Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2001-01-29
2003-05-13
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S055000, C355S077000, C430S311000, C430S312000, C430S323000, C250S492200, C250S492220
Reexamination Certificate
active
06563566
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to lithographic printing of features for forming integrated circuit (IC) patterns on a semiconductor chip, and more particularly to a method for selecting and using combinations of illumination source characteristics and diffracting shapes on a reticle mask in order to project and print an image on a semiconductor wafer that substantially matches the shape of the desired IC patterns with minimal distortion.
BACKGROUND OF THE INVENTION
Many methods have been developed to compensate for the image degradation that occurs when the resolution of optical lithography systems approaches the critical dimensions (CD's) of desired lithographic patterns that are used to form devices and integrated circuits (IC's) on a semiconductor chip. Critical dimension (CD) refers to the feature size and spacing between features and feature repeats (pitch) that are required by the design specifications and are critical for the proper functioning of the devices on a chip. When the CD's of a desired IC pattern approach the resolution of a lithographic system (defined as the smallest dimensions that can be reliably printed by the system), image distortions becomes a significant problem. Today the limited resolution of lithography tools poses a key technical challenge in IC manufacture, and this difficulty will increase in the future as critical dimensions become increasingly smaller. In order to make the manufacture of future IC products feasible, lithography tools will be required to achieve adequate image fidelity when the ratio of minimum CD to resolution of the lithographic system is very low.
The resolution &rgr; of a lithographic system can be described by the equation:
ρ
=
k
λ
NA
,
[
1
]
where &rgr; is the minimum feature size that can be lithographically printed, NA (numerical aperture) is a measure of the amount of light that can be collected by the lens, and &lgr; is the wavelength of the source light. This equation expresses the concept that the smallest feature size that can be printed is proportional to the wavelength of the light source, and that the image fidelity is improved as diffracted light is collected by the lens over a wider range of directions. Although a larger NA permits smaller features to be printed, in practice NA is limited by depth-of-focus requirements, by polarization and thin-film effects, and by difficulties in lens design. The so-called k-factor represents aspects of the lithographic process other than wavelength or numerical aperture, such as resist properties or the use of enhanced masks. Typical k-factor values in the prior art range from about 0.7 to 0.4. Because of limitations in reducing wavelength &lgr; or increasing numerical aperture NA, the manufacture of future IC products having very small CD's will require reducing the k-factor, for example, to the range 0.3-0.4 or smaller, in order to improve the resolution of the lithographic processes.
The basic components of a projection lithographic system are illustrated in FIG.
1
. An illumination source
110
provides radiation that illuminates a mask
120
, also known as a reticle; the terms mask and reticle may be used interchangeably. The reticle
120
includes features that act to diffract the illuminating radiation through a lens
140
which projects an image onto an image plane, for example, a semiconductor wafer
150
. The aggregate amount of radiation transmitted from the reticle
120
to the lens
140
may be controlled by a pupil
130
. The illumination source
110
may be capable of controlling various source parameters such as direction and intensity. The wafer
150
typically includes a photoactive material (known as a resist). When the resist is exposed to the projected image, the developed features closely conform to the desired pattern of features required for the desired IC circuit and devices.
The pattern of features on the reticle
120
acts as a diffracting structure analogous to a diffraction grating which transmits radiation patterns that may interfere constructively or destructively. This pattern of constructive and destructive interference can be conveniently described in terms of a Fourier transform in space based on spacing of the features of the diffraction grating (or reticle
120
). The Fourier components of diffracted energy associated with the spatial frequencies of the diffracting structure are known in the art as diffracted orders. For example, the zeroth order is associated with the DC component, but higher orders are related to the wavelength of the illuminating radiation and inversely related to the spacing (known as pitch) between repeating diffracting features. When the pitch of features is smaller, the angle of diffraction is larger, so that higher diffracted orders will be diffracted at angles larger than the numerical aperture of the lens.
A diagram can be constructed in direction space to indicate the diffracted orders that can be collected by a lithographic system that is based on repeating dimensions of a desired pattern. For example, the pattern illustrated in
FIG. 4
can be represented by a unit cell as in FIG.
2
. The pattern has a horizontal repeat dimension
203
, and a staggered pitch indicated by the diagonal repeat dimension
205
(alternatively indicated by the vertical pitch
201
). Assuming that this unit cell is repeated in a diffraction grating and illuminated by an on-axis beam, the diffracted orders can be illustrated in direction space as indicated in FIG.
3
. The position of a diffracted order (points
300
-
326
) is plotted as the projection of the beam diffracted at an angle &thgr; from the on-axis beam. The distance of a non-zero order from the center of the direction space diagram
300
(which represents the position of the zeroth order and is also the direction of the on-axis beam) is plotted as the sine of &thgr; which is the ratio of the wavelength of the illumination divided by the repeat distance. For example, the +2 order represented by the horizontal repeat distance
203
is represented by the point
301
and the −2 order is represented by the point
310
. Similarly, points
305
and
319
represent the +2 and −2 orders based on the vertical repeat distance
201
. Other orders are diffracted both horizontally and vertically, such as order
308
, denoted as the {−1, +1} order. For reference, the numerical aperture (NA)
350
of the lens is also plotted. The only orders collected by the lens are
300
,
301
,
310
,
303
,
308
,
313
, and
312
. Note that the amplitudes of a wave front diffracted by a reticle will be dependent on both the illumination amplitude and the diffractive properties of the mask.
Off-axis illumination has been known in the art as a technique used to improve resolution. Although off-axis illumination causes asymmetry in the projected image, the asymmetry caused by the off-axis illumination can be corrected by illuminating from mirrored directions. This technique is often used in the prior art, for example, by using an annular source configuration.
The intensity contours of light projected by the lens can depart significantly in shape from those of the input mask pattern. Two dimensional (2D) patterns have multiple critical dimensions that must be met, thus exacerbating the problem of achieving image fidelity. Moreover, with all but the simplest shapes, the errors in the different critical dimensions that comprise the printed pattern are unequal, making it impossible to correct the errors with an exposure adjustment. Quite often such unequal dimensional distortions fall into the broad category of “line-shortening”. For example, patterns such as in
FIG. 4
(for example, an isolation level of a dynamic random access memory (DRAM) design) or as in
FIG. 14
(for example, the capacitor level of a DRAM design) are prone to line-shortening. In the pattern of
FIG. 4
, the rectangular features, have width
401
equal to the basic dimensional unit of the cell F. The recta
Bukofsky Scott Josef
Rosenbluth Alan E.
Wong Alfred K. K.
Adams Russell
Brown Khaled
C. Li Todd M.
LandOfFree
System and method for printing semiconductor patterns using... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System and method for printing semiconductor patterns using..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System and method for printing semiconductor patterns using... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3003850