Disposable hard mask for phase shift photomask plasma etching

Details Disposable hard mask for phase shift photomask plasma etching Disposable hard mask for phase shift photomask plasma etching

2003-02-19

2004-01-27

Huff, Mark F. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

C430S313000, C430S316000, C430S317000, C430S318000, C430S323000, C430S396000

Reexamination Certificate

active

06682861

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a photomask, either binary or phase shift, which includes a hard mask layer, the use of which improves the uniformity of critical dimensions on the photomask.
BACKGROUND OF THE INVENTION
Photomasks are used in the semiconductor industry to transfer microscale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer. A typical binary photomask is comprised of a transparent quartz substrate and chrome (Cr) opaque material that includes an integral layer of chrome oxide (CrO) anti-reflective (AR) material. The pattern of the Cr opaque material and CrO AR material on the quartz substrate is a scaled negative of the image desired to be formed on the semiconductor wafer.
As shown in
FIG. 1
, a prior art blank photomask
20
is comprised of four layers. The first layer
2
is a layer of quartz, commonly referred to as the substrate, and is typically approximately one quarter inch thick. Affixed to the quartz substrate
2
is a layer of Cr opaque material
4
which typically is approximately 900 Å to 1000 Å thick. An integral layer of CrO AR material
6
is formed on top of the layer of Cr opaque material
4
. The layer of CrO AR material is typically approximately 100 Å thick. A layer of photosensitive resist material
8
resides on top of the CrO AR material
6
. The photosensitive resist material
8
is typically a hydrocarbon polymer, the various compositions and thicknesses of which are well known in the art.
The desired pattern of Cr opaque material to be created on the photomask may be defined by an electronic data file loaded into an exposure system which typically scans an electron beam (E-beam) or laser beam in a raster fashion across the blank photomask. One such example of a raster scan exposure system is described in U.S. Pat. No. 3,900,737 to Collier. As the E-beam or laser beam is scanned across the blank photomask, the exposure system directs the E-beam or laser beam at addressable locations on the photomask as defined by the electronic data file. The areas of the photosensitive resist material that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble. As shown in
FIG. 2
, after the exposure system has scanned the desired image onto the photosensitive resist material, the soluble photosensitive resist is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material
10
remains adhered to the CrO AR material
6
.
As illustrated in
FIG. 3
, the exposed CrO AR material and the underlying Cr opaque material no longer covered by the photosensitive resist material is removed by a well known etching process, and only the portions of CrO AR material
12
and Cr opaque material
14
residing beneath the remaining photosensitive resist material
10
remain affixed to quartz substrate
2
. This initial or base etching may be accomplished by either a wet-etching or dry-etching process both of which are well known in the art. In general, wet-etching process uses a liquid acid solution to erode away the exposed CrO AR material and Cr opaque material. A dry-etching process, also referred to as plasma etching, utilizes electrified gases, typically a mixture of chlorine and oxygen, to remove the exposed chrome oxide AR material and chrome opaque material.
A dry-etching process is conducted in vacuum chamber in which gases, typically chlorine and oxygen are injected. An electrical field is created between an anode and a cathode in the vacuum chamber thereby forming a reactive gas plasma. Positive ions of the reactive gas plasma are accelerated toward the photomask which is oriented such that the surface area of the quartz substrate is perpendicular to the electrical field. The directional ion bombardment enhances the etch rate of the Cr opaque material and CrO AR material in the vertical direction but not in the horizontal direction (i.e., the etching is anisotropic or directional).
The reaction between the reactive gas plasma and the Cr opaque material and CrO AR material is a two step process. First, a reaction between the chlorine gas and exposed CrO AR material and Cr opaque material forms chrome radical species. The oxygen then reacts with the chrome radical species to create a volatile which can “boil off” thereby removing the exposed CrO AR material and the exposed Cr opaque material.
As shown in
FIG. 4
, after the etching process is completed the photosensitive resist material is stripped away by a process well known in the art. The dimensions of the Cr opaque material on the finished photomask are then measured to determine whether or not critical dimensions are within specified tolerances. Critical dimensions may be measured at a number of locations on the finished photomask, summed, and then divided by the number of measurements to obtain a numerical average of the critical dimensions. This obtained average is then compared to a specified target number (i.e., a mean to target comparison) to ensure compliance with predefined critical dimensions specifications. Additionally, it is desired that there is a small variance among the critical dimensions on the substrate. Accordingly, the measured critical dimensions typically must also conform to a specified uniformity requirement. Uniformity is typically defined as a range (maximum minus minimum) or a standard deviation of a population of measurements.
Another type of known photomask used for transferring images to a semiconductor wafer is commonly referred to as a phase shift photomask. Phase shift photomasks are generally preferred over binary photomasks where the design to be transferred to the semiconductor wafer includes smaller, packed together feature sizes which are below the resolution requirements of optical equipment being used. Phase shift photomasks are engineered to be 180 degrees out of phase with light transmitted through etched areas on the photomask so that the light transmitted through the openings in the photomask is equal in amplitude.
One type of known phase shift photomask is commonly referred to as an embedded attenuated phase shift mask (“EAPSM”). As shown in
FIG. 10
, a typical blank EAPSM
31
is comprised of four layers. The first layer is a typically a substantially transparent material
33
(such as quartz, for example) and is commonly referred to as a substrate. The next layer is typically an embedded phase shifting material (“PSM layer”)
35
, such as molybdenum silicide (MoSi), tantalum silicon nitride (TaSiN), titanium silicon nitride (TiSiN) or zirconium silicon oxide (ZrSiO) and other known phase materials. The next layer is typically an opaque material
37
, such as chromium, which may optionally include an anti-reflective coating such as chromium oxynitride (CrON). The top layer is a photosensitive resist material
39
.
The method for processing a conventional EAPSM is now described. As with binary photomasks, the desired pattern of opaque material
37
to be created on the EAPSM
31
is scanned by an electron beam (E-beam) or laser beam in a raster or vector fashion across the blank EAPSM
31
. As the E-beam or laser beam is scanned across the blank EAPSM
31
, the exposure system directs the E-beam or laser beam at addressable locations on the EAPSM
31
. The areas of the photosensitive resist material
39
that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble.
As is done with binary photomasks and as shown in
FIG. 11
, after the exposure system has scanned the desired image onto the photosensitive resist material
39
, the soluble photosensitive resist material is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material
39
′ remains adhered to the opaque material
37
. Thus, the pattern to be formed on the EAPSM
31
is formed by the remaining photosensitive resist material
39
′.
The pattern is then transferred from the remaining photoresist material
39
′ to the opaque layer

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