a plasma process monitor/control system

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 124È133 (1998)

A Plasma Process Monitor/Control System

Joel O. Stevenson,1,* Pamela P. Ward,1 Michael L. Smith1 and Richard J. Markle21 Sandia National Laboratories, Albuquerque, New Mexico, USA2 Advanced Micro Devices, Inc., Austin, Texas, USA

Sandia National Laboratories has developed a system to monitor plasma processes for the control of industrialapplications. The system is designed to act as a fully automated, stand-alone process monitor during printed wiringboard and semiconductor production runs. The monitor routinely performs data collection, analysis, process identi-Ðcation and error detection/correction without the need for human intervention. The monitor can also be used inresearch mode to allow process engineers to gather additional information about plasma processes. Because plasmaprocesses have wide application in thin-Ðlm growth, the monitor could also be used to better understand and controlmyriad thin-Ðlm manufacturing processes. Surface interactions, thin-Ðlm interfaces, growth and stoichiometry arepotential areas of impact.

The plasma process monitor/control system consists of a computer running software developed by SandiaNational Laboratories, a commercially available spectrophotometer equipped with a charge-coupled device camera,an input/output device and a Ðber-optic cable. The tool is designed to be a versatile, multipurpose piece ofequipment allowing automated process veriÐcation and error detection/correction, as well as a research tool. Wehave used the monitor for gauging the “state-of-healthÏ of plasma processes. System 1 has been at Texas Instru-ments in Austin, TX, since February 1996, monitoring the health of printed wiring board plasma desmear andetch-back. System 2 has been at Advanced Micro Devices in Austin, TX, since July 1996, monitoring integratedcircuit multivariate oxide etch. These systems and possible beneÐts derived from in situ monitoring of plasmaprocesses will be discussed. 1998 John Wiley & Sons, Ltd.(

Surf. Interface Anal. Vol. 26, 124È133 (1998)

KEYWORDS: plasma etching/processing ; plasma diagnostics ; optical emission spectroscopy ; semiconductor plasmaprocessing ; thin Ðlm manufacturing processes

INTRODUCTION

Plasma processing is an integral part of manufacturing,not only for the printed wiring board (PWB) industrybut also for the semiconductor industry and others.1,2Plasma treatment has been used throughout industryfor many processes, including surface cleaning, roughen-ing, activation, Ðlm deposition and patterning. Histori-cally, the plasma treatment portions of manyproduction procedures have proved difficult to predictand control.3 Therefore, plasma processes are oftenunder-utilized or only used in less critical applications.The non-uniformity of most plasma processes and thesensitivity to environmental changes make it difficult tomaintain process stability from day to day. Any manu-facturer that relies on plasma as a process step, whetherfor cleaning, etching, surface enhancement, patterning,etc., is vulnerable to process non-uniformity. Plasmaprocesses are susceptible to external factors, such asenvironmental humidity, and internal factors, such aspower, pressure, gas purity, mechanical failures andelectrical failure.

Plasma processes often fail to perform as expected. Asmall deviation in any internal factor can cause largeshifts in plasma performance. Often the cause can betraced back directly to the source upon inspection of

* Correspondence to : J. O. Stevenson, Sandia National Labor-atories, Albuquerque, New Mexico, USA.

the system after failure. Depleted gas cylinder, wrongfeed gas, power supply failure and air leak are sometypical failure modes in which plasma performance maybe a†ected. These occurrences can be easily identiÐedand corrected, but often a load of PWBs or semicon-ductors is lost before the failure is detected. Productrework or further processing of improperly plasmacleaned product is costly. Other failure modes are notas easily identiÐable. At times, a plasma operation willfail for no discernible reason. These failures, althoughinfrequent, are a costly source of lost product. Processstability is absolutely necessary for reproducible results.

Plasma cleaning, etching and desmear processes areroutinely used in the manufacture of PWBs and elec-tronic components. Photoresist removal, plasma etch-back and plasma desmear of drilled thru-holes arerepresentative production processes in a carefully moni-tored and controlled production environment. Cus-tomers, such as the Defense Department, sometimesrequire very secure electrical connections to internalcopper layers. In such applications, the military speciÐ-cation is a minimum polymer etch-back of 0.02 mils,and anything less is not acceptable. However, too muchpolymer removal can result in serious plating problems,such as plating voids, solution entrapment and exposedÐbers.

Commercial customers also require reliable internalconnections. Although not as tightly controlled as com-ponents for military applications, commercial customersgenerally prefer etch-back of polymer to depths of 0.01mils, to guarantee complete desmear of all the exposed

CCC 0142È2421/98/020124È10 $17.50 Received 20 May 1997( 1998 John Wiley & Sons, Ltd. Accepted 16 October 1997

PLASMA PROCESS MONITOR/CONTROL SYSTEM 125

internal copper features. A consistent plasma process isessential in obtaining good electrical connections. Forthis reason, careful attention to detail must be main-tained throughout the production process of plasmacleaning.

To date, plasma process control has occurred usingproduction controls (i.e. PWB handling before andduring plasma treatment, precise plasma chemistriesand process parameters). A failure in the plasmadesmear or plasma etch-back step of production iscostly and time consuming. To diagnose the nature ofthe plasma failure, repair the reactor and recycle thePWBs requires costly personnel hours and results inreduced yield. Often the plasma failure results in defec-tive boards that cannot be recycled and must bereplaced. Replacing product due to plasma failure isextremely costly. Non-uniform plasma Ðeld and processvariation with each cycle require some method bywhich to test the integrity of the components treated.Most plasma users in industry rely on one of twocommon test methods. Many users incorporate bothtechniques into their quality control procedure.

One test method measures the material removed fromone or more weight loss coupons placed in the plasmachamber during the cycle. The coupons are placed inthe areas in which average plasma performance isexpected (i.e. strategic placement throughout the plasmachamber is required). The success or failure of the run isbased upon whether the amount of material removedfalls within some range of acceptable values (based ondata from past plasma performance). The difficulty withthis method is that if post-process microsectioning ofthe coupons reveal some abnormality, often the run andall components within the run must either be scrappedor undergo more rigorous testing before being acceptedor failed. Further testing is costly and time consuming.If the parts are rejected, either based on the weight lossvalue or on the results of the postmortem inspection,enormous cost can be incurred and a delay in pro-duction is likely.

Another disadvantage is the lack of reliable data fromthe weight loss coupons. Plasma Ðelds are extremelysensitive to small variations from cycle to cycle.Chamber cleanliness, chamber temperature, samplenumber, sample placement and relative humidity areonly a few factors that a†ect process performance. Pre-dicting the areas in which to sample plasma per-formance via weight loss coupons may be difficult, if notimpossible. By misplacing coupons, data may indicate afailure when none has occurred. Data may also indicatethat the cycle performance falls within guidelines when,in fact, the parts are substandard.

Another test method is destructive testing of a frac-tion of parts from each production run. With thismethod, a fraction of parts from each lot is removed,sectioned and optically examined for plasma per-formance. Many of the problems experienced with theweight-loss coupon test method are similar to the prob-lems encountered with the destructive testing method. Abatch is accepted or rejected based on post-plasmaresults ; again, this is costly and time consuming. Poorselection of test parts can yield false positive or negativeresults leading to scrapping of acceptable parts oracceptance of substandard products. Because someportion of the parts is sacriÐced for testing, the overall

price per part is increased to cover the expenses of areduced yield.

Both of the techniques discussed above evaluateplasma performance after the fact, i.e. when any damageto product has already occurred. Neither of these tech-niques is very reliable, often yielding false information.An in situ test that could diagnose plasma problems,correct the malfunction or abort the process until theproblem has been Ðxed could reduce the time andexpense of failed plasma etch or desmear. A real-timetechnique to monitor plasma progress in situ could indi-cate process failure as it occurs, before damage to theparts has occurred. This would allow for correction ofthe problem, salvage of the parts and less down time. Bymonitoring the plasma health and progress, decisionscan be made regarding plasma gas mixtures, pressures,powers, etc. that allow for a more uniform systemwithout the need for post-plasma examination. Processanomalies should be detected and corrected before theparts being treated are damaged by inadequate oroverly aggressive plasma treatment. Real-time monitor-ing would allow for near-instantaneous corrections. Asystem that could monitor several areas of the plasmafor performance mapping would also be a useful toolfor plasma users.

In the semiconductor industry, a “stop-etchÏ conditionin plasma etch tools prevents the complete etch of con-tacts and vias in microelectronic devices, resulting indecreased yield and product throughput. One of thegoals of this study is to demonstrate that, in situ, real-time monitoring can be used to predict when stop-etchwill occur in a semiconductor manufacturing environ-ment.

In this paper, optical emission spectroscopy for non-intrusive, in situ process monitoring will be exploredalong with applications of this technique for processcontrol, failure analysis, endpoint determination andstop-etch detection.

BACKGROUND/HISTORY

The lack of in situ sensors in plasma process equipmenthas been identiÐed as a major hurdle facing the semi-conductor industry.4 Numerous researchers are investi-gating sensor-based manufacturing and control ofsemiconductor tools.5 Many methods for plasma moni-toring have been studied, including mass spectroscopy,emission spectroscopy, optical reÑectometry, impedancemonitoring, pressure monitoring, Langmuir probe mon-itoring, Fourier transform infrared (FTIR) spectroscopyand laser-induced Ñuorescence (LIF).6

One method that has been heavily investigated isoptical emission spectroscopy (OES), used for endpointdetermination,6h9 chemical species identiÐcation10h13and general etching performance.14h16 Automatedclosed-loop feedback systems have also been developedusing OES.11,17h22 Numerous analytical techniqueshave been applied to the analysis of OES data.23h34

With commercial equipment (a monochromator orspectrometer), one can observe non-intrusively andidentify the species present in a glow discharge. In acleaning process, the spectral peaks of a contaminantcan be isolated from the cleaning gases. By identifying

( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 124È133 (1998)

126 J. O. STEVENSON ET AL .

the peaks due to the contaminant, the process can bemonitored for an endpoint. This is a relatively straight-forward process. A reference set of spectra may beobtained by recording the emission of the cleaning gaseswithout any sample present and then comparing it tothe emission observed when the contaminated samplesare present. The di†erences observed are due to thepresence of the contaminant in the plasma. When theplasma has consumed the contaminant, these di†erencesno longer exist and an endpoint is determined. If thesamples are themselves consumable, such as polymers,the reference spectra should be accumulated with cleansamples present and then compared to the spectra ofdirty samples. Another advantage to this process is theability to accumulate spectra from known failuremodes, such as incorrect gas mixture, air leakage, etc.,and then to store such spectra as a reference library.With a spectrophotometer linked to a computer, systemmonitoring, diagnostics and automatic self-correctionor shutdown could occur, allowing for a fully automa-ted system. Optical emission spectroscopy has beenused to monitor plasma cleaning processes for manyyears.

Optical monitoring of plasma etching and desmear isnot as easily accomplished. In an etching or desmearprocess, plasma is used either to roughen the existingsurface for enhanced adhesion or to polish a surface toobtain a smooth Ðnish. Because the material beingremoved is part of the sample and not a contaminant,such as grease, oil or photoresist, no easily identiÐableendpoint can be obtained. Spectral peaks due to thematerial being removed will always be present as longas the samples are present. Process monitoring forsystem failures, such as air leaks or wrong gas mixture,can still be performed by accumulating spectra during asuccessful cycle and comparing it to spectra of sub-sequent cycles, but reliable endpoint determination hasbeen thought extremely difficult, if not impossible.

To deÐne an endpoint for a plasma etching ordesmear operation using OES, some aspect of theplasma would have to change with time (more or less ofthe etchant gases would be consumed or more or less ofthe etch product would be present). If these phenomenawere occurring and detectable, a comparison to etchrate with plasma event could be made and some esti-mate of endpoint could be predicted. The difficulty withsuch an approach is that these time-related events rarelyoccur and are not trivial to detect and identify. An end-point indicator that would rely on such events wouldhave to be uniquely derived for each desmear or plasmaetch process. This is a time-intensive process but thepotential advantage of greater process control andproduct reliability has justiÐed the endeavor.

APPROACH

Sandia National Laboratories, in cooperation with theNational Center for Manufacturing Sciences (NCMS),Texas Instruments, SEMATECH and Advanced MicroDevices, has developed a tool to monitor plasma oper-ations. This tool was developed at Sandia NationalLaboratories in response to the initial request of thePWB industry and a subsequent request from the semi-conductor industry for a method of ensuring product

quality. The monitoring tool uses OES to gauge the“state-of-healthÏ of plasma processes. We currently havetwo prototype systems in the Ðeld. System 1 has been atTexas Instruments in Austin, TX, since February, 1996,monitoring the health of PWB plasma desmear andetch-back processes. System 2 has been at AdvancedMicro Devices in Austin, TX, since July 1996, monitor-ing integrated circuit (IC) multivariate oxide etch pro-cesses.

The tool gathers information about the health of theplasma process (obtained by OES) and performs tasksbased on real-time analysis of the information. By uti-lizing a library of spectra, including normal plasmaoperation, common plasma failures and unique di†er-ences correlated with di†erent product types, the tool iscapable of diagnosing system abnormalities. Softwareconstantly compares current plasma state against thelibrary. If any deviation from normal occurs, the soft-ware initiates a series of hierarchical commands. Theexact set of commands is outlined by the customerbased on particular needs and preferences. The resultingtool is a real-time plasma monitor with logical patternrecognition ability.

When monitoring plasma systems, one must take carenot to perturb the plasma Ðeld by introducing a newvariable into the complex plasma environment. Remotemonitoring, external to the plasma reactor, is moredesirable than a probe that resides in the plasma Ðeld.Optical emission spectroscopy is an excellent techniquefor external monitoring because it non-intrusivelyexamines the optical emission of the plasma dischargeand separates the emission into its spectral components.As the plasma condition varies from optimum, opticalindicators can be seen in the visible, UV andinfrared spectra of the glow discharge. By comparingcurrent plasma state against a library of normal andabnormal plasma spectra, information is obtainedabout the plasmaÏs current performance. By becomingfamiliar with normal plasma features, any deviationfrom normal can be identiÐed. Information about theplasma is gained via OES monitoring through an exter-nal view port or window, allowing for real-time plasmadiagnostics with resulting error identiÐcation and cor-rection whilst not disturbing the plasma environment.Because this tool monitors the plasma external to thereactor, it does not perturb the plasma Ðeld or a†ect theperformance of the reactor in any way.

Figure 1 shows a typical use of the Plasma ProcessMonitor/Control System. The plasma is created in acapacitively coupled plasma reactor (2). Two reactorsare shown as part of a plasma unit (1) and controlled byinstruments housed in a rack (8). A plasma unit mayhave multiple plasma reactors, sample handlingequipment, loading/unloading ports, etc. A view of theplasma can be obtained through the viewport (3). Lightfrom the plasma is carried to a spectrophotometer (5)via a Ðber optic (4). The spectrophotometer splits theplasma light into spectral components or wavelengths.The spectrum is imaged onto a charge-coupled device(CCD) camera (6) and read into software on the com-puter (7), which in the automation mode controls thespectrophotometer and camera, performs patternrecognition routines, identiÐes processes, countssamples and lots, diagnoses errors, performs correctiveactions, notiÐes personnel and writes daily activity

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 124È133 (1998) ( 1998 John Wiley & Sons, Ltd.


Figure 1. Schematic of the Plasma Process Monitor/Control System demonstrating typical usage of the system.

reports. In the research mode, the software interactswith a user to perform functions as desired by theresearcher.

The program, in performance of the above-mentionedtasks, communicates with discrete components via aninput/output device (12). This device is capable of com-municating with 64 independent devices. Some devicesthat the unit can control include pressure sensors, Ñowcontrollers, throttle valves and solenoid valves (9). Ifoperator notiÐcation is necessary, the device can soundan audible alarm (14), Ñash a beacon (13) or pageappropriate personnel with alert codes or phone withsynthesized voice messages (11).

The Plasma Process Monitor/Control System isdesigned to operate independently of human inter-action. The unit receives a plasma-on signal from anode (10) within the instrument rack. With this signal,the Plasma Process Monitor/Control System awakensand performs the tasks selected by the user. When theplasma cycle has been completed, the Plasma ProcessMonitor/Control System places itself into standbymode, awaiting the next signal. A recent advance allowsthe monitor to wake (even in the absence of a signal) ifit detects an event of interest.

The Plasma Process Monitor/Control System com-prises a computer running a LabVIEW 4-based soft-ware program, a spectrophotometer equipped with aCCD camera, an input/output control center and aÐber-optic cable. The instrument automatically moni-tors the various plasma recipes and steps needed tocomplete all phases of production. During normal oper-ation, the tool notes process steps as they occur, stores arepresentative set of spectral details for future analysisand counts number of products and lots being treated.If the plasma experiences an abnormality, the tool notesthe abnormality, signals appropriate personnel andattempts to correct the abnormality in situ if so directed.The tool is designed to be a versatile multipurpose pieceof equipment allowing automated process veriÐcation,notation and correction, as well as being a researchtool.

The Plasma Process Monitor/Control Systemreceives information regarding plasma performance viaa Ðber-optic cable placed in contact with a view port onthe plasma reactor. Using OES, details of plasma per-formance, etch rate or deposition rate can be obtained.The spectrophotometer and CCD camera send spectraldetails to software for analysis and action.



The software system is an object-oriented, multi-tasking system that allows a variety of activities to beexecuted concurrently. The software controls the oper-ation of the spectrophotometer and CCD camera,sending instructions to these pieces of equipment forspectral range, resolution, exposure times, etc. Thesystem also instructs the spectrophotometer and CCDcamera to perform self-evaluations allowing for self-calibration for intensity and grating position.

The typical operation mode may follow a speciÐcplan of action (plan of action and error correction pro-tocol are user selectable, with endless possibilities forperformance demands). The system makes decisions andtakes corrective actions based on a protocol establishedby the user. For example, the Plasma Process Monitor/Control System remains on at all times but is dormantuntil the plasma reactor begins a cycle. At that point,the Plasma Process Monitor/Control System receives asignal from a node within the reactor via an electro-optically isolated line. The tool sends commands to thespectrophotometer to initiate and scan a region of thespectra centered about an area containing informationregarding plasma recipe. The spectrophotometer per-forms the scan and returns information to the PlasmaProcess Monitor/Control System. Upon receiving thespectral information from the spectrophotometer andCCD camera, the tool identiÐes the recipe and recipestep and then makes decisions based on informationdetailed in the spectra. The tool may instruct the spec-trophotometer to scan a second, third or nth region foradditional information. If an error is detected, the tool

may take any number of actions based on an error-handling protocol selected by the user.

In an example situation, assume that the PlasmaProcess Monitor/Control System detects a failure in theform of inadequate process gas from gas type B in atwo-gas recipe. The error protocol preferred mayinclude the following actions as shown in Fig. 2. Thetool accomplishes this task by controlling replacementmodules. Modules, such as Ñow controllers, powersupplies, gas cylinders, etc., can be arranged in a bankand may be accessed by the input/output (I/O) device ofthe Plasma Process Monitor/Control System. The I/Odevice can send and receive information from 64 indi-vidual devices from alarms and communication devices(synthesized voice messaging or paging) to powersupplies, pumps and gas cylinders via solenoid valves.The components are addressed using electro-opticallyisolated control lines, which allow instrument controlwithout the possibility of errant signals due to randomelectrical impulses that may be experienced with RFÐelds, power surges, electrical storms, etc.

The Plasma Process Monitor/Control System is ableto diagnose plasma failures and identify plasma recipeand step by referring to a table of known spectra. Ini-tially, normal plasma conditions and some known engi-neered failures, such as air leak or gas failure, can bescanned and recorded on a chamber-by-chamber basisfor future reference. These reference spectra act as aguide for pattern recognition subroutines within thesoftware package. If plasma conditions arise that havenever before been encountered, the Plasma Process

Figure 2. An example error protocol that the Plasma Process Monitor/Control System can implement.



Monitor/Control System can be instructed to abort theprocess, contact appropriate personnel and save thenew condition into the library as “Unknown ConditionXÏ, where X is the number of unknown error conditionslogged to date. As the cause of the unknown conditionbecomes apparent from service diagnostics, theunknown Ðle can be renamed to reÑect the nature of thefailure. From service reports, error correction protocolsfor this new condition can be assigned, making thePlasma Process Monitor/Control System more useful.

Because the software is truly multi-tasking, error con-ditions can be identiÐed immediately after the plasma-on signal has been received. A multiplicity of correctiveactions can be taken soon thereafter, ensuring that theproduct can be saved before damage has occurred. Theunit can abort the process and hold products in stasisuntil corrective actions are complete, when the processcan continue without damaging products. Duringnormal operation, with no error condition present, thetool serves as a process-checking and record-keepingtool. The tool can also be used to identify endpoint forprocess veriÐcation and quality control. The amount ofcontrol the tool has is dependent upon the desires of theuser. The tool may be used for indication only, withalarm notiÐcation or a fully functioning process con-troller that adjusts recipes to maximize etch rate ordeposition.

Not only would this tool be able to save many runsby immediately correcting failures under its control, butit would also reduce reactor down time on aborted runsby performing some of the diagnostics needed to repairthe instrument. Parts being treated in the reactor wouldbe saved by the automated process correction, and inscenarios where the plasma was aborted, parts wouldnot be ruined by errant conditions and could be re-runafter repairs to the reactor. This tool is technicallysuperior to current PWB test mechanisms, whichinclude weight loss coupons and/or destructive testing.Because this method of testing is performed in situ, cor-rective actions can be taken as the failure occurs ratherthan after the cycle is complete. This test method wouldprovide an accurate evaluation of the plasma per-formance and document the quality of the productexiting the plasma chamber.

RESULTS AND DISCUSSION

Printed wiring board (PWB) application

Plasma etch-back and desmear operations are per-formed in a capacitively coupled, or parallel plate,reactor. In a reactor of this type, a series of parallel elec-trodes is used. The electrodes are positioned to formalternating ground potential and r.f.-powered planes.The powered electrodes are at D2500 W at an r.f. of13.56 MHz. The PWBs are placed directly on these elec-trodes for plasma treatment.

For typical plasma etch-back or desmear operations,carbon tetraÑuoride and oxygen Ñow(CF4) (O2)through the chamber at reduced pressures in anapproximate 2 : 1 ratio. The presence of the Ñuorine-containing molecule in the plasma discharge enables theproduction of atomic oxygen, which is the aggressivepolymer attacker in systems of this type. By monitoring

the etch-back rate of a speciÐc polymer while varyingthe percentage of to one can optimize the etchCF4 O2 ,rate for any given organic molecule susceptible tooxygen attack. Depending on reactor geometry, productloading, available power, obtainable pressures andother process-speciÐc parameters, the overall per-formance of any given gas mixture may vary. Foroptimum results, it is suggested that a series of surveyexperiments be conducted, prior to placing any newreactor into service, to identify ideal operating param-eters and limits.

Handling of the product before plasma treatment andduring the plasma cycle is extremely important formaintaining quality and uniformity. Plasma etchingperformance is dramatically a†ected by environmentalfactors and the condition of the PWBs. For example,large variations in environmental humidity can severelyreduce the performance of the plasma etch-back ordesmear operation. The amount of available water inthe system can negatively impact the expected etch rate,leading to inconsistent product from day to day. Like-wise, improper handling can increase surface water con-tamination or large-molecule oil deposits on the PWBprior to plasma treatment. Both types of surface con-tamination are harmful and potentially destructive toPWBs.

Oil residue, such as from Ðngerprints, locally slowsthe etch-back process by presenting a barrier or maskto be overcome by the plasma before actual productetch can occur. To prevent contamination due to oilresidue, caution must be taken to prevent unnecessaryhandling during production. Workers should limitdirect contact with the PWBs during assembly and anyunavoidable contact should be performed with properprotective equipment, such as gloves. Proper storageand handling of the PWBs prior to plasma processingwill help to reduce cross-contamination from externalsources.

Surface water contamination is increased by exposingthe PWBs to humid environments. To limit exposure,PWBs should be stored in dry areas away from high-humidity locations. Also, processing of the PWBsshould be performed within a timely manner withoutunnecessary delays between process steps. Althoughsurface water contamination is unavoidable, properhandling can reduce unnecessary water absorption.

To eliminate the e†ects of all unavoidable water con-tamination, pretreatment before plasma treatment isnecessary. The PWBs must be baked to remove thewater before e†ective, predictable and uniform plasmatreatment can occur. Industry uses two methods to helpdrive o† water.

A prebake in a separate oven before plasma treat-ment at D225 ¡F for a period of 1 h or more can driveo† excess water. However, without vacuum assistance,water comes o† more slowly and drying times mayvary. Another consideration is the transport betweenthe oven and the plasma reactor. When transportthrough air is necessary, some water pick-up isexpected. The boards must be transported as quickly aspossible and should be at or near the bake temperatureduring travel and subsequent loading in the plasmareactor. If the PWBs are transported in a timelyfashion, while still at an elevated temperature, addi-tional water pick-up will be minimized. One consider-



ation for this type of pretreatment is the safety issue ofhandling hot boards between oven unloading andreactor loading. Severe burns can occur if operators arenot familiar with proper handling techniques or appro-priate safety equipment.

A second method for water removal is a vacuumbake in the plasma reactor. The reactor plates can beretroÐtted to accommodate heating elements. By evac-uating the chamber to normal operating pressure andheating the electrodes and PWBs to D180 ¡F, the watercontamination can be removed in 20 min or less. Fur-thermore, because the operation is performed within theplasma reactor, no further water pick-up can occur ifthe reactor is not opened prior to the plasma cycle. Ifthe PWBs are repositioned mid-cycle for more uniformetch, as is standard procedure in many plasma pro-cesses, additional baking may be necessary to eliminateany water contamination that may occur at that time.

Plasma systems are typically non-uniform. Reactorgeometry, number and type of PWBs in the system, andinherent plasma non-uniformities such as the boundarysheath region, all a†ect the uniformity of the plasmaprocessing. The e†ects of this non-uniformity can beminimized by holding load size consistent and byadjusting the load at mid-cycle. By varying the load sizeand type, charged regions build in an unpredictablemanner. By always using the same amount of exposedpolymer (number of holes and thickness of panel) andtype of polymer (i.e. epoxy, polyimide), the e†ects can bepredicted and controlled. To accommodate loads ofvarying size, Ðller boards may need to be used. A Ðllerboard, also referred to as a dummy panel, is a PWB ofsimilar material with a known number of drilled thru-holes. By using Ðller panels along with actual product,consistent load size can be maintained.

Non-uniformity in the plasma can be predicted withthe use of consistent load size. To minimize the e†ects ofplasma non-uniformity, mid-cycle board adjustment isnecessary. A typical plasma desmear and etch-backoperation is D1 h in length, comprised of two 30-mincycles. At the mid-point between the two cycles, theboards are unloaded from the system and reloaded in adi†erent order. Depending on the reactor, the center setof electrodes may demonstrate a faster etch rate thanthose spaced further from the center. At mid-cycle, theoperator may choose to exchange the boards from thetop of the machine with those in the lower half. Theboards may also be replaced in reverse order so that theboard exposed to the center region of the discharge isnow near the edge of the discharge. By repositioning theboards in this fashion, the average etch rate experiencedby all boards is the same, resulting in a more uniformetch throughout the load. During the mid-cycle adjust-ment, Ðller boards are handled like any other product inthe load and also repositioned within the system.

Over the course of the PWB test, the monitor hasdiagnosed serious problems with the plasma process.One error detected and corrected as a result of informa-tion obtained by the plasma monitor was the e†ect ofplacing mismatched dummy boards in the system withproduct.

For more uniform plasma performance, it is standardpractice to Ðll incomplete lots of boards with dummyboards. A dummy board is a board matched to productby size, shape and type of material. A dummy board

may be reused many times before it has been destroyed.A recent change in production material (new supplier)resulted in product being made with a slightly di†erentchemical content than the earlier product. The dummyboards were made with the original materials. Thefacility assumed that the materials were identical.Because of slight di†erences in chemical content, thedummy boards and product boards did not etch atthe same rate, causing asymmetry in the process. Theplasma monitor detected the di†erence in plasma per-formance. New dummy boards were manufactured inresponse to this discovery.

Other problems detected by the monitor includeimpending failures of power supplies and pumpingsystems. The monitor can also detect air leaks and Ñuc-tuations of either of the two process gases used. Theperformance of the monitor for these failures has beentested by engineering tests of these conditions. To date,no actual failures of this nature have been observed.

Endpoint detection of plasma desmear and etch-backwas also conducted. The spectra of various types ofPWBs were monitored with time. The PWBs weretreated in a typical etching plasma at anCF4/O2approximate 2 : 1 ratio. Cycle times were extended up to80 h to observe the peaks of interest. The behaviour ofthe etchant gas as well as etch products was comparedthroughout the run. Postmortem examination of thesamples was performed to ensure that the accumulateddata were for average cycles with expected etch rates.

Careful inspection of the accumulated spectrarevealed a time-related behavior at 325 nm and 333 nm.The peaks observed at these wavelengths appeared tochange intensity relative to each other throughout thecycle in an oscillating fashion (Fig. 3). The period ofthese oscillations is not regular, but appeared todecrease in frequency with time. The peaks are pri-marily due to the etchant gases and are present in thespectra when there is no sample present. However, theirintensity does not change with time. These two peaksonly begin to toggle when samples are being etched inthe chamber. This phenomenon was observed both in

Figure 3. Variation in relative peak height observed in printedwiring board etching using OES. The X-axis is plotted in wave-length (nm) units. The Y-axis is plotted in intensity (counts) units.



the laboratory with a scaled-down plasma apparatusand in the production facility on an industrial plasmatreatment system. E†orts are in progress to correlateetch rate with peak oscillation by comparing real-timespectra with test coupons.

The most intriguing question is by what mechanismcould an oscillatory behavior of spectral peaks occurwhen etch gas and etch product should be a constant?One explanation might be that the plasma is encounter-ing some mechanical barrier so that, from time to time,etch rate is slowed until the barrier is overcome. Thebarrier in this scenario may be the glass-Ðber micro-weave fabric used within the interlayers of the PWBs.The PWBs are constructed of alternating layers ofcopper sheet and epoxy-Ðlled glass fabric (Fig. 4). Thevoids between the warp and weave of the fabric formrelatively large pockets of epoxy, which are easilyremoved by the plasma. As these pockets of epoxybecome depleted, the plasma is primarily exposed to theglass Ðbers, which do not etch and therefore become abarrier. These barriers are not impenetrable to theplasma. Even though the large pocket of epoxy hasbeen removed, the adjacent pockets of epoxy are nowexposed by a small gap in the glass fabric caused by oneÐber being placed across another. The plasma etchingprocess has been slowed but not halted. As the plasmaopens the gap between Ðbers by etching away the epoxythat seals it, again a large pocket of epoxy is revealedand the etch process continues at a faster rate. Witheach barrier the plasma overcomes, the path throughwhich the plasma must travel becomes longer and moregeometrically complex, slowing the processes with time.This may explain why the period of oscillation seems todecrease in frequency with time.

Integrated circuit (IC) application

Plasma etch tools, such as the Applied Materials(AMAT) Centura High Density Plasma (HDP) 5300Dielectric Etcher, exhibit a phenomenon known as“stop-etchÏ, which results in an incomplete etch of con-tacts and vias in microelectronic devices manufacturedin IC fabrication facilities. This results in decreases inboth yield and product throughput, both of which havenegative impact on productivity. Using OES, we havedeveloped a plasma process monitor and controller fordetermining the “state-of-healthÏ of IC plasma processes.By extending our initial tool for monitoring PWBplasma desmear and etch-back processes, we have beensuccessful in monitoring IC multivariate oxide etch pro-cesses. We have used OES to acquire spectral informa-

Figure 4. Multilayered printed wiring board.

tion during typical IC plasma etching and cleaningcycles during a beta test where an automated plasmaprocess monitor is collecting data at the AdvancedMicro Devices facility in Austin, TX.

The OES sensor has worked extremely well duringthe Ðrst phase of the beta test where spectra were col-lected over the 250È1000 nm range during all phases ofthe oxide etch process. The calibration and long-termstability of the OES sensor were found to be acceptable.The OES spectra collected from two AMAT 5300 High-Density Oxide Etchers during the polymer removalportion of the etch process show clear di†erences inseveral regions, demonstrating that the relative age ofreactors may be determined from OES analysis (Fig. 5).This type of analysis can provide useful information inthe design of reactor cleaning schedules.

Preliminary results also show clear di†erences in thespectra for processes that were identiÐed by AdvancedMicro Devices as being di†erent from one another (Fig.6). Chemometric analysis is currently being applied tothe data for improved sensitivity compared to single-peak and peak ratio analysis. The OES data are alsobeing correlated with XSEM, etch rate, contact resist-ance and microprocessor functionality results. Thesepreliminary results are being explored in the secondphase of the beta test, where the automated plasmaprocess monitor is being augmented to collect datafrom eight reactors at the Advanced Micro Devicesfacility in Austin, TX. The Plasma Process Monitor/Control System is being reconÐgured to automaticallyswitch from one reactor to another, thereby collectingOES data from the full suite of etch tools. This studywill provide information and insight on chamber-to-chamber variations within the etch bay. We will alsocharacterize these OES spectra for cycle-to-cycle varia-tions as indicators of stop-etch condition.

When the tool is Ðnalized, predictions of impendingstop-etch conditions or other malfunctions will be made

Figure 5. OES spectra recorded during polymer removal portionof semiconductor oxide etch process. The top graph is from aclean tool and the bottom graph is from a dirty tool. Note the dif-ferences in peaks at 913, 916, and 928 nm. The Y-axis is plotted inintensity (counts) units.



Figure 6. Representative spectra from semiconductor oxide etchprocess showing three of the many process steps that can be dis-tinguished using OES.

before product wafers are damaged. The tool could ulti-mately notify key personnel, take corrective action, etc.,as prescribed by the customer. The monitor continuesto perform wafer and lot counting, recipe and recipestep identiÐcation, as well as general process monitor-ing. The proprietary nature of the experiments beingconducted and the data being collected at this facilitypreclude a detailed discussion at this time.

SUMMARY AND CONCLUSIONS

Plasma processes for etching and desmear of PWB andelectronic components are critical fabrication steps thatare difficult to predict and control. To ensure productexcellence, care must be taken to optimize plasma pro-cesses and to eliminate sources of inconsistent or non-uniform plasma results. In the case of PWBs, reactorperformance can be adversely a†ected by environmentalfactors and by board condition. Board handling priorto plasma processing and during mid-cycle boardadjustment must be done with care and consistency.

Water absorption is a serious problem and must beeliminated. Either prebake in a separate oven orvacuum bake in the plasma reactor can reduce theamount of water in the system. It is suggested thatvacuum bake in the reactor is the better method foreliminating water because transport between oven andreactor can lead to recontamination. Currently, qualitycontrol is maintained through proper processing proto-col and post-process testing. The accepted method oftesting is performed using weight loss coupons andpost-process destructive testing. The unreliability ofsuch testing has led to the development of a new toolusing OES to monitor plasma performance in situ andcorrect problems as they occur and before the producthas been damaged.

The Plasma Process Monitor/Control System devel-oped by Sandia National Laboratories uses OES todetect real-time plasma failures. The technology utilizesa commercially available spectrophotometer andcamera to view the plasma. Error detection and signal-ing is performed by software developed at SandiaNational Laboratories. In an industrial process, theplasma system technology can compare normal andactual operation and note any deviations. If any devi-ation from normal occurs, the software initiates a seriesof hierarchical commands. The exact set of commandsare outlined by the customer based on particular needsand preferences. An important capability of this real-time plasma monitor is that it can allow control ofsupport systems known to inÑuence plasma behavior.The resulting tool is a real-time plasma monitor withlogical pattern recognition ability. The system isdesigned to act as a stand-alone process monitor or as aresearch tool for investigating plasma performance.

The beneÐts of identifying the end of a plasmaprocess, prescribing the amount of material to etch-back or identifying and correcting failures are apparent.In this competitive age, industry needs to work fasterand more efficiently. Post-process testing to ensure thata part is acceptable is time consuming and costly. Insitu testing for process integrity and immediate errorcorrection is preferable. Endpoint determination ofcleaning processes and known failure analysis are rela-tively easy to perform and require little to implement.Identifying spectral clues for endpoints of processessuch as plasma etching or desmear is more difficult. Thepotential payo†, however, is enormous, with fewer failedparts and greater process reliability. It is promising thatof the several types of products examined to date, allhave shown some time-related behavior and other dis-tinguishing characteristics.

Acknowledgements

The authors wish to thank Mike Kelly and Terry Guilinger for theircritical review of the manuscript. The authors also acknowledge RickSmedley of Texas Instruments for technical insight into PWB funda-mentals. Technical comments from the NCMS PWB Materials Teamwere also valuable. The authors acknowledge Michael Gatto, ChrisNauert, and Patrick Traister for technical assistance at AdvancedMicro Devices. This work was performed in collaboration with theNational Center for Manufacturing Sciences (NCMS) under CRADACR91-1030B and SEMATECH under CRADA SC92-1082 at SandiaNational Laboratories, supported by the US Department of Energyunder contract DE-AC04-94AL85000. Sandia National Laboratoriesis a multiprogram laboratory operated by Sandia Corporation, aLockheed Martin Company, for the US Department of Energy.



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a plasma process monitor/control system

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