Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-04-16
2004-06-29
Rosasco, Stephen (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C204S192250
Reexamination Certificate
active
06756161
ABSTRACT:
FIELD OF INVENTION
This invention relates to manufacture of binary photomask blanks in photolithography, using the ion-beam deposition technique. These masks can be used with short wavelength (i.e., <400 nanometer) light. Additionally, this invention relates to binary photomask blanks with single or multi-layered coating of chromium, molybdenum, tungsten, or tantalum metal and/or its compounds or combinations thereof, on the blanks.
TECHNICAL BACKGROUND
Microlithography is the process of transferring microscopic circuit patterns or images, usually through a photomask, on to a silicon wafer. In the production of integrated circuits for computer microprocessors and memory devices, the image of an electronic circuit is projected, usually with an electromagnetic wave source, through a mask or stencil on to a photosensitive layer or resist applied to the silicon wafer. Generally, the mask is a layer of “chrome” patterned with these circuit features on a transparent quartz substrate. Often referred to as a “binary” mask, a “chrome” mask transmits imaging radiation through the pattern where “chrome” has been removed. The radiation is blocked in regions where the “chrome” layer is present.
The electronics industry seeks to extend optical lithography for manufacture of high-density integrated circuits to critical dimensions of less than 100 nanometer (nm). However, as the feature size decreases, resolution for imaging the minimum feature size on the wafer with a particular wavelength of light is limited by the diffraction of light. Therefore, shorter wavelength light, i.e. less than 400 nm are required for imaging finer features. Wavelengths targeted for succeeding optical lithography generations include 248 nm (KrF laser wavelength), 193 nm (ArF laser wavelength), and 157 nm (F
2
laser wavelength) and lower.
Physical methods of thin film deposition are preferred for manufacture of photomask blanks. These methods, which are normally carried out in a vacuum chamber, include glow discharge sputter deposition, cylindrical magnetron sputtering, planar magnetron sputtering, and ion beam deposition. A detailed description of each method can be found in the reference “Thin Film Processes,” Vossen and Kern, Editors, Academic Press NY, 1978). The method for fabricating thin film masks is almost universally planar magnetron sputtering.
The planar magnetron sputtering configuration consists of two parallel plate electrodes: one electrode holds the material to be deposited by sputtering and is called the cathode; while the second electrode or anode is where the substrate to be coated is placed. An electric potential, either RF or DC, applied between the negative cathode and positive anode in the presence of a gas (e.g., Ar) or mixture of gases (e.g., Ar+O
2
) creates a plasma discharge (positively ionized gas species and negatively charged electrons) from which ions migrate and are accelerated to the cathode, where they sputter or deposit the target material on to the substrate. The presence of a magnetic field in the vicinity of the cathode (magnetron sputtering) intensifies the plasma density and consequently the rate of sputter deposition.
If the sputtering target is a metal such as chromium (Cr), sputtering with an inert gas such as Ar will produce metallic films of Cr on the substrate. When the discharge contains reactive gases, such as O
2
, N
2
, CO
2
, or CH
3
, they combine with the target or at the growing film surface to form a thin film of oxide, nitride, carbide, or combination thereof, on the substrate. Usually the chemical composition of a binary mask is complex and often, the chemistry is graded or layered through the film thickness. A “chrome” binary mask is usually comprised of a chrome oxy-carbo-nitride (CrO
x
C
y
N
z
) composition that is oxide-rich at the film's top surface and more nitride-rich within the depth of the film. The oxide-rich top surface imparts anti-reflection character, and chemically grading the film provides attractive anisotropic wet etch properties, while the nitride-rich composition contributes high optical absorption.
In ion-beam deposition (IBD), the plasma discharge is contained in a separate chamber (ion “gun” or source) and ions are extracted and accelerated by an electric potential impressed on a series of grids at the “exit port” of the gun (ion extraction schemes that are gridless, are also possible). The IBD process provides a cleaner process (fewer added particles) at the growing film surface, as compared to planar magnetron sputtering because the plasma, that traps and transports charged particles to the substrate, is not in the proximity of the growing film as in sputtering. Moreover, the need to make blanks with fewer defects is imperative for next generation lithographies where critical circuit features will shrink below 0.1 micron. Additionally, the IBD process operates at a total gas pressure at least ten times lower than traditional magnetron sputtering processes (a typical pressure for IBD is ~10
−4
Torr.). This results in reduced levels of chemical contamination. For example, a nitride film with minimum or no oxide content can be deposited by this process. Furthermore, the IBD process has the ability to independently control the deposition flux and the reactive gas ion flux (current) and energy, which are coupled and not independently controllable in planar magnetron sputtering. The capability to grow oxides or nitrides or other chemical compounds with a separate ion gun that bombards the growing film with a low energy, but high flux of oxygen or nitrogen ions is unique to the IBD process and offers precise control of film chemistry and other film properties over a broad process range. Additionally, in a dual ion beam deposition the angles between the target, the substrate, and the ion guns can be adjusted to optimize for film uniformity and film stress, whereas the geometry in magnetron sputtering is constrained to a parallel plate electrode system.
While magnetron sputtering is extensively used in the electronics industry for reproducibly depositing different types of coatings, process control in sputtering plasmas is inaccurate because the direction, energy, and flux of the ions incident on the growing film cannot be regulated (ref: The Material Science of Thin Films, Milton Ohring, Academic Press 1992, p. 137). In dual ion beam deposition proposed here as a novel alternative for fabricating masks with simple or complex, single-layered or multi-layered chemistries, independent control of these deposition parameters is possible.
SUMMARY OF THE INVENTION
This invention concerns an ion-beam deposition process for preparing a binary photo mask blank for lithographic wavelengths less than 400 nanometer, the process comprising depositing at least one layer of a MO
x
C
y
N
z
compound, where M is selected from the group consisting of chromium, molybdenum, tungsten, or tantalum or a combination thereof, on a substrate by ion beam deposition of chromium, molybdenum, tungsten, or tantalum and/or a compound thereof by ions from a group of gases;
wherein:
x ranges from about 0.00 to about 3.00;
y ranges from about 0.00 to about 1.00;
and z ranges from about 0.00 to about 2.00.
More specifically, this invention concerns a dual ion-beam deposition process for preparing a binary photo mask blank for lithographic wavelengths less than 400 nanometer, the process comprising depositing at least one layer of a MO
x
C
y
N
z
compound, where M is selected from chromium, molybdenum, tungsten, or tantalum or combination thereof, on a substrate;
(a) by ion beam deposition of chromium, molybdenum, tungsten, or tantalum and/or a compound thereof by ions from a group of gases, and
(b) by bombarding the said substrate by a secondary ion beam from an assist source of a group of gases wherein the layer or the layers are formed by a chemical combination of the bombarding gas ions from the assist source gas with the material deposited from the target or targets onto the substrate;
wherein:
x ranges from about 0.00 to about 3.00;
y ranges from ab
Carcia Peter Francis
Dieu Laurent
E. I. du Pont de Nemours and Company
Rosasco Stephen
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