Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
1999-07-19
2001-06-19
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
Reexamination Certificate
active
06248485
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to image transfer processes and systems and, more particularly, to a method and controller for controlling a process and system for patterning a feature in a photoresist on a semiconductor wafer.
BACKGROUND OF THE INVENTION
Features defined in a semiconductor may be characterized by a critical dimension CD (i.e., a target critical dimension), which is typically specified by a chip designer. A target critical dimension specification provides upper and lower limits of the critical dimension. The features formed in the photoresist (i.e., an actual dimension) must be within the target critical dimension specification.
Unfortunately, the various systems and components used in an image transfer process to form features in a photoresist on a semiconductor wafer provide a multiplicity of error sources across an image field. Further, an error at one point in the image field may be different than an error at another point in the image field.
For example, the image transfer system, which includes an illumination source and a lens, will introduce errors into the image transfer process. One source of errors is the illumination source, which may not provide a uniform output intensity, thus causing the light energy impinging on the photoresist surface to vary. The local degree of partial coherence, which is difficult to control, is another source of error in the image transfer process. Lens aberrations provide yet another source or error. Still another source of error is the product reticle or mask, which provides the feature pattern for the photoresist. Due to manufacturing device limitations, dimensions of the features defined on the reticle deviate from target dimensions. Many image transfer systems use reduction optics with a magnification ratio, M, such that the pitch (P) of reticle features is imaged into the photoresist with a pitch of P/M. The effective width (W) of a reticle feature is W/M. For purposes of this disclosure, M=1, but the present invention is not limited to M=1. Consequently, the actual dimensions of features formed in the photoresist will likely deviate from the target critical dimension and may fall outside the limits of the target critical dimension specification.
Commonly, reticles include lithographic control features (LCFs) which are used to determine the amount of deviation between target and actual dimensions. When features are formed in the photoresist, a resulting LCF formed in the photoresist may be measured to determine a deviation between its actual dimension and the target critical dimension. The deviation information is useful to determine if that resulting LCF feature has an actual dimension that is within the limits defined by the target critical dimension specification. Unfortunately, the actual dimension of that resulting LCF does not directly correlate with the actual dimensions of other features defined by the reticle pattern and formed in the photoresist. This occurs because the errors may vary across the image field. Thus, measuring the dimensional deviation of the LCF does not necessarily provide useful information on the actual dimensional deviation for non-LCF features and whether they fall within the specification limits. Clearly, the information obtained via the LCF critical dimension deviation is generally inadequate to control or adjust downstream processing steps and devices when significant reticle errors and image transfer errors are present—as they always are.
There thus exists a need in the art for a method of controlling the critical dimension of features patterned in a photoresist on a semiconductor wafer that overcomes the above-described shortcomings of the prior art.
SUMMARY OF THE INVENTION
The present invention is directed to a method and controller for controlling a process and system for patterning a feature in a photoresist on a semiconductor wafer. The present invention characterizes various components (both individually and collectively) of an image transfer system, including the illumination source, lens and product reticle, with regard to errors introduced into the image transfer process by these components. The collective error data or information provided in accordance with the present invention may be communicated to the image transfer system to control the image transfer system and the image transfer process.
The present invention is directed to a method of controlling an image transfer process for patterning a feature in a photoresist on a semiconductor wafer using an image transfer system having a plurality of image transfer components, at least one of the image transfer components introducing a critical dimension (CD) error in the image transfer process. The method preferably comprises quantifying the error introduced in the image transfer process by the image transfer system and adjusting the image transfer system to compensate for the error.
In a preferred embodiment, the present invention comprises a method of controlling a critical dimension of a feature patterned in a photoresist disposed on a semiconductor wafer. The feature being patterned has a target critical dimension and a target critical dimension specification (i.e., permissible line width deviation). The feature is patterned using an image transfer system that includes an illumination source, a lens, and a product reticle having a plurality of features defined therein and forming a reticle feature pattern. The reticle is disposed in the image transfer system, preferably between the lens and illumination source which directs light energy through the product reticle and lens to define the reticle feature pattern in the photoresist. The method comprises forming a product reticle error matrix &Dgr;(x,y) of discrete error values and forming an image transfer system error matrix &dgr;(x,y) of discrete error values. The present invention then derives an effective corrected reticle error matrix &Dgr;c(x,y) from the combination of the product reticle error matrix &Dgr;(x,y) and the image transfer system error matrix &dgr;(x,y). An effective corrected reticle size (ECRS) for the product reticle is defined as the sum of the target critical dimension (CD
Target
) and &Dgr;c(x,y). A maximum error value and a minimum error value are selected from the effective corrected reticle error matrix. To account for the fact that the discrete points of the matrices (e.g., product reticle error matrix and image transfer system matrix) may or may not be conjugate with each other (in obvious notation—CD
Target
+(&Dgr;c(max)+&Dgr;c(min))/2), a computer may be used to construct an error profile or surface for each of the reticle and image transfer system from the discrete error points in the respective error matrices. Similarly, a computer may be used to combine the error profiles to form an effective corrected reticle error profile. The present invention may then control the image transfer system, namely the exposure dose (i.e., duration and intensity), to insure that the maximum and minimum error values are within the target critical dimension specification. The exposure (E) that will place the minimum and maximum error values symmetrically within the upper and lower critical specification limits is defined by the equation:
ECD
Target
+(
dE/dCD
)((&Dgr;
c
(max)+&Dgr;
c
(min))/2).
At this exposure, the expected critical dimension of a lithographic control feature (LCF) located at an arbitrary point (x,y) in the image field is:
CD
Target
+&Dgr;c
(
x,y
)+((&Dgr;
c
(max)+&Dgr;
c
(min))/2).
The present invention thus provides maximum exposure latitude for keeping all actual dimensions within the specification limits.
The present invention is also directed to a controller for an image transfer system having a plurality of image transfer components. The image transfer system transfers a feature defined in a product reticle into a photoresist disposed on a semiconductor wafer in an image transfer process. The feature defined by the product reticle has a target critical dimension an
Lucent Technologies - Inc.
Stroock & Stroock & Lavan LLP
Young Christopher G.
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