Photocopying – Projection printing and copying cameras – Detailed holder for original
Reexamination Certificate
1999-12-29
2001-05-29
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Detailed holder for original
C355S023000, C355S025000, C355S047000, C355S075000, C353S023000, C353S095000
Reexamination Certificate
active
06239863
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to lithography, and more specifically to the protection of lithographic reticles without the use of a pellicle.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A semiconductor wafer, for example, can be used as a substrate to fabricate an integrated circuit.
During lithography, a reticle is used to transfer a desired pattern onto a substrate. The reticle is formed of a material transparent to the lithographic wavelength being used, for example glass in the case of visible light. The reticle has an image printed on it. The size of the reticle is chosen for the specific system in which it is used. A reticle six inches by six inches and one-quarter inch thick may be used, for example. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer corresponding to the image printed on the reticle.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.
As should be clear from the above discussion, the accurate location and size of features produced through lithography is directly related to the precision and accuracy of the image projected onto the wafer.
The rigors of sub-100 nm lithography place stringent demands not only on the lithography tool, but also on the reticle. Air borne particles and dust that settle on the reticle can cause defects on the wafer. Small image distortions or displacements in the reticle plane can swamp critical dimension and overlay error budgets. The conventional solution is to use a thin piece of permanently fixed transparent material as a pellicle for the reticle. This pellicle remains in place during all stages of the lithography process. A pellicle has a dual role in improving the accuracy of the image projected onto a wafer. First, a pellicle serves to protect the reticle from direct contact with particulate contamination. Particles that settle on the reticle can produce image distortion and so must be removed. Removal of particles from the reticle can cause damage to the reticle because such removal may involve direct contact with the reticle. When a pellicle is used, particles will settle on the pellicle rather than the reticle. Thus, it is the pellicle which must be cleaned. Cleaning the pellicle rather than the reticle poses fewer dangers to the integrity of the reticle since the reticle is protected during this cleaning by the pellicle itself.
The second role played by a pellicle is related to the standoff of the pellicle. During exposure, the focal plane corresponds to the location of the image printed on the reticle. By including a pellicle, any particles in the system will settle on the pellicle rather than the reticle. By virtue of the thickness of the pellicle, and thus the distance between the surface of the pellicle and the patterned surface of the reticle, these particles will not be in the focal plane. Since the pellicle lifts the particles out of the focal plane, the probability that the image projected onto the substrate will include these particles is greatly reduced.
This solution discussed above works well in many conventional lithographic processing techniques. Since materials are available for producing transparent pellicles and reticles, the use of such a system is convenient in, for example, a system in which light must pass through both the reticle and the pellicle.
The pellicle approach, however, is not well suited for use in extreme-ultraviolet (EUV) applications. Currently, there are no materials sufficiently transparent to EUV that can be used to make a pellicle. In EUV lithography, the EUV does not pass through the reticle, but is reflected off the image side of the reticle. This technique is known as reflective lithography. If a pellicle were to be used in a reflective lithography process, the EUV would necessarily pass through the pellicle twice, once on the way to the reticle and again after reflecting off of the reticle. Thus, any amount of light loss associated with the pellicle is effectively doubled with EUV processing techniques.
SUMMARY OF THE INVENTION
A pellicle material has not been disclosed within the industry that is sufficiently transparent to EUV and compatible with the vacuum environment in which EUV reflective lithography takes place. Thus, the present invention is directed to a removable cover to protect a reticle during lithography processing.
The removable cover of the instant invention minimizes friction and thus reduces particle generation. The removable cover of the instant invention can be securely mounted to a reticle during most processing and transport steps and is easily removed for the exposure step. Furthermore, the removable cover of the instant invention is designed to be held by a simple end effector and transported by that end effector between a shelf and a reticle mount. The removable cover of the instant invention is compatible with the reticle mount such that the reticle can be mounted with the removable cover in place. Thus, the removable cover of the instant invention achieves much of the advantages of conventional pellicles yet is compatible with EUV lithography.
In a preferred embodiment of the instant invention, the removable cover for protecting a reticle used in a lithography system includes a frame and a membrane supported by the frame. The membrane is transparent to an inspection wavelength such that the reticle can be inspected with the removable cover in place. This removable cover protects the reticle when the removable cover is in place and is removable for lithographic exposure. Since the cover is removable, the membrane can be formed of a material that is at least partially opaque to an exposure wavelength used in the lithography process. The removable cover can further include at least one reticle fastener that applies force to the reticle thereby preventing movement of the removable cover relative to the reticle when the removable cover is in place.
The reticle fastener can be one or more rotationally actuated bi-stable fasteners that serve to bias the reticle in a direction away from the bi-stable fasteners when the bi-stable fasteners are in a fastened position.
Furthermore, the removable cover of the preferred embodiment includes a filter that allows gas flow between the removable cover and the reticle when the removable cover is in place.
The frame of the removable cover of the preferred embodiment includes a plurality of ridges. The ridges serve to contact the reticle when the removable cover is in place. When the removable cover is in place and the bi-stable fasteners are in a fastened position, the bi-stable fasteners bias the reticle toward at least two of the plurality of ridges.
The removable cover of the preferred embodiment further includes positional locators disposed on a side of the removable cover opposite from a side that faces the reticle when the removable cover is in place. These positional locators can be selected from the group consisting of bumps and detents. Furthermore, in the preferred embodiment, the positional locators are arranged to permit contact between a first set of positional locators and a shelf, and between a second set of the positional locators and an end effector while the remov
Catey Eric B.
del Puerto Santiago
Hult David
Roux Stephen
Adams Russell
Brown Khaled
Silicon Valley Group Inc.
Sterne Kessler Goldstein & Fox P.L.L.C.
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