MultiCPM

Advanced Process Control Increases Yield of Low Open Area Via EtchesEric Bluem, Jean-Philippe Vassilakis - Application Scientists - Thin Film Division

Scaling CMOS technology into the nanotechnology regime allows hundreds of millions of devices tobe fabricated on a single chip. To integrate this many active-elements onto one IC, the on-chip metalinterconnect system must be able to incorporate up to 10 wiring-levels.

The electrical-resistance and parasitic-capacitance associ-ated with such interconnects have become the dominantfactor that limits circuit speed. In an attempt to overcomethis limitation, technology has been developed to replacethe aluminium interconnect-metal with copper (Cu). UsingCu instead of Al offers reduced resistance and enhanceselectro-migration reliability. The Cu material is integrated into the interconnect systemby using a dual-damascene (DD) process. This is necessarybecause Cu does not form a volatile by-product, and is thusdifficult to dry-etch. The DD-process slidesteps this problemby eliminating the need to etch Cu. Instead, recesses aredry-etched into interlevel dielectric (ILD) films, and these re-cesses are then filled with Cu. Any Cu that gets depositedon top of the dielectric is polished back to the surface of theILD with a CMP process.

The VIA etch process is a critical step in the fabrication oflogic devices with critical structure dimensions of 0.18 -0.13 µm with copper backend.The low open area oxide, less than 1%, the highly selectivechemistry, and the accurate detection of oxide/nitride andnitride/copper interfaces are the main challenges to over-come.

The optical emission spectroscopy is the ideal technique foraccurate and reliable in-situ real-time endpoint monitoringof thin film etch processes. This application note clearly il-lustrates the successful monitoring of the VIA etch processusing the HORIBA Jobin Yvon MultiCPM OES system.

Experimental

Processing is performed in a APPLIED MATERIAL P5200 IPSetch cluster tool. The MultiCPM was used to collect the plasma emission dur-ing the VIA etching process. This system is equipped with asturdy optical fiber which can be easily mounted on the sidewindow of the plasma chamber. It uses a 2048 CCD sensordesigned for multi channel spectroscopy, and covers thespectral range from 190 to 850 nm.

By monitoring the emission intensity of selected wave-lengths the MultiCPM tracks the amount of material in theplasma, and hence whether a particular material has beencompletely removed from the etch surface. Thus the endpoint is detected based on the changes in the spectrum ofoptical radiation emitted by the plasma.

The wafer structure is composed of three layers, including asoft mask resist. The VIA process removes the oxide layerdown to reach the Si3N4 interface using a non selectivechemistry.

To implement an in-line OES system for the CA etch proc-ess step three stages were required:

The selection of the relevant wavelengths that carry the information about the transition

The difficult plasma conditions were overcome by usingHORIBA Jobin Yvon's AutoPattern software. It is an ad-vanced Recipe Designer that automatically analyzes the fullspectral data over the complete run and classifies eachwavelength according to the shape of its intensity profile. Itthen automatically identifies the relevant emission lines. Forthe accurate monitoring of the VIA etch process, two spec-tral lines due to CN and SiF emissions were used by the al-gorithm.

0.3 µm

Oxide 410 nm

Nitride 50 nm

Cu

Resist

Starting Point Step 2: SiO2 etch

Stop at the oxi-nitride interface

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Real-time data filtering and the construction of an endpoint indicator

The endpoint recipe can be rapidly constructed to includethe CN and SiF wavelengths identified by the AutoPatternsoftware along with advanced signal arithmetic and filter-ing.

A series of tests to expose the algorithm to the rea-lity of production fluctuations

The ex-situ control of etch depth was performed using ascanning electron microscope (SEM). The SEM pictures

show that the interface is reached only when the endpointdecision is complete.

SEM pictures and data are provided with the courtesy of AL-TIS SEMICONDUCTOR.

Another test was used to validate the endpoint detection.Several wafers with different oxide thickness were proc-essed. The figure shows the excellent correlation betweenthe increase of endpoint detection time and the oxide thick-ness. It clearly illustrates the MULTICPM's powerful detec-tion capabilities of the SiN interface.

Statistics were performed on more than 30 000 wafers, and100 % successful endpoint detection was achieved. It dem-onstrates the robustness of the endpoint algorithm andhence that the optical emission spectroscopy technique is apowerful technique for accurate and reliable control ofetching process termination. Moreover the MultiCPM wasshown to be production ready by its ability to detect wafermismatches as no endpoint was called in the case of blan-ket resist and double etch wafers.

Relevant wavelengths pattern research using Autopattern software

CN and SIF spectral lines selection for endpoint decision

SIF

CN

Endpoint trace showing the process termination

SEM cross section 5s beforethe endpoint decision

SEM cross sectionat the endpoint decision

EPD time versus Oxide Thickness

0100020003000400050006000700080009000

0 20 40 60 80 100EPD time (sec)

Oxi

de T

hick

ness

(A°)

Endpoint algorithm validation

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The end point detection time varied by 15 s is explained bytwo factors:• Previous steps (CMP and Mx process) introduce oxide

thickness variation. A variation of 500 Å is introduced bythe CMP process and then 300 Å more by the Mx pro-cess. A potential dispersion of 1100 Å represents a dis-persion of 11 s on the endpoint detection with an etchrate of 100 Å/sec.

• Variability caused by different open areas (1 to 6%) frombatch to batch

Conclusion

This application note demonstrates the ability of the Multi-CPM - plasma monitor platform to provide accurate androbust endpoint detection in a production environmentwhich is known to generate fluctuations from batch to batchand wafer to wafer. Only one single endpoint recipe was used whatever thetechnology and sequence performed enabling the processto be stopped each time the oxi-nitride interface was detect-ed.

The overall performance of the MultiCPM ensures a reliableprocess control in the VIA etch step and allows a reductionin over etch time and thus an increase yield and processspeed.

Acknowledgments: the authors acknowledge the valuable sup-port of ALTIS Semiconductor, France.

3600 Å

7900 Å

8400 Å

8150 Å

4150 Å

3900 Å

4400 Å

CMP disp (± 250 Å)

Mx etch ( – 4000 Å )

4700 Å

Mx etch dispersion(± 300 Å)

Stability whatever the open areas

Mismatches

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Statistics on production wafers