Moving lens for immersion optical lithography

An apparatus for immersion optical lithography having a lens capable of relative movement in synchrony with a horizontal motion of a semiconductor wafer in a liquid environment where the synchronous motion of the lens apparatus and semiconductor wafer advantageously reduces the turbulence and air bubbles associated with a liquid environment. The relative motions of the lens and semiconductor wafer are substantially the same as the scanning process occurs resulting in optimal image resolution with minimal air bubbles, turbulence, and disruption of the liquid environment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The current invention generally relates to optical lithography, and more specifically relates to optical lithography implemented in an immersion liquid environment.

2. Description of the Related Art

The widespread utilization of computer systems to solve complex problems has created a demand for applications designed to produce solutions using increasingly complex algorithms. As the complexity of the problems has increased, so too have the computational requirements of the applications used to solve these problems. The ability of computer systems to produce accurate results in an efficient manner is determined by both the application design and computer system hardware running the application.

Increases in computer hardware performance are continuously strained by design specifications that push the physical properties of the materials that comprise a computer chip. Increased demands in performance require computer chips with more wires on more layers to perform complex computations in acceptable time frames. The number of components and wires on a computer chip required to satisfy these demands has continually increased forcing computer chip designers to create denser chip layouts or increase the physical dimensions of the chip. Chip manufacturers have chosen to keep chip size relatively constant over time and increase the density of components on chips.

As wire, component, and layer density increased on computer chips, manufacturers struggled to maintain the necessary precision for useful operation. To meet increasingly demanding precision requirements, the method of optical lithography was implemented using light to print device and wire patterns on the chip layers. Optical lithography uses a light projection device including a lens system to concentrate light of a particular wavelength onto a wafer. The wafer is first coated with a resist material sensitive to light exposure. As the light scans the wafer, the image is printed onto the wafer. Alternatively, “scanning” may be referred to as “imaging” and used interchangeably within the field of optical lithography. The wafer is then chemically bathed removing any positive acting photoresist material exposed to the light scan. In the early 1990's, optical lithography was capable of producing linewidths of 0.35 micrometers in manufacturing, and current optical lithography techniques can produce linewidths of 100 to 150 nanometers in manufacturing. Smaller lines can be produced for developmental and prototype purposes.

The foundation of optical lithography is based heavily on Rayleigh's two equations. These equations define the dependency of resolution (W) and depth of focus (DOF) on wavelength and the numerical aperture (NA) of the imaging system where numerical aperture is defined as a measure of light gathering power of a lens {Lin, B. J. “New λ/NA scaling equations for resolution and depth-of-focus.” Optical Microlithography XIII (2000): 759.}. The resolution of imaging is defined in the Rayleigh equation: W=k1·λv/NA. Resolution is the minimum feature that may be printed using optical lithography and determines the fidelity of the pattern transfer. Depth of focus can be defined as the region around the image plane in which the image will appear to be sharp. (“Depth of field and depth of focus”, 25 Jul. 2000 at URL http://www.matter.org.uk/tem/depth_of_field.htm.) Based on Rayleigh's work, depth of focus is defined as: DOF=k2·λv/NA2as derived for the paraxial case, where λvis a wavelength in a vacuum and NA=n sin θ where n is the index of refraction and θ is the acceptance angle of the lens.

For consistency in the high NA immersion case, Bum Lin has defined resolution as W=k1·λ/sin θ where λ=λv/n. Burn Lin has also shown for immersion optical lithography that DOF=k3·λ/sin2(θ/2), where λ=(λv/n), k3is an engineering constant specific to the lithographic process, θ is the angle used to define NA, and λ is the wavelength (λv/n) in the immersion media. This second form is less ambiguous for high NA and immersion optical lithography.

Optical lithography has been extended to use 193 nanometers for manufacturing patterns, but problems begin to occur below this wavelength. As components and wire dimensions become smaller, the difference in size between the wavelength of the light and the components shrinks. The components and wires at some critical point become the same size or smaller than the wavelength of the light. At this point, the implemented wavelength is no longer capable of printing the chip design with sufficient fidelity. To overcome this problem, shorter light wavelengths must be used; however, new problems arise when using shorter wavelengths. Shorter wavelengths, such as x-rays have been used to achieve smaller linewidths, but the adoption of equipment capable of producing x-rays has been hindered by difficulties associated with manufacturing lenses capable of producing sufficient imaging quality when used with x-rays. These difficulties have led to high lens costs resulting in an expensive migration path from past optical lithography equipment to x-ray optical lithography equipment. Shorter wavelengths are also higher energy wavelengths and therefore high doses of x-rays have a greater potential to damage the solid chip material, especially dielectric. Furthermore, light sensitive compounds in resist only absorb light over a specific range of wavelengths and alternative materials may not always perform as well as necessary. See “Optical Lithography”, Craig Friedrich, 1998 at URL http://www.me.mtu.edu/˜microweb/chap1/ch1-4-1.htm.

One way to improve the resolution of optical lithography is to manipulate the numerical aperture variable in Rayleigh's equation or sin θ/2 in Burn Lin's equations. The maximum attainable value for numerical aperture in conventional dry optical lithography methods is 1; however, it is known from optical microscopy and the work of E. Abbe (1878) that by filling the space between the final lens and the wafer with a high index liquid, light that would otherwise be totally internally reflected is able to pass through the liquid to the wafer surface {Switkes, M., M. Rothschild “Resolution Enhancement of 157 nm Lithography by Liquid Immersion.” Optical Microlithography XV (2002): 459.}. It is possible to achieve numerical apertures greater than one and as high as the index of the immersion liquid. The use of a liquid in optical lithography increases the depth of focus by a factor equal to the index of the immersion liquid when NA is held constant, therefore increasing the tolerable error in the process.

Immersion optical lithography permits optical lithography exposure equipment manufacturers to extend the use of their current optical lithography equipment to the next generation of chip design with minimal development cost. With potential numerical apertures of 1.25 or higher and resolutions of 50 nanometers, future chips can be produced using modern immersion optical lithography techniques without making high risk, expensive expenditures on new capital equipment and resist materials required for shorter wavelengths. Because the properties of water make it an ideal immersion liquid for 193 nanometer imaging, and relatively minor modifications to existing equipment are necessary, the transition from dry optical lithography to immersion optical lithography is an economically feasible and low risk decision. New sources of light and new resists are also unnecessary.

The advent of immersion optical lithography has also resulted in numerous additional problems. In order to achieve maximum gains in numerical aperture size, there can be no air between the final lens and the immersion liquid. This requires the final lens element to be immersed in the liquid. Throughout the process, the wafer is secured to a horizontal support surface capable of moving in the x,y, and z directions. During scanning, the final lens element and/or horizontal support surface are moved as the wafer surface is scanned. As the lens moves through the liquid, the motion of the lens translates energy from the lens into the liquid, thus creating ripples, turbulence, and disruption of the liquid environment. Gas and air bubbles may become trapped within the liquid or attach to the lens surface resulting in light scattering and poor quality imaging. Therefore, a need exists for a device capable of minimizing the ripples and turbulences associated with the energy transfer between the motion of the lens and the liquid environment.

SUMMARY OF THE INVENTION

The current invention reduces the turbulence and air bubbles associated with the relative motion of a final lens element in an immersion optical lithography environment. The lens apparatus is coupled to a motion control device capable of adjusting the final lens' angular orientation relative to the wafer. The current invention enables the lens apparatus to move such that the horizontal velocity of a portion of the final lens element relative to the wafer surface substantially minimizes air bubbles, turbulence, and other disruptions of said liquid detrimental to imaging quality as the wafer is scanned.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described in detail with reference to the following figures. It will be appreciated that this description and these figures are for illustrative purposes only, and are not intended to limit the scope of the invention. In particular, various descriptions and illustrations of the applicability, use, and advantages of the invention are exemplary only, and do not define the scope of the invention. Accordingly, all questions of scope must be resolved only from claims set forth elsewhere in this disclosure.

FIG. 1is a side-view illustration of an embodiment of the overall invention. A frame100serves as a support structure to hold the various related components. Light source104is attached to frame100and projects the light through a photomask113and a lens system101having lens elements101a,101b,101cand final lens element103. Examples of light sources for light source104are excimer lasers and mercury arc lamps. Lens system101a,101b,101cand final lens103serve to manipulate light projected by light source104. The lens shape of lenses101a,101b,and101care shown as having a circular cross section in the illustration but are not limited to this shape and may have curvatures that are spherical or aspheric and concave or convex. Often lenses101a,101b,and101chave a lenticular form. A wafer111is secured to a horizontal support surface102and is exposed by light source104via a scan. A specified pattern is printed on the wafer using a mask113through which the light from light source104is projected. Horizontal support surface102movement is controlled by a first motion control device106that controls movement of horizontal support surface102in the x, y, and z direction, relative to frame100. First motion control device106is secured to a first stationary surface108. Wafer111is immersed in a liquid112such as deionized water or perfluorinated polyethers as part of the optical lithography process.

Final lens element103ofFIG. 1is coupled to a second motion control device105at one or more coupling points110. Final lens element103then pivots on an axis defined by coupling point110when the apparatus is operably running. The pivoting is achieved by control of the second motion control device105using a computer controlled shaft, a motor having suitable torque and precision, cams, a gear system, a belt system, or manual control of the operator. The surrounding gaseous environment may consist of one or a combination of the following: air, argon, dry nitrogen, or other inert optically transparent gasses.

FIGS. 2A,2B,2C, and2D are exemplary variations of final lens element103shapes. InFIG. 2A, an embodiment of the lens element103has a first curved surface region201A and a second curved surface region201B. The horizontal line separating first curved surface region201A and second curved surface region201B is used for exemplary purposes only and does not exist in the actual embodiment. The degree of curvature of first curved surface region201A is greater than that of second curved surface region201B. Second curved surface region201B is substantially facing horizontal support surface102in this variation of final lens element103to effectively scan wafer111. Final lens element103pivots on coupling point110. In a second variationFIG. 2B, the degree of curvature of first curved surface201A is less than that of second curved surface region201B. Second curved surface region201B is substantially facing horizontal support surface102in this variation to effectively scan wafer111. In a third exemplary shape of final lens element103shown inFIG. 2C, final lens element103has a consistent degree of curvature. Final lens element103pivots on coupling point110. Wafer111is secured to horizontal support surface102and is scanned during the process.FIG. 2Dis a fourth exemplary shape of final lens element103. InFIG. 2D, second curved surface region201B has a radius significantly smaller than first curved surface region201A, as shown.

FIG. 3is a simplified, side view ofFIG. 1illustrating final lens element103shown as a cylindrical lens, an extension connector302, and second motion control device105. Examples of extension connectors include a threaded solid shaft, one or more flexors, or a combination of a solid shaft and an adhesive material. Final lens element103contains end covers301A and301B which enable extension connector302to couple final lens element103to second motion control device105. Final lens element103effectively pivots on the axis created by extension connector302whose motion is driven by second motion control device105.

FIGS. 4A,4B, and4C illustrate the relative motion of finals lens element103. InFIG. 4A, final lens element103is oriented to a starting position by second motion control device105(shown inFIG. 3) coupled to coupling point110capable of rotating on an axis. Final lens element103is partially submerged in liquid112and positioned above wafer111. Wafer111is secured to horizontal support surface102capable of moving in an x (horizontal in figures), y (into the paper in figures), and z (vertical in figures) direction under control of first motion control device106. A proximal lens surface exists on the lens between point401A and401B. Proximal lens surface401A,401B is that portion of a surface on final lens element103nearest the wafer during a scan of the wafer. Note that, as final lens element103moves (e.g., pivots about an axis), proximal lens surface401A,401B moves along the surface of final lens element103. As wafer111is scanned and synchronous motion of proximal lens surface401A,401B and horizontal support surface102in the same direction occurs, the velocity of final lens103element and/or horizontal support surface102are adjusted such that the relative velocity between proximal lens surface401A,401B and wafer111substantially minimizes air bubbles, turbulence, and other disruptions of said liquid detrimental to imaging quality.

As the scanning of wafer111occurs, shown from startFIG. 4Ato mid-scanFIG. 4B, final lens element103moves such that turbulence and air bubbles associated with the transfer of energy from final lens element103moving through liquid112are substantially minimized. During this scanning process the velocity of proximal lens surface401A,401B and/or horizontal support surface102are adjusted, keeping the relative velocity between proximal lens surface401A,401B and wafer111sufficiently low to minimize air bubbles, turbulence, and other disruptions of said liquid detrimental to imaging quality. Horizontal support surface102may also move in a z-direction to vary the distance between final lens element103and wafer111thus optimizing the image focus and image resolution. First motion control device106responds to signals from an autofocus device (not shown), moving horizontal surface102in the z-direction to optimize the image focus. Auto-focus mechanisms moving in the z-direction are well-known in the art. Therefore, three possible adjustment scenarios exist. In a first scenario, horizontal support surface102moves at a constant velocity while final lens element103moves at a varying angular velocity in a relative motion. In a second scenario, final lens element103moves at a constant angular velocity and horizontal support surface102moves at a varying velocity in a horizontal direction. In a third scenario, final lens element103and horizontal support surface102both move at varying velocities such that the relative motion reduces air bubbles, turbulence, and other disruptions of said liquid detrimental to imaging quality enough to keep them from significantly interfering with the imaging.FIG. 4Cillustrates final lens element103motion nearing the end of the relative motion, where final lens element103is in a final position. Depending on the degree of relative motion and the shape of the lens, constant angular velocity and constant horizontal velocity of horizontal support surface102may give sufficiently small relative velocity to reduce formation of bubbles and turbulence enough to keep them from interfering with imaging.

FIG. 5Ais an exemplary case of a final lens element103being a sphere or cylinder having a cylindrical cross section as depicted inFIG. 2C. As wafer111is scanned, a proximal lens surface401A,401B of final lens element103moves a distance XLand wafer111secured to horizontal support surface102moves a distance XW. As the lens moves at an angular velocity

ⅆθⅆt,
the x-component of the velocity vector for proximal lens surface401A,401B of final lens element103nearest the wafer is

ⅆxLⅆt=r1⁢ⅆθⅆt.
To sufficiently reduce the turbulence and air bubbles associated with this relative motion between final lens element103and wafer111, the x-component of the velocity vector of horizontal support surface102,

ⅆxWⅆt
should be sufficiently close to

r1⁢ⅆθⅆt
to reduce the turbulence and air bubbles from forming to the degree that they significantly interfere with the imaging. It will be understood that equations hereinafter, for simplicity, show the horizontal component of velocity of the proximal lens surface401A,401B being equal to the horizontal velocity of horizontal support surface102(upon which wafer111is fastened), the invention contemplates any relative velocity difference sufficiently low as to reduce turbulence and bubbles such that the turbulence and bubbles do not significantly interfere with the imaging.

The mathematical process for x-dimension velocity adjustment of proximal lens surface401A,401B of final lens element103(as shown inFIG. 2AandFIGS. 4A,4B,4C) relative motion and horizontal support surface102motion is illustrated here with reference toFIG. 5B. While the following mathematical process is illustrated for exemplary purposes, and other mathematical processes that produce similar or the same end result are contemplated, this discussion is not deemed to be restrictive as to the scope and spirit of the invention. θ is an angular rotation of final lens element103about coupling point110(O1), defined positive when moving in a clockwise direction and thus for velocity of proximal lens surface401A,401B of final lens element103in the x-direction. O1is the center of a first spherical or cylindrical surface (as shown in5A., not shown in5B) with radius r1. O2is the center of a second spherical or cylindrical surface with radius r2.

ⅆxLⅆt
is the x-direction velocity component of proximal lens surface401A,401B on final lens element103as it moves in the x-direction (horizontal) as shown inFIG. 5B. The distance h is the radius created between center O1and point x1,y1on circle with center O2.

ⅆxLⅆt
can be computed by first noting that

ⅆxLⅆt=h⁢ⅆθⅆt
for proximal lens surface401A,401B. Note that, in the present example, h is a function of angle theta, and therefore changes as final lens element103is rotated.
(h+c1)2+(c2)2=r22, and then substituting for c1and c2:
(h+ccos θ)2+(csin θ)2=r22and, therefore:

The distance h represents the z-direction (vertical) distance between coupling point110(i.e., O1is coupled to coupling point110) of final lens element103and the point on final lens element103surface intersecting with ray v1(i.e., proximal lens surface401A,401B), which is in the immersed portion of final lens element103through which light from light source104exposes wafer111.

During a portion of the scan, wafer111travels a distance XWin time t. Proximal lens surface401A,401B of final lens element103and wafer111move in the same direction at a small enough relative velocity so as to reduce or eliminate turbulence and/or bubble formation in liquid112: (equation (2) is shown as an “equality”, but it will be understood that the invention contemplates any relative velocity between proximal surface401A,401B and horizontal support surface102low enough to reduce turbulence and bubbles to the degree that the turbulence and bubbles do not significantly interfere with the imaging).

While the previous mathematical explanations have been computed using the exemplary case of final lens element103shape as shown inFIG. 2A, the apparatus and motion are not limited to this example shape. Other lens shapes may be implemented and are within the scope and spirit of the invention.

FIG. 6is a plot of the length of h versus the size of the angle created by the relative motion of the final lens element103according to the embodiment depicted inFIG. 2A, using equation (1) above with exemplary parameters R2=2 and C=1.5, and letting θ vary from −30 degrees to +30 degrees. As final lens element103moves in a relative motion from a first position as shown inFIG. 4Ato a mid-scan position as shown inFIG. 4B, and theta decreases, h decreases as well, reducing a horizontal velocity of proximal lens surface401A,401B. As final lens element103continues to move in a relative motion from the mid-scan position as shown inFIG. 4Bto a final position as shown inFIG. 4C, h increases, with a proportional increase in horizontal velocity of proximal lens surface401A,401B. As described earlier, the horizontal velocity of proximal lens surface401A,401B can be maintained constant by changing the angular velocity versus theta to compensate for the changing length, h. As described earlier, the angular velocity of final lens element103can be maintained constant and first motion control device106controls horizontal velocity of horizontal support surface102at a varying rate to account for the varying horizontal velocity of proximal lens surface401A,401B. The current invention contemplates varying motions of both horizontal support surface102and angular velocity of final lens element103as an alternative mechanism to maintain the relative velocity of proximal lens surface401A,401B substantially the same as the horizontal velocity of horizontal support surface102. Alternatively, if turbulence and bubbles are minimal when the angular velocity of final lens element103is constant and the horizontal velocity of horizontal support surface102is constant, such constant angular velocity and horizontal velocity is acceptable as an embodiment of the present invention.

FIG. 7is an illustration of the method steps comprising the optical lithography process. In step701, a semiconductor wafer is placed on a horizontal support surface capable of moving in the x, y, and z dimensions. In step702, the wafer is secured to the horizontal support surface such that the horizontal and vertical movement of the wafer and the horizontal support surface are identical. Step703involves immersing the wafer into a liquid by creating a reservoir of liquid on the horizontal support surface. The liquid generally is deionized water or perfluorinated polyethers. The final lens element is immersed in the liquid environment in step704near a top surface of the wafer by lowering the final lens element into the liquid, raising the horizontal support surface towards the final lens element, or both. The top surface of the wafer is the surface of the wafer to be scanned which faces the final lens element when resting on the horizontal support surface. In step705the final lens element is oriented to a start position by a motion control device coupled to the final lens element. A computer system, a motor or motors having suitable torque and precision, a cam system, a gear system, or manual control of an operator, controls the movement of the final lens element.

Step706involves scanning the wafer by moving the final lens element in synchrony with the horizontal support surface, thus substantially reducing the turbulences and air bubbles associated with the movement of the final lens element through the liquid. Adjustments to the velocity of the final lens and horizontal support surface velocity are made during the process to ensure the relative velocity between the wafer and proximal lens surface is substantially small. The proper adjustment can be made using a computer system, one or more precision motors, a cam system, or manual control of the operator.

In step707, the process of scanning is repeated for each field of the wafer until the necessary scanning of the wafer is complete. After each field is scanned, the final lens element may be lifted out of the liquid and repositioned to the next field or the final element may be repositioned without removing the final lens element from the liquid. At the beginning of each new scan, the final lens element and mask are re-oriented back to the start position and the process is repeated. After scanning a field on the wafer, the final lens element is at an end position which may also become the start position of a scan in the opposite direction for the next new field. When all scanning is complete, the final lens element is removed from the liquid environment and the liquid environment is eliminated by suction or draining off of the liquid in step708. The wafer is then released and removed in step709. In step710, the final lens element may be cleaned by various methods including but not limited to a cloth, air pressure, or a liquid cleaner.