Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-02-22
2004-05-18
Goudreau, George A. (Department: 1763)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C438S725000, C438S704000, C438S717000, C438S951000, C438S670000
Reexamination Certificate
active
06737202
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor processing techniques, and more particularly to the fabrication of a tiered structure using optical lithography methods and base layer stabilization to prevent interlayer mixing and a method of forming a semiconductor device with the tiered structure.
BACKGROUND OF THE INVENTION
Gate structures in FET devices are a critical component affecting device performance. Gate structures on many current metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs) use metals, such as gold (Au), to achieve the low noise, low resistance performances required. Due to the non-reactive nature of these metals and the difficulty in etching them, additive fabrication techniques, such as evaporative metal deposition and lift-off processing, are typically employed in the manufacturing of these gates.
During operation, the speed of a transistor is inversely related to the length of the gate with smaller gates providing faster switching times. This is because a gate with a small foot or stem (length) offers less gate capacitance. However, one drawback is that such a gate having a conventional rectangular cross sectional area is more resistive as the gate length decreases, because its cross sectional area decreases. Improved gate performance can be attained if a gate is fabricated to have a small length or foot dimension, but a larger top section having a larger cross sectional area. The small foot or stem (gate length) attached to the substrate minimizes gate capacitance, while the larger structure or head on top allows for low gate resistance. This tiered structure now resembles a T and so this type of gate is commonly referred to a T-shaped gate or simply a T-gate. In many instances, this structure is also referred to as a Y-gate or a mushroom gate due to its final shape, which depends upon the method used to deposit the metal. Deposition methods such as evaporation or sputtering result in a gate structure which is dimpled at the top and thus take the shape of a Y. Metal deposition which is done from the bottom up such as by plating, results in a gate structure which is more rounded in profile, thus resembling a mushroom. In either case, the cross-sectional dimension is not uniform and thus is deemed a tiered structure. In yet another instance, a tiered structure known as a gamma-gate can be produced. A gamma-gate is fabricated to include a top tier which is not centrally aligned with the bottom tier, or stem portion. This can be easily done by purposeful and controlled misalignment during lithography of the top tier with respect to the bottom tier. Accordingly, use of the terms T-gate, Y-gate and gamma-gate are all synonymous terms, since all apply to tiered structures having slightly different cross-sectional shapes. In this disclosure the term T-gate, the most common term in the art, is intended to encompass all of these structural variations.
Typically, T-gate structures are used in high performance devices, as they give lower noise performance, higher gain, and higher cutoff frequencies as compared to simple rectangular gates. In a metal T-gate the upper, wider part of the gate increases the cross-section of the gate, which reduces the gate resistance. So where a small gate length reduces noise, a T-gate with a small gate length reduces noise even more.
Fabrication of a metal structure such as a gate using an additive metallization process, requires that a liftoff resist process be used. In a liftoff resist process, special techniques and resist materials are used to form a retrograde or re-entrant resist profile. Here the top of the resist is opened more narrowly than the bottom of the resist which is processed to open more widely, forming the desired profile. This is in contrast to conventional resist processing such as when resist is used as an etch mask. Here, it is advantageous that the resist profile be either vertical or have a slightly positive slope. There are many known resist processes used for liftoff fabrication, but all have in common this retrograde profile. The most commonly used processes employ two different resist materials which are layered one on the other to form a bilayer stack. The two materials have different exposure and dissolution characteristics which cause each to have a different development rate. The bottom layer is generally a somewhat isotropically developing, low contrast resist and is made to develop laterally more than the top layer. The top layer is a more anisotropically developing, high contrast layer used to define the opening for metal deposition and ultimately define the metal structure's dimension (length). Other processes have made clever use of resist chemistry and have achieved a similar retrograde profile in only a single layer. This is done by controlled diffusion of a neutralizing agent into the top surface of the resist to a limited depth. This neutralizing agent (typically a base) slows the dissolution rate of the resist near the top of the resist layer where the concentration of the agent is greatest. Dissolution rate is unaffected below a certain depth into the resist, where development occurs at a normal rate. The slowed rate for the top compared to the bottom portions of the resist layer combine to create a retrograde profile.
A T-gate is a special type of structure, and like conventional rectangular gates, can be fabricated using additive metallization and liftoff processing. However, since a T-gate is an example of a tiered structure, the resist process is more complicated since an additional tier of resist is needed to form the stem of the T shape. As with conventional liftoff processing, many types of T-gate processes have been proposed. All have in common a high resolution opening in the base resist layer used to form the narrow part, the stem. A conventional bilayer or single layer liftoff resist process is built on top of the base layer which forms the wider, current flowing, top portion of the gate. Once the complete stack is built, metal is deposited into the opening and fills the structure starting from the bottom (base or stem section) and proceeds through the top section, stopping when the desired thicknesses are achieved.
Electron-beam lithography has been often used for the fabrication of T-gate resist profiles. E-beam offers the highest resolution possible, a factor which is necessary for creation of small gate lengths which play a key role in gate performance. However, many other factors are beneficial and naturally inherent to e-beam lithography, and are in contrast, absent from optical lithography. Such factors add additional favor to e-beam lithography as a means of fabricating T-gate resist profiles. The two most important of these are, the transparency of e-beam resists to high energy electrons, and the insolubility of e-beam resists which allow for multi-level stacking.
Polymeric materials used as e-beam resists are largely transparent to high energy electrons used for exposure. For this reason, it is well known in the art that a single e-beam exposure, coupled with the correct tri-layer resist stack and selective developers is adequate to expose a completed T-gate resist profile. This is not true of other types of lithography such as optical lithography where resist materials are largely absorbing making it impossible to pattern, with the highest resolution, the base layer through the adjacent overlayers. In addition, many commonly used e-beam resists such as PMMA (poly methylmethacrylate) have a very high molecular weight (MW) (often>1,000,000) and achieve development contrast through e-beam induced chain scissions which dramatically reduce MW in exposed areas. Rapid development of exposed areas occurs since lower molecular weight polymers dissolve much more rapidly than high molecular weight polymers. The relative insolubility of unexposed high MW e-beam resists also allows them to be coated in adjacent laminar layers without interlayer mixing or solvent attack from the next layer to be coated.
Gehoski Kathleen Ann
Mancini David P.
Popovich Laura
Resnick Doug J.
Goudreau George A.
Koch William E.
Motorola Inc.
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