Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2002-07-27
2004-06-22
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C204S192110, C204S192320, C360S131000
Reexamination Certificate
active
06753538
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to the field of charged particle beam processing and, in particular, to a method and apparatus using an electron beam to create or alter microscopic structures, such as to repair photolithography masks.
BACKGROUND
Photolithography processes are widely used in the fabrication of integrated circuits. In a typical photolithography process, a thin layer of a photoactive material called “photoresist” is deposited onto the surface of a semiconductor substrate, such as a silicon wafer. The photoresist can be composed of any of a number of different materials whose chemical structure changes when exposed to a certain type of radiation. This change in chemical structure can cause the photoresist to become more soluble, in the case of a positive photoresist, or less soluble, in the case of a negative photoresist, in a chemical solution referred to as a developer.
A photolithography mask in the shape of a desired circuit pattern is used as a template to transfer the circuit pattern to the surface of the semiconductor substrate. A typical transmission mask has a pattern of clear and opaque areas, repeated over its surface, that is used to fabricate a layer of a circuit. The mask, when positioned between an appropriate radiation source and the photoresist-coated semiconductor substrate, casts a shadow onto the photoresist and thus controls which areas of the photoresist are exposed to the radiation.
After exposure, the photoresist is removed from either the exposed or the unexposed areas by washing with an appropriate developer. This leaves a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent process step, such as etching, deposition, or diffusion. After the subsequent process step in completed, the remaining photoresist is removed. This photolithography process ultimately allows the actual circuitry to be integrated into a semiconductor chip.
Obviously, the mask is a key element in this process since it is the mask that determines the circuit pattern that is transferred to the semiconductor substrate. A mask comprises a patterned layer of an opaque absorber material, such as a metallic film of chromium or tungsten, on a substrate of a transparent material, such as quartz. Features on a mask can be as small as a few millionths of an inch. When the pattern is formed on the mask, typically by using computer controlled laser or electron beam systems to expose the desired mask pattern in a photoresist material, it is not unusual for the mask to have defects. There are essentially two defect types, opaque and clear. Clear defects are areas where absorber is missing from areas that should be opaque; opaque defects are areas having absorber material deposited in areas that should be clear. Since any defect in the mask will ultimately be transferred to any semiconductor chip manufactured using that mask, these defects must be repaired before the mask can be used.
Traditionally, focused ion beam systems (FIB) have been used to repair defects in photolithography masks. A finely focused beam of gallium ións from a liquid metal ion source is scanned across the mask surface to form an image of surface. The intensity at each point of the image is determined by the current of secondary electrons ejected by the ion beam at the corresponding point on the substrate. The defect is identified on the image, and the ion beam is then scanned over the defect area in order to remove the excess absorber material from a mask surface or to deposit missing absorber material.
When used to remove material, the heavy gallium ions in the focused ion beam physically eject atoms or molecules from mask surface by sputtering, that is, by a transfer of momentum from the incoming ions to the atoms at the surface. The momentum transfer mechanism is considered to function through a series of collisions with nuclei in the substrate lattice, the process being referred to as a “collision cascade.”
When a FIB is used to deposit material to repair a clear defect, a gas is directed toward the defect area, and material is deposited by using an ion beam to decompose gas molecules absorbed on the substrate surface. A process for depositing a metal material using a FIB is described, for example, in U.S. Pat. No. 5,104,684 to Tao entitled “Ion Beam Induced Deposition of Metals.”
There are several problems with the use of gallium ion FIB systems to repair masks, particularly when used to repair opaque defects. First, gallium ions become implanted into the substrate surrounding the defect area. This phenomenon, commonly referred to as “staining,” causes the stained substrate to lose some of its transparency. This loss of transparency, in turn, introduces defects in the mask image that is transferred to the semiconductor substrate. The loss of transparency is particularly severe for the very short exposing light wavelengths used in modern photolithography processes, with the loss of transparency typically being between three and ten percent.
Second, the sputtering process of the focused ion beam is relatively unselective. While an opaque defect is being removed by the ion beam, substrate material at the edge of the defect is also attacked, and the result is a trench of damaged substrate material around the defect. This type of substrate damage is known as “riverbedding” because the etched edges resemble riverbeds when viewed with an electron microscope. Riverbedding results in an altered intensity and phase for the light traversing the quartz surrounding the defect.
Third, the sputtering of material by the ion beam leads to ejection of material in all directions, and some of this ejected material comes to rest on adjacent surfaces. This effect, known as redeposition, limits the precision of the microstructure fabrication.
Lastly, because the mask substrate is typically made of an insulating material, a positive electrical charge tends to accumulate on isolated defects when they are bombarded by the positive gallium ions. Each positively charged gallium ion not only brings a positive charge to the area, each massive ion also ejects multiple electrons from the surface. As this positive charge accumulates, it will reduce the emission of secondary electrons by which an image of the defect is attained. Ion beam systems used for mask repair typically include a charge neutralizer, such as an electron flood gun as described in U.S. Pat. No. 4,639,301 to Doherty, et al. for “Focused Ion Beam Processing.” It can be difficult to adjust the flood gun to just neutralize the surface charge, especially as the surface composition under bombardment is changing as the absorber material is removed.
Sputtering by an FIB system can be further enhanced and some of the previously described problems can be minimized by using an etching gas that adsorbs onto the surface and forms volatile compounds with the surface atoms under impact of the ion beam. The surface atoms are then more readily removed and less likely to redeposit. The gas atoms react with the surface molecules when energy is provided by the incoming ions. The incoming ions do not significantly react directly with the adsorbed gas molecules. The ions typically react in a series of collisions with atoms in the substrate, the collisions providing energy back through the lattice to knock atoms from the surface and instigate chemical reactions with the adsorbed gas molecules. Some gases cause the ion beam to preferentially etch one material over another. Although the use of a gas can reduce the enumerated problems associated with gallium-based FIB systems, the problems still remain.
Some materials are known to be etched by an etchant chemical in the presence of an electron beam. For example, Matsui et al. in “Electron Beam Induced Selective Etching and Deposition Technology,”
Journal of Vacuum Science and Technology B
, Vol. 7, No. 5 (1989) describes electron beam induced etching of silicon, gallium arsenide, and polymethylmethacrylate using xenon difluoride, chlorine, and CIF
3
. Matsui et al. also d
Casey, Jr. J. David
Chandler Clive
Da Xiadong
Gannon Thomas J.
Musil Christian R.
FEI Company
Hashmi Zia R.
Lee John R.
Scheinberg Michael O
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