Photocopying – Projection printing and copying cameras – With developing
Reexamination Certificate
2000-11-14
2003-08-05
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
With developing
C396S611000, C356S237400, C356S237500, C356S630000, C356S632000, C356S388000
Reexamination Certificate
active
06603529
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a monitoring apparatus and method particularly useful in photolithographically processing of substrates, particularly useful in the manufacture of semiconductor devices,
BACKGROUND OF THE INVENTION
The principal process of production of semiconductor devices is photolithography, which includes three main serial steps or operations:
(a) coating a semiconductor wafer with photoresist material (PR);
(b) exposing the PR through a mask with a predetermined pattern in order to produce a latent image of the mask on the PR; and
(c) developing the exposed PR in order to produce the image of the mask on the wafer.
The satisfactory performance of these steps requires a number of measurements and inspection steps in order to closely monitor the process.
Generally speaking, prior to a photolithography process, the wafer is prepared for the deposition of one or more layers. After a photolithography process is completed, the uppermost layer on the wafer is etched. Then, a new layer is deposited in order to begin the aforementioned sequence once again. In this repetitive way, a multi-layer semiconductor wafer structure is produced.
FIG. 1
schematically illustrates a typical set-up of photocluster tools of the photolithography process in a semiconductor fabrication plant (Fab). The photocluster (or link) is composed of two main parts: a phototrack
5
, and an exposure tool
8
. The phototrack includes a coater track
6
having a cassette load station
6
a
, and a developer tack
10
having a cassette unload station
10
a
. Alternatively, both coater and developer functions may be combined and realized in the same stations (not shown). The wafer W is placed in the cassette station
6
a
. From there, the wafer is loaded by a robot
2
to the coater track
6
, where the coating step (a) commences. After step (a), the wafer is transferred by the robot
2
to the exposure toot
8
, where the exposing step (b) is executed. Here, using optical means installed inside the exposure tool, the pattern on the mask is aligned with the structure already on the wafer (registration). Then, the wafer W is exposed to electromagnetic radiation trough the mask. After exposure, robot
2
transfers the wafer to the developer track
10
where the micro-dimensional relief image on the wafer is developed (step (c)). The wafer W is then transferred by robot
2
to the cassette station
10
a
. Steps (a)-(c) also involve several different baking and other auxiliary steps which are not described herein.
As shown in
FIG. 1
, the coater track
6
, the exposure tool
8
, and the developer track
10
, are tightly joined together in order to minimize process variability and any potential risk of contamination during photolithography, which is a super-sensitive process. Some available commercial exposure tools are series (MA-1000, 200, 5500) of Dainippon Screen MFG. Co. Ltd., Kyoto, Japan, PAS-5500 series of ASM Lithography, Tempe, Ariz., series FPA 3000 and 4000 of Canon USA Inc., USA, and Microscan of SVGL, Wilton, Conn. Some available phototracks are series 90s and 200 of SVGT, San-Jose, Calif., Polaris of FSI International, Chaska, Minn., and phototracks D-spin series (60A/80A, 60B, 200) of Dainippon Screen MFG. Co. Ltd., Kyoto, Japan, Falcon of Fairchild Technologies Inc., USA and of Tokyo Electric Laboratories (TEL.), Japan.
It is apparent that in such a complex and delicate production process, various problems, failures or defects, may arise or develop during each step, or from the serial combination of steps. Because of the stringent quality requirements, any problem which is not timely discovered may result in the rejection of a single wafer, or of the entire lot.
In modem photolithography processing, especially using DUV exposure, a wafer cannot be taken out of the photocluster for measurement or inspection before the process is completed and the wafer arrives at the cassette station
10
b
. As a result, any process control based on measuring processed wafers cannot provide ‘real time’ process malfunction detection. Therefore, there is an urgent need for an approach based on integrated monitoring, i.e., a monitoring apparatus physically installed inside or attached to the relevant production unit, dedicated to it, and using its wafer handling system. Such integrated monitoring can provide tight, fast-response and accurate monitoring of each of the steps, as well as complete and integrated process control for the overall semiconductor production process, in general, and for photolithography, in particular.
However, the existing monitoring and control techniques typically utilize ‘stand-alone’ monitoring systems. A ‘stand-alone’ monitoring system is installed outside the production line, and wafers are transferred from the production unit to this system using a separate wafer handling arrangement than that of the production process.
In general, three different monitoring and control processes are performed at the present time during semiconductor fabrication process. These are monitoring of (a) overlay registration, (b) inspection and (c) critical dimension (CD) measurement. A brief description of each of these processes is given below:
(a) Overlay Registration Control
The overlay registration (hereinafter—“overlay”) is a process executed in the exposure tool
8
in which the pattern on the mask is aligned with respect to the pattern features existing already on the uppermost layer on the wafer. The shrinking dimensions of the wafer's features increase the demands on overlay accuracy.
An overlay error or misregistration (hereinafter—“overlay error”) is defined as the relative misalignment of the features produced by two different mask levels. The error is determined by a separate metrology tool from the exposure tool.
FIG. 2A
illustrates a typical overlay error determination site on a wafer. It is composed of two groups of target lines, one on the uppermost feature layer of the wafer
11
and the second is produced on the new PR layer
16
. Target lines
16
are similar but smaller than target lines
11
; thus they can be placed in the center of target lines
11
. Therefore, these overlay targets are called “bars in bars”.
FIG. 2B
is a top view of the same overlay error determination site. The lines of these targets, such as
11
a
and
16
a
are typically of ~2 &mgr;m width, and 10-15 &mgr;m length, respectively.
According to a common method, the overlay error is defined as the relative displacement of the centers of target lines
11
with respect to lines
16
, in both the X- and Y-axes. For example, in
FIG. 2B
the displacements between lines
11
a
and
16
a
,
11
b
and
16
b
are denoted as
14
a
and
14
b
, respectively. Thus, the overlay error in the X-axis is the difference between the lengths of lines
14
a
and
14
b.
FIG. 3
illustrates a common configuration of photocluster tools and a ‘stand-alone’ overlay metrology system composed of a measurement unit and an analysis station. It should be noted that wafers to be examined are taken out of the photolithography process-line, and handled in the measurement tool. This is associated with the following features of the available overlay technology: (i) closed loop control in ‘real time’ is impossible; (ii) not all the wafers as well as all the layers within a wafer are measured for overlay error; (iii) additional process step is needed; and (iv) a ‘stand alone’ tool is needed. It should be noted that it is a common situation in the Fab, especially in advanced production processes, flat during ‘stand alone’ overlay measurement, the processing of the lot is stopped. This break may even take a few hours.
The results of the measurements are sent to the analysis station, and a feedback is returned to the stepper in the photocluster tool.
U.S. Pat. No. 5,438,413 discloses a process and a ‘stand-alone’ apparatus for measuring overlay errors using an interferometric microscope with a large numerical aperture. A series of interference images are at different vertical planes, and artificial images thereo
Browdy and Neimark
Mathews Alan A.
Nova Measuring Instruments Ltd.
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