Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2000-07-31
2002-04-30
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S296000, C430S942000
Reexamination Certificate
active
06379851
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the general field of energy beam lithography and, more particularly, to prediction and correction of the lithography process to compensate for proximity heating of the resist and, most particularly, to correction of proximity heating occurring during resist exposure by means of vector scanning or stencil exposure.
2. Description of Related Art
The fabrication of integrated circuits (“ICs”) requires ever more accurate methods for creating patterns on a wafer substrate, typically involving exposure of a resist-coated wafer to a pattern of energy in the form of electromagnetic radiation, electrons or other particle beams. “Positive resists” require exposure of the resist to energy in those areas in which resist removal is desired. “Negative resists” require exposure of the resist to energy in those areas in which resist retention is desired. Both positive and negative resists are commercially useful. Exposure to a pattern of energy may occur through a mask having a pattern of energy-transmissive regions therein, typically fully transmissive or fully opaque although “gray” regions are not inherently excluded. “Photolithography” commonly denotes the exposure of a resist-coated substrate through a patterned mask by means of electromagnetic radiation.
Another general method of writing patterns on a resist makes use of a beam of energy directed only to those regions of the resist-coated surface requiring exposure without screening by an intervening mask. A suitable beam steering mechanism is typically employed along with suitable controls to insure that only the regions of the surface requiring exposure are contacted by the incident beam. The beam may be electrons, ions, neutral particles, collimated laser light or other electromagnetic radiation. However, to be definite in our discussion, we will emphasize the example of a beam of electrons impacting the resist-coated substrate (e-beam lithography), not excluding thereby other forms lithography by means of directed energy beams. The control mechanism may be a simple on-off control to expose the pattern pixel by pixel. The control mechanism may be more complex, controlling the beam intensity, shape, dwell time or other beam parameters, the control of which leads to precise patterning.
In addition to direct beam writing and lithography through a mask, circuit components (e.g. a memory cell) may be exposed in a single flash through a “stencil.” Stenciling in this manner may make use of either electromagnetic radiation or particle beams, and numerous uses of the same and different stencils may be necessary to fully pattern the entire surface of the wafer.
Direct beam writing of patterns onto a resist-coated surface is the method presently preferred for creating the masks used in photolithography, but direct writing offers other advantages as well. Among these other advantages of direct beam writing are the avoidance of complications of alignment and registration of the mask with the substrate and the possibility of creating more precise patterns with the use of accurately focused beams. One disadvantage of direct beam patterning in comparison with photolithography is the relatively smaller throughput possible with direct beam writing.
Considering e-beam lithography by way of example and not limitation, the presently employed writing techniques may be classified into general categories as vector scan, raster scan or stenciling. Vector scan typically directs the beam while off to a region of the substrate requiring exposure, then exposes a contiguous region of the substrate to the energy of the beam before moving to another region for exposure. Simply stated, vector scanning “paints” or “tiles” a region of the substrate with beam energy before moving on to expose another region. Most conveniently, beam direction, scanning trajectories, flash size, shape and/or intensity are under computer control, defining the pattern to be written.
Raster scanning directs the beam to all regions of the substrate no matter what pattern requires exposure and adjusts the beam intensity at each point scanned to effect the correct pattern of exposure. The simplest beam control during raster scanning entails having the beam on or off as each pixel is scanned. However, adjustment of beam intensity to numerous levels between full-on and full-off (gray scales) is also feasible in some raster scanning procedures. Stenciling combines use of one or more masks for patterning a single circuit component (such as a memory cell), with a flash exposure of the entire stencil.
Precise exposure of resist requires a detailed understanding of the sensitivity of the resist to e-beam exposure. The exposure of resist to an e-beam, called the dose, is typically measured in microcoulombs per square centimeter (&mgr;C/cm
2
). The sensitivity of the resist means the electron dose (in &mgr;C/cm
2
) necessary to create the desired pattern in the resist upon development. This sensitivity is a function of the resist composition, the energy of the incident electron beam, the temperature of the resist, the resist development process and other factors as well. The changes of resist sensitivity with its temperature at the time writing occurs is a particular concern of the present invention.
It is helpful to emphasize that exposure of the resist by e-beam impact and heating of the resist are two conceptually distinct phenomena. The chemical activity of the resist leading to its useful lithographic properties is initiated by e-beam impact. The efficacy of electrons in causing this chemical activity is defined as the sensitivity of the resist. The sensitivity of the resist to e-beam impact depends in turn on many factors including the temperature of the resist at the time it is exposed. Thus, changing the temperature of the resist changes its sensitivity which may require changing the dose of electrons, typically by changing the dwell time or beam current (or both) in order to achieve proper exposure. Failing to take into account changes in resist sensitivity with temperature may lead to overexposure of the resist, exposure of the resist in regions not intended to be fully exposed, and less precise patterns. Pattern “blooming” is the undesired result.
Heating of the resist occurs in two ways: 1) As an inherent adjunct effect to the impact by electrons intentionally directed onto the resist for exposure. This heating is always present in e-beam lithography and is taken into account when the resist is calibrated to specify the correct exposure dose. 2) In high voltage lithography, most of the electron beam energy passes through the resist and the underlying mask layer (typically very thin) and penetrates the substrate where most of the beam energy is deposited. (An exception occurs when thin substrates are used, typically in the manufacture of X-ray masks, where the substrate is itself a film so thin that most of the beam energy passes through without significant diminution.) Electron diffusion in a thick substrate deposits the heat from a single e-bean flash in the substrate, typically in a volume 10 or more microns (&mgr; or micrometers) in lateral extent (perpendicular to the e-beam direction). Subsequent thermal conduction transports a portion of this heat to the substrate surface where it heats the resist in a zone that may be tens of microns in lateral extent a few microseconds following the flash, increasing to a millimeter across after several milliseconds. (Exact numbers will depend on beam energy, the composition of the substrate and its thermal properties). Thereafter the heat has diffused so much as to have no significant effect on resist temperature or, consequently, on resist sensitivity. It is this second type of heating that this invention addresses and denotes as “proximity heating.” Such proximity heating depends on the previously written pattern and the time history of the pattern writing. This variability makes proximity heating particularly challenging to estimate in designing a process for high accuracy
Applied Materials Inc.
Young Christopher G.
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