(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of repairing Phase Shift Masks (PSM) such that a normal light transmission rate and light phase angle can be maintained.
(2) Description of the Prior Art
A photolithographic mask is one of the essential components that is used for creating patterns of exposure on semiconductor surfaces. The photolithographic mask contains a pattern of device features that must be transposed from the mask to underlying layers of semiconductor material such as for instance a layer of photoresist.
A standard photo mask contains a transparent substrate, typically made of quartz. A patterned layer of opaque material has been created over a surface of the transparent substrate. Chromium is typically used for the opaque material, deposited to a thickness of about 1,000 Angstrom. Alternate opaque materials for the creation of the patterned layer on the surface of a photolithographic mask are nickel and aluminum.
More sophisticated photo masks apply the principle of phase shifting of the light as the light passes through the mask. Phase shifting masks are used to create device features of sub-micron dimensions.
It is well known in the art that adjacent light beams, which are in extreme close proximity to each other while the light beams are used to create sub-micron device features, mutually influence each other, having a detrimental effect on the formation of the exposed pattern. The phase shift mask counteracts this mutual influence that closely spaced light beams have on each other.
As a further advance, alternate phase shifting masks can be used, where the phase shifting characteristic of the phase shifting mask is alternately affected as the light passes through the photo mask.
A further level of sophistication is introduced by the use of gray-tone masks where light passes through the photomask in a graded manner. Light passing characteristics of the gray-tone mask can be adjusted so that not only complete passing or complete blocking of light takes place but so that the mask provides a graded exposure that may for instance be of use in creating dual damascene structures, where depth of light exposure can be used for non-uniform removal of a layer of photoresist over the thickness of the layer of photoresist.
The photolithographic mask is created using conventional methods (including sputtering) of depositing a layer of opaque material over the substrate of the mask and patterning this layer using conventional high resolution methods of exposure such as E-beam exposure.
Due to the frequently used high density of the pattern that is developed in the layer of opaque material, this formation is exposed to a number of problems that result in bad and unusable masks.
These masks are frequently repaired, based on a cost analysis as to whether it is best (most cost effective) to repair the mask or whether it is best to scrap the defective mask.
Some of the defects that can occur in the creation of an opaque pattern over the substrate are opaque material remaining in place where it should be removed and visa versa, the formation of an interconnect or bridge between closely spaced adjacent lines of the opaque pattern, extensions of the opaque material into transparent surface regions of the mask, the occurrence of an isolated opaque spot in a transparent region, the formation of pin holes in either the opaque or the transparent surface area, and the like.
A number of methods have been provided for the repair of photo masks, in view of the density and the potential complexity of the pattern that has been created on the substrate of the photo mask, this process however can be cumbersome, time consuming and expensive. The invention provides a method that is reliable and can be readily made part of a semiconductor manufacturing environment.
U.S. Pat. No. 5,965,303 (Huang) shows a method for fabricating a phase shift mask utilizing a defect repair tool.
U.S. Pat. No. 6,207,328 BI (Lin) shows a method for forming a phase shift mask.
U.S. Pat. No. 5,432,484 (Hosono) shows a method to correct defects in a PSM including Ga stains.
U.S. Pat. No. 6,042,738 (Casey et al.) discloses a pattern film repair method using a focused particle beam system.
A principle objective of the invention is to provide a method of repairing Phase Shift Masks (PSM) whereby the function of repair has no detrimental effect on the transmission rate and the phase angle of the PSM.
Another objective of the invention is to provide a method of repairing a PSM that avoids damage to the phase shifter of the mask.
Yet another objective of the invention is to provide a method of repairing a PSM whereby no permanent detrimental effect of a Ga stain on the quartz substrate of the mask is experienced.
In accordance with the objectives of the invention a new method is provided for the repair of a Phase Shifter Mask. The phase shifter of the PSM has been created over the active surface of the mask as a pattern of phase shift material, the pattern of phase shift material comprises at least one faulty element. A layer of photoresist is deposited over the active surface of the mask. A backside exposure of the PSM is performed in order to define the pattern of the mask in the layer of photoresist. The layer of photoresist is developed and remains in place over the pattern of the mask, protecting the phase shifter of the mask during repairs of the PSM. Repairs of the mask are performed using Focused Ion Beam techniques for this repair. A plasma etch removes Ga stain from the surface of the quartz substrate. The developed layer of photoresist is then removed from the pattern of the PSM using a wet strip process.