1. Field of the Invention
The present invention generally relates to electron beam lithography tools and, more particularly, to limiting temperature-dependent changes in performance and operation of electron beam lithography tools.
2. Description of the Prior Art
Numerous lithography techniques are known and are in widespread use for manufacture of integrated circuit devices, in particular. Essentially, such lithography processes define potentially very minute areas and shapes on a surface through selective exposure and removal of portions of a layer of resist to expose areas of the surface for further processing by, for example, etching, implantation and/or deposition. Exposure has generally been accomplished in the past by use of radiant energy such as visible wavelength light.
There is a strong incentive in the manufacture of integrated circuits to increase integration density to the greatest possible degree consistent with acceptable manufacturing yields. Device arrays of increased density provide increased performance since signal propagation time is reduced with reduced connection length and capacitance. Further, increased device density on a chip allows greater chip functionality as well as greater numbers of devices which can be manufactured on a chip of a given area. Accordingly, increased integration density reduces manufacturing costs by maximizing the number of devices (or the number of chips of a particular functionality) which can be formed in the course of processing a single wafer of a given size since, for a given processing schedule, the cost of processing each wafer (including amortization and maintenance of the tools and processing reactor vessels) is substantially constant regardless of the number of devices or chips simultaneously formed.
Unfortunately, device size and density is a function of the minimum feature size which can be reliably produced in the course of patterning the resist. Minimum feature size is limited by the resolution of the exposure which is, in turn, limited by the wavelength of the exposure radiation. Accordingly, while deep ultraviolet (DUV) steppers now use 248 nm or 193 nm radiation, it is common to print features at sub-wavelength resolution. For example, 180 nm and 150 nm features can be exposed using 248 nm and 193 nm sources, respectively, by using a combination of small numerical aperture, off-axis illumination and optical proximity correction schemes. At shorter wavelengths of light, however, transmission becomes a problem requiring an entirely new class of optics to extend the present capabilities of optical lithography.
Similar severe difficulties exist for extending x-ray lithography to feature dimensions less than 130 nm. Although the wavelength of x-rays is sufficiently short (on the order of nanometers) to resolve feature sizes below 100 nm, x-ray lithography relies on using defect-free masks of the same size (1.times.) as the features to be exposed and thus relies on the perfecting of mask-making technology and shrinking the mask-wafer gap to reduce error due to diffraction. Such masks have been made utilizing electron beam lithography machines.
To form feature sizes in accordance with design rules incorporating sub-one-tenth micron region and below, it has been necessary to perform resist exposures with charged particle beams. Electron beams (e-beams) are favored since particles of the low mass of an electron can be more readily manipulated (e.g. accelerated, focused, guided and the like) at lower power levels preferably using magnetic lenses, correctors, deflectors and the like. Accordingly, while the invention will be discussed below in connection with an electron beam tool, it is to be understood that all such references are intended to be inclusive of tools using any charged particle species (e.g. ions as well as electrons). By the same token, the principles discussed herein are also applicable to other charged particle beam devices including control of a charged particle beam such as electron microscopy devices and references to an e-beam tool should be understood as inclusive of such other charged particle devices.
When charged particles are in close proximity to one another, such as at a cross-over in the optical system, the resolution of the beam can be degraded by the Stochastic Coulomb interaction. The interaction is dependent on the numerical aperture, beam current and drift space of the charged particles. The numerical aperture is a trade-off between the geometric aberrations (which increase with increase of numerical aperture) and the Coulomb interaction (which decreases with increase of numerical aperture). The beam current is determined by both the throughput requirement of the tool for a given resist sensitivity and the resolution loss through the Coulomb interaction (which increases with increasing current). Finally, Coulomb interactions are reduced for short drift spaces, particularly in the region where the size of the beam is small such as near the focal plane.
Practical implementation of shortening of the electron beam column is often difficult to achieve. There are often mechanical constraints in the location of magnetic lenses, correction elements, deflection and alignment yokes and the like. U.S. Pat. No. 5,635,719, which is hereby fully incorporated by reference, discusses the difficulties presented by a shortened electron-optical column on deflection fields when lens fields overlap in an electron beam projection lithography tool. Additional complications arise due to heat transfer between magnetic lenses, deflection yokes and correction elements which are in close proximity.
Practical implementation of electron beam lithography tools generally include magnetic lenses comprised of windings of several thousand ampere turns with a steel enclosure and pole pieces made of steel, permador, ferrite or other suitable ferromagnetic material used to concentrate the field of the lens. The lens windings can be significant sources of heat and are often liquid-cooled to reduce radiant heat exchange to other column elements. Depending on the pole piece material and its temperature/permeability characteristics, the strength of a magnetic lens, for a given excitation current, can be dependent on pole piece temperature resulting in variations in beam focus and/or position. Non-axially-symmetric thermal expansion may lead to large astigmatism.
In the past, cooling the lens windings removed enough heat to keep the pole piece temperature nearly constant. However, in modern, high-performance e-beam tools, it is often the case that significant sources of heat can occur near a lens pole piece by, for example, the windings of a deflection or compensation yoke of the type described in the above-incorporated U.S. Pat. No. 5,635,719. The pole pieces are relatively more remote from the cooling arrangement of the lens than the windings and the standard cooling arrangement is thus somewhat less efficient in regulating the temperature of the lens pole piece. Further, depending on the pole piece mounting geometry, the effective position of the lens or other element can shift with temperature, causing variations in beam position below the element, astigmatism and/or focus changes of the beam.
It should be appreciated that focus, astigmatism and beam position become more critical with the reduction of the minimum feature size written because the overlay and critical dimension variation specifications are all typically more stringent with reduction of feature size. Electron beam projection systems which are to be capable of writing minimum feature sizes of less than 0.13 microns will have specifications similar to the next generation of tools for fabricating x-ray masks. While it is known to liquid-cool deflection yokes and thus limit heat radiated to pole pieces of other elements such as lenses, the criticality of focus, astigmatism and beam position at smaller feature size regimes has been found to require a degree of temperature control which is not possible with known liquid cooling arrangements, particularly if temperature dependence is strong. The expense of developing a liquid cooling system providing greater temperature control than is available from known systems is considered to be prohibitive and, in any event, would only provide a marginal improvement which would rapidly become obsolete as further reductions in minimum feature sizes are made in integrated circuit designs.
Electrical correction for temperature-dependent performance variations has not been feasible since the inductance and hysteresis in the lens is sufficiently large to prevent real-time adjustments in lens excitation in response to changes in pole piece temperature. Accordingly, it can be appreciated that temperature-dependent changes of e-beam tool performance are a major impediment to the increase of integration density and for which no solution currently exists.
Conversely, it the absence of a capability of correction for temperature-dependent performance variations severe design constraints are placed on the design of magnetic optical elements such as material choice and liquid cooling arrangements in order to minimize temperature dependent effects. These constraints severely complicate e-beam tool design and limit the reduction of charged particle column length which, in turn, limits performance of charged particle beam tools.