In projection-exposure methods and apparatus as used for microlithography, the pattern to be transferred to the substrate is defined on a mask or reticle. Transfer is normally effected using an "energy beam" such as a light beam (e.g., ultraviolet light, X-ray) or a charged particle beam (e.g., electron beam). When a charged particle beam is used, the mask is normally segmented, by which is meant that the mask "field" (usually coextensive with a single die) is subdivided into multiple mask "subfields" (portions of the mask field) that are individually exposed in an ordered fashion. The image of each mask subfield (i.e., the corresponding "transfer subfields") are aligned with each other to produce each die on the substrate. Subdividing the mask in such a way is necessary because conventional charged-particle-beam (CPB) optical systems cannot project an entire die in one "shot" while sufficiently controlling aberrations.
For pattern transfer using a charged particle beam, either a "stencil mask" or a "scattering mask" (also termed "membrane mask") can be used. In a stencil mask, the features (elements) of the pattern portion in the various mask subfields are defined in part by voids extending through the thickness dimension of the mask and in part by the intervening regions of the mask that are situated between the voids, in the manner of a conventional "stencil". The intervening regions of the mask typically comprise a thin CPB-scattering membrane made of, e.g., silicon.
Charged particles in the beam encountering the voids pass directly through the voids. Charged particles in the beam encountering the CPB-scattering membrane are scattered. Charged particles passing through the mask pass through a CPB projection-optical system to the sensitive substrate. The CPB projection-optical system includes an aperture defined by a stop. Charged particles that are scattered by the scattering membrane tend to be blocked by the stop and thus do not propagate to the sensitive substrate; charged particles passing through the voids tend to pass through the aperture to the sensitive substrate. Thus, an image of the features defined in the mask subfield is formed on the sensitive substrate.
With a stencil mask, certain pattern features can only be defined as a scattering region surrounded by a void. Such features are termed "island" features. However, with an island feature, the surrounding void leaves the surrounded scattering region without physical support. Hence, with a stencil mask, it is necessary to divide island features into two or more complementary subfield patterns that must be separately exposed onto the corresponding transfer subfield (hence, each such transfer subfield receives at least two separate exposures). A similar problem arises with a "peninsular" feature that is longitudinally extended in at least one dimension such that it has insufficient physical support over its length. Hence, with a stencil mask, a peninsular feature must also be transferred by two or more complementary subfield patterns requiring that the respective transfer subfield receive multiple exposures to complete transfer of the respective pattern segment.
A conventional scattering mask comprises a thin (e.g., 3 .mu.m thick) membrane that defines regions (corresponding to pattern features) that scatter charged particles very effectively and other regions (analogous to the voids in a stencil mask) that scatter charged particles much less effectively. Charged particles passing through such a membrane form an image of the pattern on the wafer. Unlike a stencil mask as summarized above, a scattering mask normally does not require splitting of certain mask subfields into two or more subfields having complementary patterns.
However, with a scattering mask, approximately 10 to 30 eV of energy is lost whenever the charged particle beam passes through the membrane. Thus, approximately 20 eV of the energy of the incident charged particle beam is lost. Due in part to such problems, scattering masks exhibit certain characteristic phenomena such as increased chromatic aberration in the CPB projection-optical system.
With stencil masks that include complementary mask subfields, it is preferable that all the subfields, including the complementary ones, be located on the same mask. Such a configuration allows the mask pattern to be transferred with less alignment error and other problems than with masks in which complementary subfields are situated on separate masks (e.g., two masks).
The 4-gigabit DRAM represents the new standard in "chip" (die) size. The chip size of a 4-gigabit DRAM is about 18 mm.times.36 mm as formed on the wafer. The size of a stencil mask for such a die must include sufficient area to accommodate the chip size multiplied by the reciprocal of the demagnification factor (e.g., multiplied by 4), boundary regions (non-patterned regions) located between adjacent mask subfields to separate the mask subfields from one another, and any complementary subfields. Accommodating all these requirements can make it difficult to fit such a mask on a single 8-inch diameter wafer.
If the chip size of a device is even larger than that of a 4-gigabit DRAM, serious concerns are raised as to whether the respective mask can be fit on a 10-inch diameter wafer or a 12-inch diameter wafer.