APPLICATION NOTEIn Situ Process Monitoring of RTP Cluster Tool withFabGuard® and Transpector® CPM
INTRODUCTION
RTP (Rapid Thermal Processing) is a key step in thesalicidation process. The metal silicide layer formed bysalicidation creates a high-conductivity interface
between the gate polysilicon and metal contact that isessential to modern high performance CMOS devices.
Unfortunately, in many cases, a detrimental oxide layer
forms during RTP that remains undetected until finaldevice test. At best, this testing reveals slower speedbin, lower value parts, and at worst yield loss. For thevery common salicidation of Cobalt with a RTP Cluster
Tool, the problem can be solved by the combination of
an INFICON Transpector CPM RGA and INFICON
FabGuard Sensor Integration and Analysis Systemwith a return on investment time on the order of amonth. Once the monitoring system is in place,
additional throughput and yield-enhancing, real-time
analyses can be implemented. The following benefits
result:Ability to perform rapid queue time optimization
study

Real time detection of oxide formation during RTP
step

Significant Yield Improvement (ROI time on theorder of 1 month has been demonstrated)Demonstrated 14% increased tool throughput due
to load lock optimization

PROCESS OVER VIEW
The relevant layers of a modern gate stack areillustrated in Figure 1. A metal film, Cobalt (Co) in thisapplication, covers the gate polysilicon. The film iscapped with either a Ti or TiN capping layer. Rapid
Thermal Processing (RTP) is used to form a metal
silicide layer. The capping layer is subsequently
removed, and the device moves on to furthermetalization. Metal Silicides play a critical transitionrole in the gate stack between polysilicon and metalcontact. Cobalt Silicide is the material of choice indevices at the 130 nm to 65 nm technology nodes.
Although the sub 65 nm technology node has
motivated a transition to Nickel Silicide and lower
temperature RTP tools, the challenges addressed inthis application are anticipated to be present in new
generation processes. The need for easy removal of
the capping layer, combined with the throughput
mismatch of the RTP and capping layer CVD tools
results in a sub-optimal process flow.
Figure 1 Film Stack Prior to RTP Salicidation

GatePolyOxide
N+N+Co Film
TiN Capping LayerA capping layer is used to protect the Co film fromoxygen contamination. In this regard, pure Ti has better
properties than TiN. However, the Ti film is muchharder to strip, and can often result in damage to theunderlying Co film. This problem does not appear to besolved, and most fabs opt for TiN. The ease of removalof the TiN film comes at the cost of higher oxygen
permeability, which becomes problematic whencombined with the throughput mismatch of the CVD
and RTP chambers. With enough time at atmosphericpressure, oxygen will diffuse to the TiN-Co interface
layer, and oxidize during RTP. The resulting highly
resistive oxide layer reduces the overall conductivity of
the gate stack, defeating the purpose of the salicidation
step. If TiN CVD to RTP salicidation queue times islong enough, significant end of line device performance
impact results, to the point of limiting yield. Performing
the two steps in the same cluster can eliminate the1 of 3

The nature of the RTP process makes identification ofexcess queue times impossible prior to end of line
testing. Since oxygen loading is a property of the wafer
and not the process tool, the qualification run gives noinformation about this specific problem. In addition, no
information from this process tool, or from any of thedata normally gathered in prior or subsequent steps
indicates that oxidation has occurred.

APPLICATION DESCRIPTION
The solution to this problem is the combination of an
INFICON Transpector CPM RGA and INFICON
FabGuard Sensor Integration and Analysis Systemused for both process development and real-timeprocess monitoring. Although the RGA does notdirectly measure oxidation, it does directly measure theconcentration of oxygen absorbed by TiN cap layer ofthe wafer being processed. The amount of oxygen isstrongly correlated to queue time and directly
correlated to oxide formation during RTP.
A simple process optimization study can be performed
to identify maximum acceptable queue time as well as
in-process O2 levels. As shown in Figure 2, the longer
the queue time, the higher the O2 released during
processing, and the worse the end of line performance.
The more rigorous approach to the study is to track endof line performance as a function of total O2 during
RTP. In this way, precise sensitivity to TiN O2absorption as a function of device can be determined.
A simpler and far less time and resource consuming
approach is to use a total O2 of roughly 3 or 4 times theshort-queue level as a maximum acceptable level, andselect the queue time based on this O2 level.
Controlling queue time is not sufficient to eliminate theproblem. TiN O2 absorption is not completelycorrelated to queue time. Some percentage of wafers
with acceptable queue times will still have high O2
loading, impacting device performance and yield. A key
advantage of controlling a sensor with FabGuard is theease with which real-time monitoring can beimplemented. It is very straightforward to trigger theRGA to measure total O2 released during the process
step and associate that amount with the specific wafer
being processed. Furthermore, FabGuard allows anumber of fault and alarm configurations, ranging fromnotification to direct tool interdiction. Real-timemonitoring at a minimum allows the wafers with excess
oxygen to be removed from the process flow so that
subsequent steps are not wasted on them. A more
likely scenario is that the entire lot has excess O2
absorption, and that the problem can be caught on
first-wafer. The entire lot can then be diverted andpossibly de-gassed. Existing implementations of thissolution have resulted in return on investment times on
the order of a month through yield loss prevention.
The atmospheric pressure RTP process requires adifferentially pumped RGA such as the Transpector
CPM. The RTP Cluster Tool application offers thefurther advantage that the optimal RGA placement forprocess chamber monitoring also gives visibility into
the loadlock chamber. This can be exploited for
additional throughput improving applications.
Figure 2 Queue Time Impact on O2 Concentration in RTPO  in N  ppm by Queue Time
605040O  in N  ppm

problem, but this approach is highly inefficient.
Throughput of the RTP chamber is on the order of 10x
higher than that of TiN CVD, but the chamber ratio in acluster tool is at most 3:1. To achieve an optimal ratioof chambers, separate tools must be used, and queue
times must be managed.

24 hour Queue Time

0 Queue Time
302010002004006008001000

1200-10Run NumberALTERNATIVES
Without in-situ RGA monitoring, the RTP salicidation
step will be sub-optimal in a number of ways. Othersensors can be tried, but the sensitivity requirements
are daunting. Both FTIR and Oxygen sensors candetect Oxygen, but the extremely small O2
concentrations, on the order of 20 ppm, that result inend of line failures are below the capability of both.
With no Oxygen measurement, no alternative to aIn Situ Process Monitoring of Centura RTP Cobalt Salicidation with FabGuard and Transpector CPM

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In Situ Process Monitoring of Centura RTP Cobalt Salicidation with FabGuard and Transpector CPM

IMPACTS
There are some capital and recurring costs involved in
the solution, but ROI has been demonstrated forspecific applications. In addition, INFICON offers
worldwide application support to address the setup andmaintenance of the new systems.
A positive consequence of the sensor installation isthat several additional throughput and yield enhancingapplications can easily be implemented. These
applications improve ROI by adding further FDC
capabilities without additional costs. Like many otherprocess tools, the RTP cluster tool goes through apump down and purge step prior to processing. This
contamination elimination step is currently performed
open loop, making it sub-optimal for both throughput
and yield. From a throughput standpoint, our data has
shown that default recipe settings waste significant
time targeting too low a base pressure and holding for
too long a time. As seen in Figure 3, the 10 Torr basepressure of the standard recipe can be raised to 30
Torr with no ill effect. All but the most contaminatedwafers reach steady state levels long before the end of

the conventional step. Optimizing the load lock recipe
resulted in a 14% improvement in tool throughput.
Furthermore, when incoming wafers are significantly
contaminated, the default recipe is insufficient toremove the contamination, and a full PM may berequired to recover from the damage. The combinationof wide mass sensitivity of the RGA and control powerof FabGuard makes it straightforward to automaticallyscan for all likely contaminants at the time of pumpdown in order to both optimize the recipe forthroughput and generate an alarm if a contaminatedwafer enters the tool.
Custom applications derived from this technology areavailable from INFICON and adjustment from theoriginal design may be required depending on
specifications and requirements for a particularcustomer application.
Figure 3 Contamination vs Load Lock Base Pressure
O  Contamination by Load Lock Set Point
120

High O  spikes when using 50 Torrload lock pump down setpoint100

No Problem when using 30 Torrinstead of standard 10 Torr setpoint80O  ppm

time-consuming end of line measurement study isavailable to determine acceptable queue times.
Without in-situ monitoring, only queue time can beused as an indicator of absorbed O2. This leads toexcessively short queue times, impacting overallthroughput, and some oxidation-induced end of linefailures, since queue time is not a perfect predictor ofoxide growth.

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RGA: O2 in N2+NH3 ppm at 10 Torr
RGA: O2 in N2+NH3 ppm at 20 Torr
RGA: O2 in N2+NH3 ppm at 50 Torr

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