Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
1999-09-03
2002-10-15
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
C356S237500
Reexamination Certificate
active
06466315
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of automatic optical inspection techniques, and relates to a method and system for inspecting reticles or masks in a manner to simulate the operation of a specific photolithography tool in which this reticle is to be used.
BACKGROUND OF THE INVENTION
Photolithography is one of the principal processes in the manufacture of semiconductor devices, and consists of patterning the wafer's surface in accordance with the circuit design of the semiconductor devices to be produced. More specifically, a circuit design to be fabricated on the wafer is first patterned on a mask or reticle (for simplicity, the terms mask and reticle will be used here interchangeably, although in actuality they refer to somewhat different techniques). The wafer is coated with a photoresist material, and is then placed in a photolithography tool to be exposed to light passing through the reticle to produce a latent image of the reticle on the photoresist material. Thereafter, the exposed photoresist material is developed to produce the image of the mask on the wafer. After the completion of the photolithography process, the uppermost layer of the wafer is etched, a new layer is deposited, and the photolithography and etching operations are started again. In this repetitive manner, a multi-layer semiconductor wafer is produced.
As is well known, photolithography tools utilize a lamp or a laser as a light source, and utilize a relatively high numerical aperture (NA) objective to achieve a relatively high resolution. The optics of such tools are generally designed to produce reduction (negative magnification) of the image of the reticle, e.g., ⅕ onto the wafer. Different models use different NA and magnification combination, as designed by the manufacturer of the tool.
It should be appreciated that in order to obtain operating semiconductor devices, the reticle must be defect free. Moreover, in most modern processes, the reticle is used in a repeated manner to create many dies on the wafer. Thus, any defect on the reticle will be repeated multiple times on the wafer and will cause multiple devices to be defective. Therefore, various reticle inspection tools have been developed and are available commercially. One type of such inspection systems, to which this invention pertains, scans the entire reticle using an illumination spot technique to inspect the reticle for defects. Examples of such systems are provided in U.S. Pat. Nos. 4,926,489, 5,838,433, and 5,563,702, and an example is schematically depicted in FIG.
1
.
As shown in
FIG. 1
, a reticle
10
is placed on an x-y stage
20
. A laser
30
produces an illumination beam of a relatively narrow diameter. A scanner
40
, e.g., a rotating mirror or an acousto-optic deflector (AOD), is used to scan the beam in one direction, generally referred to as the “fast scan” direction. The stage
20
is moved in a direction perpendicular to the fast scan direction in a serpentine manner, so that the entire surface of the reticle is scanned. The scanned beam passes through the dichroic mirror
50
and is focused by objective lens
60
onto the reticle. Light transmitted through the reticle
10
is collected by the objective lens
70
and focused onto light sensor
80
, e.g., a photo-multiplier tube (PMT). Reflected light is deflected by the dichroic mirror
50
to be collected by the lens
95
and focused onto the light sensor
90
. Shown by a dotted line is an optional optics and tilted mirror assembly that can be used to obtain an interferometer image of the reticle for inspection of phase shift designs (see, e.g., the cited U.S. Pat. No. '702).
Conceptually, the inspection systems exemplified in
FIG. 1
generate a highly magnified image of the reticle. Each pixel in the image corresponds to a sampled illuminated spot on the reticle, and has a grey level corresponding to the amount of light received by the light sensor. This grey level can be either compared to a corresponding pixel from an adjacent die on the reticle, or binarized and compared. to a database or compared to a gray scale image calculated from the database. When a discrepancy above a designated threshold is encountered, the location is identified as suspected of having a defect.
Recent advancements in photolithography technology have introduced another factor which may cause the latent image on the wafer to be defective. Specifically, the reduction in design rules necessitates various measures to counter changes in the latent image caused by the interaction of the light with the design on the reticle. Such interactions are generally referred to as “optical proximity effects”, and result in, for example, comer rounding, a difference between isolated and semi-isolated or dense patterns, a lack of CD linearity, etc. Whilst not being detected as potential defects. in a particular reticle by the conventional inspection system, these effects could produce real defects on the wafer. On the other hand, these effects should not cause the system to issue an alarm if they will not be transferred as defects onto the wafer. Moreover, there is a need to inspect the countermeasures, such as optical proximity correction OPC and phase shift etching on reticles, and test their design and effectiveness.
Conventionally, in designing and evaluating reticles, especially advanced reticles having OPC and phase shift features, one has to create the reticle, expose a wafer using the reticle, and check that the features of the reticle have been transferred to the wafer according to the design. Any variations in the final features from the intended design necessitate modifying the design, creating a new reticle, and exposing a new wafer. Needless to say, such a process is expensive, tedious, and time consuming. In order to short-cut this process, and to assist in design and evaluation of advanced reticles, IBM has recently developed a microscope called the Aerial Image Measurement System (AIMS).
The AIMS system is disclosed, for example, in European Patent Publication No. 0628806, and in the following articles: Richard A. Ferguson et al. “
Application of an Aerial Image Measurement System to Mask Fabrication and Analysis”, SPIE Vol
. 2087
Photomask Technology and Management
(1993) pp. 131-144, and R. Martino et al. “
Application of the Aerial Image Measurement System
(AIMS™)
to the Analysis of Binary Mask Imaging and Resolution Enhancement Techniques”, SPIE Vol
. 2197 pp. 573-584. The Microscope is available commercially from Carl Zeiss, GmbH of Germany, under the trade name MSM100 (standing for Microlithographhy Simulation Microscope).
Conceptually, rather than obtaining a highly magnified image of the reticle, as is done by inspection systems, the AIMS system emulates a stepper and creates a highly magnified image of the latent image produced by the reticle. Specifically, the operational parameters of illumination and light collection in the AIMS, such as wavelength and NA, can be adjusted by the user to simulate the tool which will be used to expose wafers using the reticle. The illumination is provided in a manner which simulates exposure in a stepper, so that a latent image of the reticle is created. However, rather than placing a wafer at the location of the latent image, a sensor is placed so as to produce an aerial image of the latent image produced by the reticle. Also, rather than providing reduction of the image like a stepper, the AIMS magnifies the latent image to enable easier image acquisition.
The AIMS is basically an engineering tool, which is intended for development and testing of various reticle designs. It is also helpful for checking how OPC and phase shift features would print on the wafer. Additionally, the system can be used to study various defects discoverred by a reticle inspection systems, and test whether those defects would actually print on the wafer. However, the MSM 100 is not intended to be used as a general reticle inspection system, and lacks any of the technology required for rapid inspection of reticles.
U.S.
Karpol Avner
Kenan Boaz
Rosenberger Richard A.
Sughrue Mion Zinn MacPeak & Seas LLP.
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