Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-05-26
2001-09-11
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06287731
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to apparatus and methods for the manufacture of microlithography masks.
BACKGROUND OF THE INVENTION
Recent developments in semiconductor integrated-circuit technology have been remarkable with the miniaturization of the constituent semiconductor elements and trends toward increased integrated-circuit density. Up to the present, so-called optical lithography steppers have conventionally been used for performing lithographic exposure (projection-transfer) of integrated-circuit patterns onto semiconductor wafers. Unfortunately, current optical lithography techniques are (or soon will be) unable to provide the image resolution necessary to satisfy anticipated demands for ever decreasing miniaturization of semiconductor elements and increases in integrated-circuit density. Consequently, effort has been expended to develop microlithographic equipment employing a charged particle beam (e.g., electron beam or ion beam) or an X-ray beam rather than a light (ultraviolet) beam. That is, a charged particle beam or X-ray beam is used to project a pattern, defined by a mask or reticle (both terms are used interchangeably herein), with demagnification, onto a sensitive substrate, such as a semiconductor wafer coated with a suitable resist.
One type of mask used in conventional charged-particle-beam (CPB) projection-transfer systems is a stencil mask. Referring to
FIG. 3
, a stencil mask typically comprises a silicon membrane
6
having through-holes
6
a
formed therein. The silicon membrane
6
is formed from a silicon substrate
5
. The silicon substrate
5
acts as support for the membrane. The through-holes
6
a
, together with the remaining mask membrane, define a pattern. Stencil masks are frequently used in ion-beam lithography and cell-projection electron-beam lithography apparatus.
Referring to
FIG. 2
, full-size X-ray lithography masks typically comprise a patterned material
3
(typically tantalum) formed on a silicon nitride membrane
4
. The silicon nitride membrane
4
is formed on and supported by a silicon substrate
2
.
As used herein, a “mask workpiece” is a mask in the process of manufacture. Mask manufacture typically involves one or more anisotropic etching steps that facilitate the formation of the pattern defined by the mask.
During manufacture of masks such as shown in
FIGS. 2-3
, process temperatures typically increase to levels that result in damage to the mask membrane or in inaccurate mask patterns. If the mask workpiece is not maintained below a specific temperature level during manufacture, certain steps may not be executed properly. For example, excessive temperature may cause an anisotropic etch process to become isotropic, resulting in undesirable angled pattern features that reduce the mask's resolution. Thus, the mask workpiece should be maintained below a specific temperature during all steps of the mask-manufacturing process (the specific temperature is determined by the particular etching process and etching equipment used to make the mask). Maintaining the temperature of the mask workpiece below such a level yields stable etch results and allows performance of anisotropic etching. Accordingly, the resolution of the resulting mask pattern is increased if the temperature of the mask workpiece is controlled as the mask is formed (i.e., during the etch processes).
Conventional temperature control for dry-etch processes involves control of the temperature of the mask membrane as it is etched (i.e., membrane
4
in FIG.
2
and membrane
6
in FIG.
3
). A stream of coolant gas (at a pressure of several tens of mTorr) is flowed directly on the lower surface of the membrane (i.e., membrane surface
4
′ in FIG.
2
and membrane surface
6
′ in
FIG. 3
) during etching. stencil masks in which through-holes
6
a
are etched in the thin membrane
6
(or, as shown in
FIG. 2
, where a patterned material
3
formed on the membrane
4
is etched), the membrane is often deformed or destroyed due to the pressure of the coolant gas impinging on it.
Additionally, conventional temperature-maintenance methods are often ineffective because the coolant gas flows through the through-holes
6
a
as the holes are being etched. As a result, the gas does not act to sufficiently and consistently cool the mask workpiece. Accordingly, conventional methods of maintaining the temperature of a mask workpiece during manufacture do not consistently maintain the temperature of the mask workpiece below a specified temperature during performance of the etch processes necessary to form the mask.
Previous attempts to resolve the problem of damage to the membrane by the cooling gas include strengthening the membrane (i.e., providing a thicker membrane) to allow for direct cooling of the membrane. Alternatively, membranes have been strengthened by application of a resin (e.g., a resist) to the lower surface of the membrane. Such cooling methods are insufficient to maintain the mask below required temperature levels. Further, whereas thinner membranes provide better pattern resolution, thinner membranes are less resistant to thermal damage than thicker membranes. Moreover, application (and subsequent removal) of the reinforcing resin on the membrane increases the amount of processing required to manufacture a mask and increases the risk of damaging the mask membrane.
Accordingly, there is a need for methods for making charged-particle-beam microlithography masks in which the methods provide consistent cooling of the mask workpiece especially during etching and other steps as required without causing damage to the resulting mask.
SUMMARY OF THE INVENTION
In light of the foregoing deficiencies in the prior art, apparatus and methods for making charged-particle-beam (CPB) microlithography masks are provided. The apparatus and methods provide for consistent maintenance of the temperature of a mask workpiece during performance of etch processes (and/or other process steps as required) used to form the mask.
According to a first aspect of the invention, methods are provided that include placing a mask workpiece, comprising a substrate upon which a membrane is formed, on a temperature-controlled support. The mask workpiece is not secured to the support. The mask workpiece is cooled by conduction of heat to the temperature-controlled support. As a result, the mask membrane is consistently cooled but is not damaged by the flow of coolant gas during the etch processes. Because the mask workpiece need not be secured to the support, removal of the workpiece from the support does not add to the process time necessary to make the mask.
According to another aspect of the invention, mask-manufacturing apparatus are provided. A representative embodiment of such an apparatus includes a support that preferably comprises a material having a relatively high thermal conductivity, such as a conventional silicon wafer. A mask workpiece is placed on (so as to contact), but is not secured to, an upper surface of the support. The apparatus further includes a dry-etch device in which the mask workpiece and support are placed. The apparatus further includes a cooling mechanism that includes a coolant source and a flow device. The coolant source is a supply of fluid, comprising a gas, a liquid, or a mixture thereof. The flow device is operable to direct a flow of coolant in a direction toward (preferably directly on) the lower surface of the support, thereby cooling the support and thus the mask workpiece.
According to yet another aspect of the invention, methods are provided that include pausing the etch process at regular intervals for predetermined periods of time (i.e, providing a cooling period). The etch processes are paused for a cooling period before the temperature of the mask workpiece increases to a level that may cause damage to the mask membrane and/or material being patterned thereon. After the mark workpiece has cooled, the etch process is resumed until the next cooling period.
The foregoing and other objects, features, and advantages o
Huff Mark F.
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Mohamedulla Saleha R.
Nikon Corporation
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