediate step in a fabrication process includes a material layer
12
which is to be patterned by photolithography, and a photoresist layer
14
which is formed on the material layer
12
. The material layer
12
can be polysilicon which is being patterned into a field effect transistor gate or interconnect, or any other applicable material. The layer
12
is formed on a silicon or other semiconductor substrate which is not shown.
A ray of light is illustrated as entering the photoresist layer
14
at an off-normal angle as indicated at
16
, and passing through the layer
14
as indicated at
18
. The ray is reflected from the surface of the layer
12
back into the layer
14
as indicated at
20
. The reflected ray
20
enlarges the exposed area of the photoresist layer
14
to include an area that was not intended to be exposed, resulting in a larger area being removed during development than was desired.
This phenomenon in general degrades the resolution or definition capability of the photolithographic process. Although
FIG. 1
illustrates only unwanted reflection caused by off-normal incident light, light can also be reflected off underlying device features to produce similar results.
In order to inhibit reflection of light back into a photoresist layer, Bottom Anti-Reflective Coatings (BARC) have been developed as illustrated in
FIG. 2
, in which similar elements are designated by the same reference numerals used in FIG.
1
. As viewed in the figure, an anti-reflective coating
22
, also known as an anti-reflective layer or film, is provided between the material layer
12
and the photoresist layer
14
.
A ray of light is incident on and passes through the photoresist layer
14
as indicated at
16
,
18
, and a portion of this light is reflected back into the photoresist layer
14
as indicated at
20
. Another portion of this light passes through the anti-reflective coating
22
as indicated at
24
, is reflected from the surface of the layer
12
, and passes back through the layer
22
into the layer
14
as indicated at
26
.
The index of refraction n, extinction coefficient k, and thickness t of the anti-reflective coating
22
are selected such that the light
26
will be 180° or ½ wavelength out of phase with the light
20
. Destructive interference will occur between the light
20
,
26
in the photoresist layer
14
causing the light
20
,
26
to mutually cancel. In this manner, reflection of light from the surface of the material layer
12
into the photoresist layer
14
is effectively inhibited.
An anti-reflective coating
22
which is effective for deep ultraviolet lithography is described in U.S. patent application Ser. No. 08/479,718, entitled “SILICON OXIME FILM”, filed Jun. 7, 1995, by D. Foote, and is incorporated herein by reference in its entirety. This coating is formed using Plasma Enhanced Chemical Vapor Deposition (PECVD) over the material layer to be patterned as described above.
The material of the coating is called silicon oxime, and has the generic chemical formulation SiNO:H, being a compound of silicon, oxygen, nitrogen, and residual hydrogen in varying proportions which are selected in accordance with a particular application. The index of refraction n, extinction coefficient k, and thickness t of the layer are selected to provide half-wavelength cancellation of light reflected into an overlying photoresist layer
14
as described above.
FIG. 3
illustrates the desired appearance of the structure
10
after a portion of the photoresist layer
14
has been exposed and developed to form an opening
14
′. The layer
14
acts as an etch mask for subsequent processing such that the portions of the layer
22
and layer
12
underlying the opening
14
′ can be selectively removed by etching using a substance that will not significantly affect the photoresist layer
14
. The walls of the opening
14
are substantially vertical as desired.
FIG. 4
is similar to
FIG. 3
, but illustrates “footing” which is caused by chemical interaction between the anti-reflective layer
22
and the photoresist layer
14
. The footing appears as horizontally enlarged portions
22
′ which extend from the lower edges of the photoresist layer
14
into the opening
14
′. The footing is undesirable because it degrades the resolution or definition of the photolithographic process.
Where the anti-reflective layer
22
is formed of silicon oxime as described above, the footing is caused by chemical interaction between amines (hydrogen-nitrogen bonds) in the silicon oxime layer
22
and the material of the photoresist layer. This is facilitated by the fact that silicon oxime is a base, whereas the photoresist material is an acid.
It is known that this interaction, and thereby the footing, can be inhibited by forming a silicon dioxide barrier layer on the surface of the silicon oxime layer before forming the photoresist thereon. The barrier layer acts as a seal or cap which separates the amines in the silicon oxime layer from the photoresist layer
14
.
Prior art methods include forming a silicon dioxide barrier layer on a silicon oxynitride (SiON:H) layer, which is another substance used as an antireflective coating. This method includes deposition in the reactor used to form the silicon oxynitride layer itself. Such deposition is limited by conventional techniques to forming a silicon dioxide layer having a thickness in excess of 100 angstroms. A silicon dioxide layer of such thickness is difficult to remove if required in the processing environment to which the present invention relates.
Another method includes growing a silicon dioxide layer on a silicon oxime layer in a downstream plasma system using oxygen gas. This method is limited in that it is only capable of forming a silicon dioxide layer which is too thin (less than 10 angstroms) to eliminate footing.
FIG. 6
illustrates a detrimental situation which can result from removing a thick (100 angstroms or more) silicon dioxide barrier layer from a field-effect transistor structure. An exemplary semiconductor struct
Foote David K.
Gupta Subhash
Advanced Micro Devices , Inc.
Baxter Janet
Lee Sin J.
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