Abrading – Precision device or process - or with condition responsive... – By optical sensor
Reexamination Certificate
2000-12-04
2004-07-20
Rose, Robert A. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
By optical sensor
C451S041000
Reexamination Certificate
active
06764379
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally in the field of controlling the process of semiconductor manufacture, and relates to an apparatus and method for in-situ endpoint detection during various processes applied to semiconductor wafers, such as Chemical-Mechanical-Polishing (CW), Chemical Vapor Deposition (CVD), etching, photolithography, and others.
BACKGROUND OF THE INVENTION
The manufacture of semiconductor articles, such as wafers, consists of forming various materials layers and structures of certain different thicknesses. Usually, this process includes deposition and removal of different materials using such techniques as CMP, CVD, etching, photolithography, etc. An important step in these procedures is terminating the process after the desired thickness is reached. For example, when dealing with CMP or etching, this process should be terminated after the layer being etched or polished is removed (e.g., partly removed such that a required remaining thickness of this layer is reached), or before the next, underlying layer is removed. A technique of determination of that process point at which the processing should be stopped is called “endpoint detection”.
The term “processing” used herein signifies at least one of the following: removing an uppermost layer or depositing a layer of a different material onto the wafer's surface. An endpoint detector serves to determine whether the desired thickness of the layer being removed or deposited is reached, aimed at terminating the removing or deposition process. In most cases, the process is terminated in response to a predetermined signal generated by such an end-point detector (or a plurality of such detectors).
CMP is a known process aimed at the planarization of the surface of the uppermost wafer's layer. CMP is basically a mechanical polishing of the wafer's surface using a pad pressed against the wafer, rotating one with respect to the other, all in a chemical liquid environment, which enhances the polishing. Like any semiconductor process step, tight control of the CMP process is required to maintain high yield levels. The polishing removal rate, which is the main process characteristic, is a complex function of different parameters which are partly controlled or understood. These dependencies, when combined with requirements for high uniformity levels and tight process reproducibility and control, dictate intensive thickness measurement procedures, notably in oxide polishing that has no natural end-point. As a result, monitoring systems and methods are a crucial part of the CMP process.
Chemical Vapor Deposition (CVD) and etching are two other major sub-processes in the semiconductor production. The former is aimed at depositing thin films (e.g., oxides, metals) on a semiconductor wafer, whereas the latter is aimed at patterning thin films according to a developed three-dimensional image on the films. In a similar manner to CMP, both CVD and etching are influenced by various parameters, and should therefore be tightly monitored and controlled in order to achieve the set targets of the process. As for the photolithography technique, similar processes, namely, photoresist coating (e.g., by spinning) and photoresist development (i.e., selective removing by etching) take place during the photoresist processing step.
The following are three major techniques used for controlling one of the above processes of semiconductor manufacture, discussed with respect to CMP:
(1) Stand Alone (SA) Systems
SA systems is-are installed outside the production line (‘off-line’) and wafers to be measured by this system are supplied thereto from the production line after the wafer processing is completed. The known SA systems for CMP are OptiProbe 2500, commercially available from ThermaWave, USA, and UV1250, commercially available from KLA-Tencor, USA. SA systems have excellent capability to provide full and accurate information concerning the measurement parameters. However, SA systems suffer from several drawbacks such as lag in response time, large foot-printing, and clean room and additional handling of wafer issues.
(2) In-situ Detectors
These are various sensors (optical, electrical, mechanical, etc.) which are installed in the working area (situ) of the processing tool (e.g., the area between the wafer and the rotating pad of the polisher) and are capable of real-time detecting the process end-point (e.g., motor current) and of continuously detecting the product parameters (e.g., thickness) and both product and process parameters (e.g., removal rate). Such an in-situ end-point detector (EPD) to be used with CMP equipment is disclosed, for example, in U.S. Pat. No. 5,433,651. The end-point detector comprises a window, which enables in-situ viewing of the polishing surface of the workpiece from an underside of the polishing table during polishing. Reflectance measurement means are coupled to the window on the underside of the polishing table. A prescribed change in the in-situ reflectance corresponds to a prescribed condition of the polishing process.
EPD reduces the time required to qualify a process, and shortens conditioning time whenever pads are replaced. EPD are mainly used in processes such as plasma etching. The known EPD tools for CMP are models 2350/2450 Endpoint Controllers, commercially available from Luxtron, Santa Clara, USA, and ISRM, commercially available from Applied Materials, Santa Clara, USA.
Unfortunately, EPD suffers from the following drawbacks: When applying the CMP to dielectric layers (which is a so-called “blind stop” process), additional frequent post-polish measurements on SA systems are needed. This is associated in the following. The EPD sensor is located in the interior of the processing area, and measures average data over a relatively large area comprising different and variable patterns. As a result, it cannot provide information concerning local planarization, and is therefore less informative as compared to an SA tool. The average data generated by the EPD does not allow for mapping the wafer's plan, whereas the latter may be of high importance. Additionally, the interpretation of in-situ sensor data is complex and less accurate, since it is also affected by irregular environment characteristics such as electrical noise, slurry, mechanical movement, etc. The in-situ EPD has low accuracy due to low optical resolution and strong signal dependency on wafer's pattern.
To demonstrate problems arising from the detection of the layer's end of polish with an in-situ EPD, reference is made to
FIGS. 1 and 2
.
FIG. 1
illustrates a common structure, generally designated
1
, of stack layers on a semiconductor wafer W, which structure is to be polished. The structure
1
contains a silicon substrate
2
, a Silicon Nitrate layer (Si
3
N
4
)
4
, and a top Silicon Oxide layer (SiO
2
)
6
.
FIG. 2
illustrates possible signal time changes determined by an EPD sensor during the CMP process applied to the two upper layers
4
and
6
. In this example, the part A presenting a substantially “flat” graph indicative of slow signal variations corresponds to the signal detected from the upper Silicon Oxide layer
6
being polished. When the layer
6
is almost completely removed, a varying signal (part B) is detected, which changes faster with the layer's disappearance. At last, when the Silicon Nitrate layer
4
is being polished, a substantially slow changing signal is observed (part C). The signal boundaries between the parts A and B, and B and C are not sharp and clear. Hence, simple threshold-based signal analysis may cause failures, either because of “early detection” (the layer to be polished is not sufficiently removed) or because of “late detection” which means that the undesirable removal of the lower layer has started.
The main difficulty in obtaining high accuracy in optical EPD is signal dependency on wafer pattern, since EPD spot size includes a lot of features with different layers structure. The effect may be stronger than signal change during polishing. There
Greer Burns & Crain Ltd.
Nova Measuring Instruments Ltd.
Rose Robert A.
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