Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2000-04-17
2002-03-26
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
06361911
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of improved Critical Dimension control for various E-beam and laser beam pattern exposure systems.
(2) Description of the Prior Art
The creation of semiconductor devices is critically dependent on the formation of numerous patterns that form device features while other patterns provide the means of interconnecting these device features. Photolithography has long been the mainstay technology that has been used for the creation of these patterns, the art of photolithography depends on transferring patterns that are contained in a mask onto a target surface whereby the most frequently used target surface is a layer of photoresist that is sensitive to impact of photo-electric energy. By placing a source of light behind the mask and exposing the layer of photoresist via the interception of the mask that contains the to be created pattern, the light that is provided by the light source only partially penetrates the mask and consequently only partially impacts the surface of the layer of photoresist. The chemical composition and molecular structure of the photoresist is changed due to the impact of the photo energy such that the impacted or non-impacted photoresist, dependant on whether the photoresist is positive or negative type photoresist, can be removed thereby leaving the pattern that is present in the exposure mask imprinted and reflected in the layer of photoresist. The remaining photoresist can then be used to shield an underlying layer of material, such as conductive materials, to create a desired pattern in these materials. It is well known in the art that for instance patterns of interconnect metal lines can be created in this manner.
The methods of creating images onto a target surface have been expanded to where not only visible or ultraviolet light energy is used for the exposure of the layer of photoresist (optical lithography using proximity or projection exposure) but where other sources of energy, such as electron beam projection, ion bombardment and X-rays can also be used as the source of energy.
Electron beam exposure and laser beam scanning have initially been applied for the creation of masks that are used for photolithographic exposures. The process of creating patterns whereby a photolithographic mask is used depends on accurate and easy alignment of the mask with underlying target surfaces, this in order to enable accurate exposure in a readily reproducible and repeatable manner. Since the image that is contained in the mask is projected onto a target surface, the method whereby the mask image is projected onto the target surface is frequently referred to as projection printing. These photolithographic technologies have advanced from earlier methods of proximity printing using a negative photoresist to methods of projection printing using a positive photoresist and a wafer stepper.
The methods of photolithographic are simple to apply but suffer from defect formation for methods where the mask is in direct contact with the wafer onto which an image is to be created. This contact is cause for high mask damage resulting in the requirement for frequent mask replacement. The major advantage of direct contact printing is that it offers high resolution, for shrinking device features contact printing becomes less attractive due to among others the impact that the (relatively larger) thickness of the layer of photoresist has on image resolution and critical dimensions of pattern images. Proximity printing offers the advantage of decreased mask damage, this advantage however is obtained at the cost of reduced image resolution.
Two of the more important parameters that apply to image creation are the Depth Of Focus (DOF) that can be achieved using a particular technology and the therewith corresponding Numerical Aperture (NA). For high feature resolution it is required that the DOF parameter is as large as possible, which implies that NA must be relatively large since the DOF is inversely proportional to (NA)
2
. There is however a compromise that must be made between the values of DOF and NA. For narrow line width, NA must be large since the line width is inversely proportional to the NA. This would lead to the conclusion that a (very) large value of NA leads to dense feature or line patterns. A relatively large value for NA however decreases the value of DOF to the point where image resolution suffers. This then leads to the conclusion that an optimum (relatively high) value for NA must be selected such that the DOF is still acceptable, bringing small feature size in balance with acceptable feature resolution. The wavelength of the light source that is used to create the pattern is also of importance whereby DOF is directly proportional to this wavelength while the aperture width is also directly proportional to the wavelength. Because of this dependency, it is clear that one of the approaches that can be used for improved line resolution is the use of smaller wavelength energy sources. Electron lithography makes use of this relation in using smaller wavelength at relatively high source energy. E-beam apparatus makes use of high-energy electrons to directly write a pattern onto a target surface. This leads to two concerns: the electrons that make up the E-beam must be propagated in a narrow and densely controlled beam while the sequential process of writing the pattern consumes a relatively large amount of time that is required to complete the writing of the pattern. The electrons, once they reach the target surface, may also be prone to scatter in a manner that is not conducive to creating a pattern of sub-micron dimensions.
A typical hardware and software configuration that makes up an E-beam system is rather elaborate and requires a considerable amount of software support to create image patterns with the capabilities of fast and error free patterns generation, pattern modification, and the like. Due to the nature of an E-beam configuration, the supporting hardware contains a number of components that are designed for electron beam control (such as the electron gun, which creates the electron beam) and a magnetic configuration that controls the electrons after release by the electron gun and before reaching the target surface. The control of the electron beam, once the beam has been emitted by the electron gun, takes place in an electron beam control column that contains such items of control as one or more lenses for the focusing of the electron beam in addition to one or more aperture masks to restrict the flow of electrons. The main operational parameters of an E-beam apparatus are the accelerating voltage that is applied to the stream of electrons, typically between about 10 and 50 KeV, and the cross section or size of the electron beam, typically between about 0.05 to 3 um. The E-beam apparatus thereby can be of a continuous write nature or it can be of a step and repeat nature containing various optical constructions. The performance parameters of an E-beam system are the feature size resolution, typically less than 0.1 um and the overlay accuracy, typically between about 0.005 and 0.4 um.
E-beam systems differ in the method that is used to project a desired pattern onto a target surface, two methods are essentially used for this purpose, that is raster scanning and vector scanning.
For the raster scanning approach, both the (scanning) E-beam and the target surface move with respect to each other such that the E-beam sweeps in an Y-direction while the underlying target surface moves in an X-direction, whereby the X and the Y direction perpendicularly intercept. The direction of the E-beam during its sweep in the Y-direction is controlled and made to follow the movement of the underlying target surface so that, during one complete sweep of the E-beam across the target surface, the vertical sweep of the E-beam addresses the same X-axis component of the underlying target surface. During the sweep of
Chou Wei-Zen
Tsai Fei-Gwo
Taiwan Semiconductor Manufacturing Company
Young Christopher G.
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