Lithography reflective mask

Details Lithography reflective mask Lithography reflective mask

1998-12-04

2001-01-23

Porta, David P. (Department: 2876)

X-ray or gamma ray systems or devices

Specific application

Lithography

C378S034000, C378S035000, C430S005000

Reexamination Certificate

active

06178221

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to a reflective mask for use in a lithography method, such as an extreme ultraviolet lithography method.
BACKGROUND OF INVENTION
Integrated circuit fabrication techniques vary greatly depending on the specific chip structure being made, the exact processes being used, and/or the available equipment.
However, almost all fabrication methods include a lithography process during which certain regions of a wafer (i e., a silicon slice coated with a photoresist material) are exposed to radiation to delineate a latent image corresponding to the desired circuit pattern. The radiation-exposed wafer is then developed, etched, and processed to form an integrated circuit.
The technical advances in lithography processes have been significant. Integrated circuits built to design rules at or slightly below 0.25 &mgr;m are common with the use of radiation in the deep ultraviolet wavelength. Radiation in the extreme ultraviolet (EUV) range (3 nm to 50 nm wavelength—also referred to as “soft x-ray”) has been found useful for the fabrication of devices having design rules of 0.18 &mgr;m and is prospectively useful for even smaller design rules, such as 0.10 &mgr;m and smaller.
During the past eight years, EUV lithography has evolved from a simple concept into a possible candidate for mass commercial production of integrated circuits. Projection lithography, and particularly reflective (rather than transmission) projection lithography, is believed to be the best route to industrial production of integrated circuits by use of EUV lithography. In such a system, EUV radiation is projected onto a lithography mask having reflective regions and non-reflective regions corresponding to the desired circuit pattern. The beams reflected from the mask are then demagnified and projected onto the wafer.
Of particular interest in the present invention is the reflective mask used in EUV projection lithography. A reflective EUV lithography mask typically comprises a substrate, a reflective coating on a top surface of the substrate, and a plurality of absorbing blocks covering certain regions of the reflective coating in a manner corresponding to a desired circuit pattern. A reflective mask may also include buffer blocks situated between the covered regions of the reflective coating and the absorbing blocks.
A method of making a reflective lithography mask typically comprises the steps of applying the reflective coating onto the substrate and then applying a buffer layer on the reflective coating to create a reticle blank. The absorber blocks are then arranged on the buffer layer in a manner corresponding to a desired circuit pattern by, for example, depositing an absorber layer on the buffer layer and etching certain portions of the absorber layer to form the absorber blocks. The uncovered regions of the buffer layer are then removed, usually by etching, to create a plurality of buffer blocks situated between block-covered regions of the reflective coating and the absorbing blocks. Accordingly, the buffer material must remain intact during creation of the absorbing blocks while at the same time must be able to be removed from the uncovered regions of the reflective coating without damaging the coating.
Reflective masks may have a tendency to accumulate and retain static electric charge when exposed to intense actinic radiation, such as during the EUV lithography session. At the very least, this static charge attracts dust which may interfere with the exposure of the mask. Moreover, perhaps more importantly, an electrostatic discharge (“ESD”) will occur when the electrostatic charge becomes substantial enough to overcome a dielectric material between the charge and another surface of lower electrical potential. Electrostatic discharge can cause permanent or costly damage to an already expensive EUV reflective mask.
Accordingly, the inventor appreciated that a need remains for a reflective lithography mask which is designed to reduce the risk of electrostatic discharge damage.
SUMMARY OF THE INVENTION
The present invention pertains to a reflective lithography mask which is designed to reduce the risk of electrostatic discharge by providing a buffer layer that is electrically conductive. An oxide material is usually the buffer material of choice because it remains intact during absorber-etching while at the same time may be removed (i.e., etched) from the uncovered regions of the reflective coating without damaging the coating. However, since such oxide materials are insulators, they increase the risk of electrostatic discharge damage by providing a dielectric material between the charged absorber blocks and the coating/substrate. Accordingly, the present invention reduces this risk by replacing the insulating buffer layer with an electrically conductive buffer layer.
More particularly, the present invention provides a reflective lithography mask comprising a substrate; a reflective coating on a top surface of the substrate; a plurality of absorbing blocks covering certain regions of the reflective coating in a manner corresponding to a desired circuit pattern; and a plurality of buffer blocks situated between the covered regions of the reflective coating and the absorbing blocks. The buffer blocks are made of an electrically conducting material, such as carbon in an electrically conductive graphite form; tin oxide and indium oxide and materials based on these compounds).
Preferably, the reflective coating is a multi-layer mirror comprising layers of high and low refractive index materials introducing a phase delay of one or more wavelengths so that a composite reflection is single phase. For example, the multi-layer mirror may comprise layers of silicon and molybdenum; molybdenum and beryllium; or ruthenium and boron carbide. Additionally or alternatively, the absorbing blocks are made at least partially of a high absorption material, such as gold; germanium; tungsten; tantalum; titanium; or titanium nitride.
The reflective mask may be used in a lithography method (preferably, but not necessarily an EUV lithography method) to delineate a latent image of a desired circuit pattern (preferably having design rules of 0.25 &mgr;m and less) onto a wafer. In this lithography method, the mask is illuminated with radiation (preferably having a wavelength of 3 nm to 50 nm) and the radiation is reflected from the reflective regions of the mask onto the wafer
A method of making a reflective lithography mask according to the present invention comprises the steps of applying the reflective coating onto the substrate and then applying an electrically conductive buffer layer onto the reflective coating to form a reticle blank. The absorber blocks are then arranged on the buffer layer in a manner corresponding to a desired circuit pattern. This block-arrangement is preferably accomplished by depositing an absorber layer on the buffer layer and then etching certain portions of the absorber layer to form the absorber blocks. The uncovered regions of the buffer layer are then removed, preferably by etching, to create the electrically conductive buffer blocks situated between the block-covered regions of the reflective coating and the absorbing blocks.
The step of removing (ie., etching) the absorber layer is performed without substantially etching the buffer layer. The step of removing (i. e., etching) the buffer layer is performed without substantially etching (or damaging) the reflective coating. Accordingly, the electrically conductive buffer material and/or the etching techniques are chosen to accomplish these objectives. For example, with a carbon graphite buffer layer, the absorber layer could be removed with a non-oxygen plasma etching technique and the buffer layer could be removed with an oxygen plasma etching process.
These and other features of the invention are fully described and particularly pointed out in the claims. The following description and drawings set forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but

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