Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-05-30
2003-12-09
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06660437
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a phase mask for exposing a photosensitive layer in a photolithographic process. The phase mask contains a T-patterned structure composed of transparent surface segments with mutually displaced phases and a surface boundary segment whose phase is situated between the phases of the adjacent surface segments.
Such phase masks are used in photolithographic processes to produce integrated circuits, in particular to produce, conductor tracks for wiring integrated circuits.
Such conductor tracks are usually incorporated in isolator layers that are seated, directly or with the interposition of a metal layer, on a substrate that contains the integrated circuits. Such substrates usually are formed of silicon layers, while the isolator layers are formed of oxide layers, preferably made from silicon oxides.
Trenches, running in a plane or in a plurality of planes, and contact vias are incorporated in the isolator layer in order to produce the conductor tracks, it being preferred to use etching processes, in particular plasma etching processes, for this purpose.
In order to incorporate the trenches and contact vias into the isolator layer, a resist mask with a via pattern corresponding to the trenches and/or the contact vias is applied to the isolator layer. A plurality of resist masks are usually also applied successively in a multistage process, in order to incorporate contact vias and/or trenches in a plurality of planes of the isolator layer.
The individual trenches and contact vias are etched with prescribed depths through the corresponding openings in the resist masks. Thereafter, the resist masks are removed from the isolator layer. Finally, metal is deposited into the trenches and/or contact vias in order to produce the conductor tracks.
The production of resist masks on the isolator layers is performed by known photolithographic processes. The first step in this case is to apply a radiation-sensitive resist layer to the isolator layer. Radiation, in particular optical radiation, is applied to the resist layer at prescribed points by the application of stencils or the like. Thereafter, either only the exposed or only the unexposed regions of the resist layer are removed in a suitable developer. In the first case, a so-called positive resist is present, in the second case a negative resist is present. The resist layer with the via pattern thus generated then forms the resist mask for the subsequent etching processes.
In the exposure process, the beams, in particular light beams, are intended to be projected onto the surface of the resist layer as accurately as possible in accordance with a prescribed via pattern. The aim in this case is as high a resolution as possible, whose goal is to obtain as abrupt a transition as possible of exposed and unexposed points in the photoresist layer.
The exposure is performed in this case in such a way that the radiation source emits radiation that is focused via a lens onto an image plane in which the resist layer is located. Individual substrates with the resist layers applied thereon are positioned in the image plane by a stepper in the beam path of the beams emitted by the radiation source.
During the exposure, the radiation is guided through a mask, it being possible to prescribe a specific exposure pattern by the structure of the mask. The mask is usually constructed as a binary mask, for example in the form of a chrome mask. Such chrome masks have an alternating structure of transparent regions that are preferably formed by a glass layer, and nontransparent layers that are formed by the chrome layers.
A phase mask is used instead of a chrome mask in order to increase the contrast of exposed and non-exposed regions on the resist layer.
Such a phase mask can be constructed, in particular, as a half-tone phase mask. In such half-tone phase masks, the opaque layers are replaced by a semitransparent layer with a transmission of typically 6%, whose layer thicknesses are constructed such that the traversing radiation experiences a phase-angle deviation of 180°.
Furthermore, the phase mask can also be constructed as an alternating phase mask. Such an alternating phase mask has neighboring transparent regions, separated in each case by a chrome layer, which have phases respectively displaced by 180°. That is to say, the radiation traversing a transparent region is offset in phase by 180° by comparison with the radiation that is guided through a neighboring transparent region.
An exact and contrasty optical imaging is obtained with the aid of such alternating phase masks when, in particular, the chrome layers are disposed as chrome webs running parallel to one another at a spacing. The transparent regions then likewise form webs that run between the chrome webs and have alternating phases of 0° and 180°.
However, a structure of phase masks that have branched opaque segments constructed as chrome webs is problematical, two chrome webs forming a T-shaped structure in each case, in particular. With such a T-shaped structure, a second chrome web opens out at the longitudinal side of a first chrome web such that the first chrome web is subdivided into two partial segments. The transparent regions that surround the chrome webs are then to be constructed, in particular, as rectangular surface segments, the lengths and breadths of the surface segments being adapted in each case to the lengths of the adjacent opaque segments or parts thereof. The transparent surface segments are then preferably disposed such that in each case two surface segments situated opposite an opaque segment have phases differing by 180°. However, there are then always two surface segments remaining with phases differing by 180° that are directly adjacent. The light beams that pass the phase mask at the boundary line of these surface segments are extinguished by interference effects, the result of this being that a non-exposed zone is obtained on the resist layer in the corresponding position.
This entails a second exposure process, with the aid of which the zone must be exposed subsequently. This constitutes an undesired additional processing step, and thus an extra expenditure on time and costs.
U.S. Pat. No. 5,840,447 discloses a phase mask that has transparent surface segments with different phases. A periodic sub-wavelength structure is provided along the boundary line between two surface segments with different phases. The sub-wavelength structure contains alternating thin layers of materials for the two adjacent surface segments. The sub-wavelength structure results in a virtually continuous transition of the refractive index during the transition from one surface segment to another. Interference is thereby prevented from extinguishing light beams that penetrate the boundary line between the surface segments.
U.S. Pat. No. 5,635,316 discloses a phase mask that has a plurality of transparent surface segments with phases of 0° or 180°. Light beams that penetrate the boundary line between two surface segments of different phase are canceled by interference effects. A closed network of unexposed lines is obtained by a suitable configuration of the surface segments and of the boundary line structure thereby produced. A partial re-exposure of the unexposed lines is performed with the aid of a second mask in a second method step.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an alternating phase mask which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which has branched structures with a high contrast and a high imaging quality.
With the foregoing and other objects in view there is provided, in accordance with the invention, an alternating phase mask for exposing a photosensitive layer in a photolithographic process. The alternating phase mask contains at least two opaque segments including a first opaque segment with a longitudinal side and a second opaque segment ending at the longitudinal side of the first opaque segment. The se
Friedrich Christoph
Griesinger Uwe
Heissmeier Michael
Ludwig Burkhard
Moukara Molela
Greenberg Laurence A.
Infineon - Technologies AG
Mayback Gregory L.
Rosasco S.
Stemer Werner H.
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