endpoint detection by acoustic emissions in chemical–mechanical polishing of metal overlay

306 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 20, NO. 3, AUGUST 2007

In Situ Endpoint Detection by Acoustic Emissions inChemical–Mechanical Polishing of Metal Overlay

Hong Hocheng and Yun-Liang Huang

Abstract—Chemical–mechanical polishing (CMP) has been rec-ognized indispensable to achieve the global planarity in removalof metal overlay across the wafer, when the number of intercon-necting metal layers and the size of wafer increase and the line rulereduces to nano scale. CMP has to stop at the endpoint when theoverlay metal has been removed, or dishing will occur which af-fects the subsequent lithography in IC manufacturing. An effectivein situ endpoint monitoring method essentially improves the yieldrate and throughput of CMP. One notices the coefficient of frictionbetween the pad and dielectric layer is distinguishably lower thanthe one between the pad and the copper overlay, based on that anendpoint monitoring method using acoustic emissions during thechemical mechanical polishing is feasible. The proposed methodis tested in the experiment, and the comparison with the previousthermal monitoring technique shows consistent results.

Index Terms—Acoustic emission, chemical–mechanical pol-ishing (CMP), copper overlay, endpoint detection, pad, thermalmonitoring.

I. INTRODUCTION

CHEMICAL–MECHANICAL polishing (CMP) hasemerged recently as an indispensable processing tech-

nique for planarization in submicron multilevel very large-scaleintegration (VLSI). Table I shows the number of interconnectingmetal layers and the size of wafer increase constantly. With theshrinking device scale and higher performance requirement ofintegrated circuits, the utilization of the advanced photolithog-raphy is essential. The increasing number of interconnectionlayers requires a highly planar surface on every layer to ensurethat the accumulated nonplanarity of the layers is less than thedepth of focus (DOF) of the lithographic stepper. The copperwiring was introduced to the micro processor units (MPUs)where the performance had been limited by the conductivityof the aluminum alloy. Copper possesses lower resistance andhigher lifetime against electron migration. Copper has replacedaluminum as the mainstream metallization for ICs.

In the metal CMP process, the previously deposited metallayer is removed, leaving the metal line and the metal in thecontact vias or plugs. Fig. 1 shows a schematic diagram of the

Manuscript received February 18, 2005. This work was supported by NationalScience Council, Taiwan, R.O.C., under Contract NSC91-2212-E007-048.

H. Hocheng is with the Department of Power Mechanical Engineering, Na-tional Tsing Hua University, Hsinchu, Taiwan, R.O.C. (e-mail: [email protected]).

Y.-L. Huangwas with the Department of Power Mechanical Engineering, Na-tional Tsing Hua University, Hsinchu, Taiwan, R.O.C. He is now with AU Op-tronics Corporation.

Color versions of some of the figures in this paper are available online athttp://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TSM.2007.901406

TABLE IROADMAP BY SEMICONDUCTOR INDUSTRY ASSOCIATION

Fig. 1. Schematic diagrams of planarized layers in integrated circuits.

planarized layers in integrated circuits. An evenly polished sur-face produces the good metal step coverage and provides a fieldwithin the lithography depth of focus, that contact vias and metalwires can be well patterned [1]. The accumulated variation infilm thickness will impair the process. CMP has become a keyprocess for the Cu interconnections, low- materials, and dualdamascene process.

During a CMP process, as shown in Fig. 2, the wafer is heldby a rotating carrier that brings the wafer against a polishingpad. The pad is mostly in circular motion. A down force to thewafer and a relative velocity between the wafer and the pad willprovide the desired removal rate. The slurry made of nano-sizedabrasive particles suspended in a chemical solution is dispensedon the pad. In order to ensure the wafers are properly planarizedand to control the constant yield of CMP, the online monitoringof the process endpoint is needed.

Since the wafer surface is in full contact with the polishingpad, detecting its endpoint is a challenge. There are patentedmethods to answer the need for in situ monitoring and end-point detection. The reported working principles can be cate-gorized into a list of seven: optical, electrical, acoustical/dy-namic, thermal, frictional, chemical/electrochemical methods,and others [2]. The optical method is among the most often em-ployed methods in monitoring CMP processes. For a patternedwafer, however, the existence of the metal lines often interfereswith the optical measurement. Besides, the number of points

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HOCHENG AND HUANG: In Situ ENDPOINT DETECTION BY ACOUSTIC EMISSIONS IN CHEMICAL–MECHANICAL POLISHING 307

Fig. 2. Schematic diagrams of CMP equipment.

on the wafer of the measurement may also affect the correct-ness of the information collected from the wafer. The electricalmethods mostly use the embedded electrodes in the pad or car-rier to measure the changes in the electrical conductivity, ca-pacity, impedance, or resistance during CMP processes to de-tect the process endpoint. These installations require a modifiedcarrier and platen to accommodate the electrodes and a com-plex wiring route, which seriously limit the acceptance in prac-tice. As for the acoustic emissions (AE), the merit is that theinherent high-frequency feature can keep the acquired signalsaway from the low-frequency noise. The temperature on pad hasbeen used to monitor metal CMP process. Quite a few CMP ma-chines are equipped with a platen temperature controller or a padtemperature-measuring module to ensure stable chemical reac-tion during polishing. It can perform the temperature measure-ment at a distance from the pad without attaching sensors withthe entangled wires. The thermal method is limited for metalCMP due to the low sensitivity of the temperature change ofthe oxide layer on wafer. The frictional methods measure thechange of the torque, the motor current, or the motor voltageapplied to the carrier or the platen to monitor the endpoint ofremoval of the metal layer. However, the acquired signals oftencontain considerable noise, or the signal is too weak, that limitsits applications. The chemical methods mainly measure the par-ticle size, the presence of certain vapor, the electrochemical po-tential, zeta potential, or conductivity to detect the endpoint. Thedisadvantage is the probes have to contact the moving wafer.The response time is also a concern.

Many of these methods require the rearrangement of the ma-chine structure and wires, some can only be implemented oncertain types of machines. Besides, each monitoring method hasits advantages and disadvantages; a multisensor system com-pensating the individual drawbacks is considered a good ap-proach to the accurate in situ endpoint detection. In fact, someCMP machines now are equipped with optical as well as thermalor frictional sensors. Hence, the exploration of various moni-toring techniques is essential for the advancement toward accu-rate CMP endpoint detection. The purpose of the current studyis to introduce the thermal and acoustic emissions as monitoringsignals for CMP endpoint. The result of this study is expectedto be feasible for an effective multisensor monitoring system ondifferent CMP machines without considerable machine modifi-cations.

II. PRINCIPLES OF ENDPOINT DETECTION

A. Acoustic Monitoring

Acoustic emission has been used for tool condition mon-itoring since the late 1970s [3]–[6]. In metal cutting, thecommon AE sources are attributed to plastic deformation inthe shear zone and the tool/chip interface, rubbing of the toolon the tool/chip interface and the machined workpiece surface,chip breakage and entanglement, and chipping and breakageof cutting tool [7], [8]. AE can also be used to investigate theabrasive process [9]. The frequency spectrum and the signalmagnitude are obtained from acoustic sensors attached to thepolishing equipment for further analysis [10]–[16], [2]. Tang etal. used AE sensors to monitor a dielectric CMP process. Theroot-mean square (RMS) signals were found closely relatedto the average material removal rate. Microscratching duringCMP can be also detected [11], [12]. The results are encour-aging for the current research on the metal CMP process, whichuses different acoustic emissions from polishing the metal anddielectric layers for endpoint detection.

One notices the coefficient of friction between the pad anddielectric layer, 0.35, is distinguishably lower than that betweenthe pad and the copper overlay, 0.7 [17], [18]. Secondly, themetal layer on the wafer surface is scratched by the abrasiveparticles in the slurry giving out acoustic emission signals.When the upper copper layer is completely removed, namelythe process endpoint, as shown in Fig. 3, the slurry particlesstart to scratch the dielectric material beneath, which willsend out different acoustic emission signals. Thus, the emittedacoustic signals during CMP of metal overlay can be used todifferentiate the borderline.

B. Thermal Monitoring

Most CMP machines are equipped with a pad temperaturesensor to monitor the stability of the chemical reaction duringpolishing. The pad temperature has also been used to monitorthe endpoint of a metal CMP process with little analytical ap-proach except a kinematic view point [19]. During a metal CMPprocess, the excess metal layer on dielectric film will be re-moved. At the beginning of the process, the area of the metalfilm being polished by the pad equals the wafer surface area.When the area of the metal film is reduced to that of metalvia, the endpoint is reached. Meanwhile, the polish arm exerts adownward pressure onto the wafer. In association with the coef-ficients of friction between the pad and the metal layer and thedielectric layer, the frictional forces are produced. Multiplied bythe polishing speed, the kinematic polishing power is requiredfor the CMP process. The kinematic power is consumed for pol-ishing work and eventually becomes measurable thermal emis-sions. A correlation between the kinematic parameters and thepad temperature with the derived monitoring technique has beenproposed [19]–[21]. One can see that the pad temperature rise isdetermined by the down force, coefficients of friction, polishingspeed, and the area of metal and dielectric layers. During theprocess, the down force and the kinematic parameters (such asthe platen speed) are constant, while the area of the metal layer(or the dielectric layer) and the associated coefficients of frictionvary. At the beginning of the process, the pad polishes the metal


Fig. 3. Schematic of endpoint in metal CMP.

TABLE IIPOLISHING PROCESS CONDITIONS

layer only, as shown in Fig. 3, until the endpoint of the CMPprocess when the pad polishes the dielectric layer with a rela-tively small portion of metal via (less than 1% of the whole di-electric area). As mentioned, the coefficient of friction betweenthe pad and dielectric layer is distinguishably lower than thatbetween the pad and the metal layer; therefore, the pad temper-ature increases more mildly than polishing the metal layer. Inuse of a mathematic regression applied to the measured temper-ature rise on the pad, the transition from polishing the copperlayer to the dielectric layer in the process can be detected. Thefirst derivative of the thermal emissions keeps constant at thebeginning phase when the metal overlay is being polished. Itdecreases in the transition when metal layer and dielectric layerare both being polished simultaneously across the wafer and be-comes constant again when the dielectric layer is being polishedeventually. The polishing endpoint is thereby identified.

The following assumptions were made for the thermalmonitoring method: 1) the amount of heat produced from theexothermic chemical reaction was insignificant compared tothe heat generated by mechanical friction, since the amount ofpolished copper material is very small; 2) the wafer was in fullcontact with the pad during the process; and 3) the pressureexerted by the carrier and the rotational rates of both platen andcarrier was constant through the process.

III. EXPERIMENTAL SETUP

The CMP machine in the experiment is an IPEC 372 M waferpolisher carrying a 150 mm wafer with 1 m of oxide dielec-tric layer and 1500- copper overlay film. The polishing con-sumables include a grooved pad, the slurry abrasive of 0.1 MHNO solution, and Al O particles of 0.1 to 0.3 m in sizeand weight percentage of 3%–5%. The down pressure and thebackside pressure were set 4.0 and 0.2 psi, respectively. Theslurry flow rate was 100 ml/min. The polishing conditions areshown in Table II.

The AE sensor is required to be placed as close to the acousticsource, i.e., the polished wafer, as possible. In the current study,a Physical Acoustics Corporation (PAC) wide-band AE sensorS9208 is placed at the backside of the carrier in a hole drilledon the plastic cover of the carrier assembly, thus the AE sensorcan be mounted on the metal part of the carrier. A slip ring isspecially designed, as shown in Fig. 4, and put in the carrier as-sembly so that the tongues inside the slip ring transfer the AE

Fig. 4. Slip ring.

signal from the inner ring on rotating carrier to the stationaryouter ring. The inner and outer rings are kept connected all thetime. The inherent high-frequency feature of AE signals canseparate with the low-frequency noise. The AE noise caused bythe friction of the slip rings is about 50–80 kHz, which is wellbelow the higher-than-200-kHz signals produced in polishingwafer. The O-ring between the plastic cover and the metal partof the carrier can further isolate the undesired noise. The AE sig-nals acquired by the sensor are transferred to the 49-dB pream-plifier with a built-in bandpass filter of 95 kHz to 1 MHz. Thesampling rate is five mega samples per second. A threshold levelwas set according to an online pretest to cancel the backgroundnoise and to serve as a reference for the waveform properties.Two preliminary calibration tests verify that the AE signals aresent from the wafer surface and immediately correctly receivedby the AE sensor at the backside of the carrier. The acousticemissions do not transfer via the pad and slurry.

In the experiment of monitoring using thermal emissions, theAGEMA Thermovision® 900 System is installed by the waferpolisher to measure the pad temperature during CMP processes.This image system captures the two-dimensional thermal emis-sions on the polishing pad with the temperature resolution of 0.1K and 272 136 pixels in each image. The measured thermalimage is calibrated before each experiment to ensure the coher-ence of the measurement [19], [21].

IV. EXPERIMENTAL RESULTS AND DISCUSSIONS

The energy index and RMS value of the acoustic emissionsare used. They are calculated as in the following:

Energy Index

RMS

where is the integration time, is the electrical resistance ofthe measuring circuit, and is the level of measured acousticsignal. The calculated AE energy is proportional to the true AE


Fig. 5. Energy index of acoustic emissions during copper CMP. (a) Platen speed = 70 r/min, carrier speed = 50 r/min. (b) Platen speed = 70 r/min, carrierspeed = 30 r/min. (c) Platen speed = 50 r/min, carrier speed = 50 r/min. (d) Platen speed = 50 r/min, carrier speed = 30 r/min.

energy in the event [22]. It is later normalized in the experi-mental analysis.

Fig. 4 shows the energy index versus the polishing time. Themetal film is first reacted with the chemical in the slurry, fol-lowed by the removal of copper by the slurry abrasives sendingout significant elastic waves. It took a few seconds to pick upthe preset down force and backside pressure at the beginning.When the metal film is fully removed, the chemical in the slurrydoes not react with the exposed dielectric oxide layer. The par-ticles in the slurry scratching the surface remove little of theamount of the hard oxide material, and the AE signals decreasecorrespondingly to a lower level. At this moment, the processendpoint is called. The polished wafer is unloaded and exam-ined for the removal of the copper overlay.

As indicated in Fig. 5, the energy index can identify theendpoint at varying polishing conditions of rotational ratesof platen and carrier. The four condition sets well representthe polishing recipe in practice. The time of the endpoint isdetermined by these conditions. The correlation between theprocessing parameters and the time of endpoint involves bothmechanical and chemical aspects and remains an empiricalobservation and practice. In general, slower polishing speed(at lower rotational rates of platen and carrier) needs longertime due to lower removal rate, based on the widely acceptedempirical Preston equation [23]. Hence, the case (d) of platenspeed r/min and carrier speed r/min shows the latestendpoint. The numerical normalization of the signal values wasperformed by dividing the difference between each measure-ment and the minimum measured value in the four experimentalconditions with the difference between the maximum and the

minimum of the measured values. The RMS of the AE signalshows the same results, as shown in Fig. 6. For the patternedwafer, the overlay copper film on the dielectric layer will beremoved, left with the copper filled in vias. The AE signalscan nevertheless tell the endpoint in time because they areinsensitive to the quite small area of the exposed copper ondielectric layer (less than 1% of the whole dielectric area).

Polishing the metal film sends out more acoustic emissionsthan the dielectric film. In Fig. 4, one further notices the ratiois about 2. The number is found to correspond to the ratio ofthe coefficients of friction between the polishing pad and thesefilms, 0.7 and 0.35, respectively.

The temperature rise can tell the progress of metal film re-moval, since the temperature rise is a function of the coefficientsof friction between the pad and the metal film as well as the di-electric film [21]. In the beginning phase, the pad polishes themetal layer only, thus the temperature rises steadily with time.When part of the metal film is removed in the transition phase,the pad polishes both metal and dielectric film. Since the coef-ficient of friction for dielectric film is lower than copper, thetemperature rises less rapidly than in the previous phase andvaries with the relative amount of metal film left on wafer sur-face. When the metal film on wafer surface is fully removed inthe end phase, the temperature rises steadily and more mildly ascompared to polishing metal film in the beginning period. Thesecond-order curve can happen when a very thick Cu film is pol-ished. However, in most cases of CMP processes (including thecurrently investigated case), the Cu film is so thin that the tem-perature increases approximately linearly until the underlyingoxide emerges.


Fig. 6. RMS of acoustic emissions during copper CMP. (a) Platen speed = 70 r/min, carrier speed = 50 r/min. (b) Platen speed = 70 r/min, carrier speed = 30

r/min. (c) Platen speed = 50 r/min, carrier speed = 50 r/min. (d) Platen speed = 50 r/min, carrier speed = 30 r/min.

A monitoring strategy can be developed in use of the re-gression technique based on this thermal behavior [21]. Whenmonitoring, the pad temperature is constantly measured at sev-eral points on the scanning path. The least square regressionsof the first and second order are continuously carried out forall existing temperature measurement. The errors of two re-gressions are compared to the other continuously. In the be-ginning period, the linear regression indeed shows less errorthan the second-order regression until the transition phase whenboth metal and dielectric films are polished and the behavior oftemperature rise changes. After the error of linear regressionis found larger than the second-order regression for a periodof five continuous measurements, the beginning of transition isidentified. Similarly, after the error of the linear regression isfound again less than the second-order regression for a periodof five continuous measurements, the end of transition is identi-fied. Namely, the metal film is fully removed, or the endpoint isreached. The error of all regression is less than 5%. Fig. 7 showsthe monitoring strategy schematically. Fig. 8 shows the historyof temperature rise on the pad at the location of radial distance110 mm from the pad center at different polishing conditions.The temperature rise at these points was found to follow thecharacteristic pattern described above. At different locations,the time of reading the endpoint was found to be a little dif-ferent. Monitoring of more locations on the pad is necessary, sothat the actual endpoint of the process is identified as the latestone.

The results of monitoring using acoustic and thermal emis-sions are compared to each other under the same process con-ditions, as shown in Table III, [19], [21]. The slight delay of0%–15% in use of the thermal method is attributed to the less

Fig. 7. Schematic of thermal monitoring strategy [21].

agile response of the heat transfer in the system (including theslurry and polymer pad) than the acoustic wave propagation, aswell as the required check for the regression errors along BC andbeyond (in Fig. 6) as mentioned previously. The experimentalresults demonstrate the endpoint in copper CMP can be moni-tored in situ by both methods used individually or in a fusionmanner for the nano-scale integrated circuits manufacturing.

V. CONCLUSION

CMP has become an indispensable process in deep submicronsemiconductor manufacturing. In a metal CMP process, the end-point is reached when the overlay metal layer is removed. Aneffective in situ endpoint monitoring method is needed to im-prove the yield rate and throughput of CMP. Both the acousticand thermal emissions during polishing the metal layer and the


Fig. 8. Monitoring of temperature rise on pad passing wafer (radial distance at 110 mm from pad center) [21]. (a) Platen speed = 70 r/min, carrier speed = 50

r/min. (b) Platen speed = 70 r/min, carrier speed = 30 r/min. (c) Platen speed = 50 r/min, carrier speed = 50 r/min. (d) Platen speed = 50 r/min, carrierspeed = 30 r/min.

TABLE IIIENDPOINT TIME DETERMINED BY THERMAL AND AE METHODS

dielectric layer are different due to the inherent difference inthe tribological behavior. In fact, the coefficient of friction be-tween the polishing pad and the dielectrics is lower than that be-tween the pad and the metal. The acoustic emissions decreaseto a lower level when the metal film is removed. The measuredRMS signal as well as the energy index can be used to indicatethe process endpoint agilely. Since the frictional forces betweenthe pad and the metal layer and the dielectric layer consume thepolishing power and cause the pad temperature to rise, the samefact of the difference in the coefficients of friction causes the padtemperature to increase mildly compared to polishing the metallayer. By use of a simple mathematic regression, the endpointof the process can be identified from the change of temperature

rise. The experimental results can be applied to supplement thecurrent plant monitoring practice at no significant costs for mod-ifications of existing CMP equipment to improve the yield rateof nano-scale IC manufacturing.

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Hong Hocheng was born in 1958. He receivedthe B.Sc. degree from National Taiwan University,Taiwan, in 1980, the Diplom-Ingenieur from Tech-nische Hochschule Aachen, Germany, and the Ph.D.degree from the University of California, Berkeley,in 1988.

Presently, he is a Professor at National Tsing HuaUniversity, Taiwan, R.O.C. His fields of interest areinnovative manufacturing processes.

Yun-Liang Huang was born in 1973. He receivedthe B.Sc. degree in mechanical engineering fromNational Chiao Tung University, Taiwan, R.O.C., in1995, and the Ph.D. degree from National Tsing HuaUniversity, Taiwan, in 2002.

Presently, he is a Senior Engineer at AU OptronicsCorporation. His major research field is chemical me-chanical planarization and intellectual property.

endpoint detection by acoustic emissions in chemical–mechanical polishing of metal overlay

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