Lithographic techniques are essential to semiconductor manufacturing. In a nutshell, a photo-resist material is applied to the surface of a semiconductor substrate, and a high definition image of a layer of circuitry is exposed onto the photo-resist. The exposure of light onto portions of the photo-resist causes those portions to either be easily washed away or prevents those portions from being washed way, depending upon the type of photo-resist used. Because a patterned version of the photo-resist remains, the next application of a material or etching solution may be masked by the patterned photo-resist, resulting in a patterned application of the material or etching solution. This allows one to add material, dope, or etch in a controlled manner to form, for example, transistor gates or other conductive pathways, doped source/drain regions, or trenches.
However, because of the high accuracy required during lithography, the process is extremely sensitive to the topographic heights of the substrate being exposed. Unless the topographical variations of the surface of a semiconductor substrate are accounted for, they can significantly deteriorate lithographic performance by reducing the common depth of focus for the entire chip field. This problem has become more serious with the increasing use of high numerical aperture (NA) lithography, which uses an extremely thin optical focal plane. Conventional scan exposure systems deal with the topography of the chip by leveling. This method can reduce the focus difference across the entire chip field by changing stage height and tilt during scanning to help even out the topography. In conventional leveling, the stage moves in a z-direction and follows the topography profile. Leveling is used for correcting for changes in topography in the scan direction.
However, in the slit direction, the focus difference is reduced only by stage tilting. Stage tilting results in portions of the semiconductor device being intrinsically outside of the focal plane. This deteriorates lithographic performance by causing critical dimension (CD) error or failures such as pattern collapse or pattern scumming. In some cases, stage tilting can also cause the exposure in general to be out of focus.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter.
There is a need for a system of leveling the topography of a semiconductor surface in the slit direction. Accordingly, a method and system of leveling in the slit direction is presented. The system may include an illumination system, a reticle scan stage, a wafer scan stage and at least one projection lens. In addition, the system can include a leveling sensor, an analyzer and a controller.
In one arrangement, the system levels the topography by inducing a low-order lens aberration in addition to stage tilt. In order to induce such aberration, a system may include multiple independently adjustable projection lenses. In addition, the system may include a focal plane control lens to further improve response time.
Further to this arrangement, the topography of the chip may be measured via a leveling sensor. The measured topographical information (e.g., height of the wafer at a given location) is then sent to an analyzer that determines the aberration to induce based on the topographical information. This information is then transmitted to a controller that adjusts the lenses and stage tilt to induce such aberration. The controller is able to dynamically adjust one or more of the lens positions to provide a focal plane that closely approximates the topography of the portion of the wafer being exposed.
Additionally or alternatively, the exposure slit may be split into multiple smaller slits to provide a focal plane that approximates the topography of the portion of the wafer being exposed. The analyzer may receive the height information from the leveling sensor and use it to determine the appropriate number of slits to implement. This information may then be transmitted to the controller that then may control stage height and tilt dynamically for each slit. In addition, dose profile can be controlled in order to compensate for flare.
These and other aspects of the disclosure will be apparent upon consideration of the following detailed description of illustrative embodiments.
A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the following description in consideration of the accompanying drawings, in which like reference numbers indicate like features, and wherein:
The various aspects summarized previously may be embodied in various forms. The following description shows by way of illustration various embodiments and configurations in which the aspects may be practiced. It is understood that other embodiments may be utilized and structural and functional modifications may be made, without departing from the scope of the present disclosure.
One illustrative configuration of a system of leveling is shown in
According to one arrangement, the system 100 of
The lenses 110 may each be tilted independently of the others of the lenses 110, in order to introduce the desired aberration. In the arrangement shown in
In the three lens system 110 of
As each of the lenses 110a, 110b, 100c is tilted, it induces an aberration in the focal plane. The accumulated aberration results in (in the present example) a second-order aberration in the focal plane. The order of the curve that the focal plane follows depends upon the number of lenses being used and adjusted. For instance, two lenses can be used to provide a first-order curved focal plane, three lenses can be used to provide a second-order curved focal plane, and four lenses can be used to provide a third-order curved focal plane, etc.
In order to determine the amount of adjustment for each lens, the analyzer 112 may receive height information from the leveling sensor 122. As the wafer is scanned, portion by portion, the leveling sensor 122 may determine the heights of the wafer surface at the various portions across the wafer. Leveling sensors are generally known in the art and exist in conventional lithographic systems. The height of the wafer surface at various locations is then transmitted from the sensor 122 to the analyzer 112 where the information may be processed to determine the adjustments to be made to the lenses 110 to produce the appropriate focal plane.
Once the appropriate focal plane has been determined, the analyzer 112 outputs this information to the controller 114, which may adjust one, some, or all of the lenses 110 to introduce the appropriate aberration based on the wafer height information, to closely match the topography of the wafer portion to be illuminated. Such controlling may be done by the Piezo-electric devices 118a, 118b, 118c, 118d. Controlling such devices may be generally known in the art. In another example, the Piezo-electric devices 118a, 118b, 118c, 118d may adjust an additional aberration lens 120. This analysis and control system 112 and 114 can dynamically calculate and control the lenses 110 in real time, as the scan is performed, to provide a focal plane for each wafer portion that approximates the topography of the surface of that wafer portion.
In contrast,
As previously mentioned, such height differences may deteriorate lithographic performance because the distance from the surface 130 to the lens system 110 changes depending upon the location on the surface 130. An illustrative second-order focal plane formed using the system of
In order to improve response time, a fourth lens 120 may be added to the system, as shown in
In step 208, the wafer is removed from the system 100. In one configuration, the aberration analysis may be performed once on a certain wafer or wafer portion to determine the dynamic adjustments to be made to the focal plane. Information indicating the adjustments made over time may be stored in memory or another computer-readable medium, such that when further identical wafers are exposed by the system 100, these adjustments need not be re-determined but instead read from memory. This can speed up the exposure process dramatically when a large number of identical wafers are being exposed in series. Alternatively, the aberration adjustments may be determined from scratch for each individual wafer.
In another configuration, the exposure slit may be divided or split into more than one smaller slit. This configuration may allow the focal plane to more closely follow the topography of the wafer surface.
The surface 330 of the wafer portion is seen to have a step change 330(a) in height. In order to accommodate these height differences, the exposure slit may be split into two or more smaller slits to expose only sub-portions of the wafer surface at any given time. In one arrangement, the size of the slit is adjusted using blind aperture to physically adjust the size of the slit.
Broken line 332 indicates an example of a focal plane that may be produced when two slits are used. Solid line 334 shows an example of a focal plane that may be produced using three slits. Although the system is shown with two or three slits being used, any number of slits may be utilized. In one arrangement, the number of slits correlates to the number of inflection points for a given focal plane. For example, the slit number Sn and the number of inflection points for a given aberration plane An can be correlated using the following equation:
S
n
=A
n+2
That is, in an arrangement where there is no inflection point, the number of slits would be 2. In another example, if there is one inflection point, the number of slits would be 3.
As before, a threshold value 336 may be defined such that the focus difference between the wafer surface and the focal plane should be no larger than the threshold value 336 at any given location.
The analyzer 112 may be used to determine the appropriate number of slits and the information may be transmitted to a controller 116. The controller 116 then controls the stage height and tilt for each split to provide a focal plane that closed matches the topography of the chip surface. The controller 116 may also control the dose profile within each slit to compensate for flare. Flare may be a kind of leakage dose from the edge of the slit. The dose profile may be controlled by a filter, such as a neutral density (ND) filter.