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
06756160
ABSTRACT:
FIELD OF INVENTION
This invention relates to manufacture of phase shift photomask blanks in photolithography, known in the art as the attenuating (embedded) type, using the ion-beam deposition technique. More specifically, this invention relates to photomask blanks to be used with short wavelength (i.e., <400 nanometer) light, which attenuate and change the phase of transmitted light by 180° relative to the incident light, and which provide tunable optical transmission. Additionally, this invention relates to photomask blanks with single or multi-layered coating of the general formula MxSiOyNz or MxAlOyNz 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. Commonly, 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 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. <400 nm is required for imaging finer features. Wavelengths targeted for succeeding generations of optical lithography include 248 nm (KrF laser wavelength), 193 nm (ArF laser wavelength), and 157 nm (F
2
laser wavelength) and lower. However, as the wavelength of the incident light decreases, the “depth of focus,” (DoF) or the tolerance of the process also decreases according to the following equation:
DoF=k
2
(&lgr;/
NA
2
)
where k
2
is a constant for a given lithographic process, &lgr; is the wavelength of the imaging light, and NA=sin &thgr;, is the numerical aperture of the projection lens. A larger DoF means that the process tolerance toward departures in wafer flatness and photoresist thickness uniformity is greater.
Resolution and DoF can be improved for a given wavelength with a phase shift photomask which enhances the patterned contrast of small circuit features by destructive optical interference. Often a phase-shift mask can increase DOF. Therefore, as the minimum feature size in integrated circuits continues to shrink, the “phase-shift mask” becomes increasingly important in supplementing and extending the applications of traditional photolithography with “binary” masks. For example, in the attenuating (embedded) phase-shift mask, the electromagnetic radiation leaks through (attenuated) the unpatterned areas, while it is simultaneously phase-shifted 180°, instead of being blocked completely. Compared to “chrome” on quartz masks, phase-shift masks improve printing resolution of fine features and the depth of focus of the printing process.
The concept of a phase shift photomask and photomask blank that attenuates light and changes its phase was revealed by H. I. Smith in U.S. Pat. No. 4,890,309 (“Lithography Mask with a Pi-Phase Shifting Attenuator”). Common categories of known attenuating embedded phase shift photomask blanks include: (1) Cr-based photomask blanks containing Cr, Cr-oxide, Cr-carbide, Cr-nitride, Cr-fluoride or combinations thereof; and (2) SiO
2
- or Si
3
N
4
-based photomask blanks, where SiO
2
or Si
3
N
4
are doped with an opaque metal such as Mo to form a molybdenum silicon oxide, nitride, or an oxynitride.
Physical methods of thin film deposition are preferred to manufacture 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 elemental such as silicon (Si), sputtering with an inert gas such as Ar produces films of Si on the substrate. When the discharge contains reactive gases, such as O
2
, N
2
, or CO
2
, they combine with the target/or at the growing film surface to form a thin film oxide, nitride, carbide, or combination thereof, on the substrate.
Whether the mask is “binary” or phase-shifting, the materials that comprise the mask-film are usually chemically complex, and sometimes the chemistry is graded through the film thickness, or is layered. Even a simple “chrome” mask is a chrome oxy-carbo-nitride (CrOxCyNz) composition that can be oxide rich at the film's top surface and more nitride-rich within the depth of the film. The chemistry of the top surface imparts anti-reflection character, while the chemical grading provides attractive anisotropic wet etch properties.
In the ion-beam deposition process (IBD), the plasma discharge is contained in a separate chamber (ion “gun” or source) and ions are typically extracted and accelerated by an electric potential impressed on a series of grids at the “exit port” of the gun (other ion-extraction schemes without grids 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. 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 the IBD 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 all sorts of coatings, process control in sputtering plasmas is n
E.I. du Pont de Nemours. and Company
Rosasco Stephen
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