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CMP: Where Does It End? Ron Allen, Charles Chen, Tom Trikas, Kurt Lehman, Robert Shinagawa, and Vijay Bhaskaran, KLA-Tencor Corporation Brian Stephenson and David Watts, Ebara Technologies Inc.
We describe the design, operation, and algorithms for an in-situ CMP endpoint detection and control system, with particular emphasis on copper polishing. The system’s eddy current-based sensor gives absolute surface metal thickness. Its multi-angle reflectometer gives eight optical reflectance measurements. The endpointer improves on existing sensors and techniques in several ways. It can process reflectance traces individually according to their endpoint sensitivity, which applies to dielectric polishing and copper barrier removal processes. Also, it merges reflectance signals for higher signal-to-noise ratios, which benefits copper CMP. Finally, the system can fuse the reflectance data with thickness readings for more robust endpoint detection.
Introduction
Chemical Mechanical Planarization (CMP) is a widely accepted polishing and patterning method in microelectronics fabrication. Though CMP is indeed crucial to some processes—copper (Cu), for instance, for which plasma etching remains problematic—process engineers must still cope with underpolish and overpolish problems. Stopping a metal polish step too soon (underpolishing) leaves metal or barrier material residues, which cause electrical shorts in the target layer. Underpolishing dielectric films causes open circuit defects. Polishing too long (overpolishing) results in metal dishing and dielectric erosion, ultimately leading to metal pooling and short circuits in higher metal layers. Simple time-based polishing is widespread in fabs. But Cu electroplating produces film thickness variations that thwart timed recipes, and the requisite rate monitor wafers are becoming prohibitively expensive. CMP tool conditions such as temperature, pad condition, and wafer pressure profile also affect the polish rate and uniformity. To facilitate CMP integration into largevolume production, process controllers must have cross-wafer information available and monitor each wafer’s polish profile to 54
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determine the process endpoint (the precise time at which the target material has been removed) or the remaining requisite layer thickness. KLA-Tencor’s Precision In-Situ CMP Endpoint (PRECICE™) system addresses these issues by providing real-time film thickness measurements, reflectance data, and endpoint detection for a variety of polishing processes, including Cu CMP. Design: sensors, controller, and communications links
The system contains an eddy current-based sensor; a single wavelength, multi-angle reflectometer; a data acquisition and control processor; and communication links to the CMP host computer. The sensors mount beneath the CMP tool’s rotary platen (Figure 1). The eddy probe has a drive coil that induces a current in the wafer, a sense coil to find in-phase and quadrature components of the induced voltage, and signal generation and data acquisition electronics1. The control computer’s calibration curves give absolute metal thickness values, independent of temperature and pad wear. The standard optical sensor wavelength is 808 nm. Two methods exist for creating an optical path to the wafer: a flexible polyurethane window inserted into the pad and a self-clearing objective (SCO) that uses a timed flow of deionized water (DIW). Thus, wafer incidence angles vary; with a SCO they range from 6.7° to 56.3°. A rotary union and slip ring on the table shaft bring fluid lines and electrical paths to table-mounted sensors.
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Table rotation CCW Eddy current probe Reflectometer
Sensor path Wafer Acquisition electronics Carrier ring Slip ring
Endpoint control computer
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where φm is the magnetic flux (webers/m2) linking the coil.3 Let εo be the open coil EMF generated when the sense coil is not over the wafer. As the probe scans the wafer, primary flux enters the Cu layers, inducing an EMF, and thus an eddy current by Ohm’s law. The eddy currents reduce the primary flux. As the sense coil passes over thicker Cu regions, the flux moves through deeper layers of metal, decreasing the linkage flux, and reducing the voltage magnitude across the sense coil from εo (Figure 2). Calibration of the sensor involves an estimate of εo and a measurement of ε for a Cu sample wafer of known thickness, such as may be measured with a four-point probe. A referencing scheme is employed to compensate for temperature and pad wear. For pass k over the wafer and each wafer sample n, the metal thickness is
TABLES AND FIGURES
Proximity sensor
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Polisher host computer
T(n) = Scal[||ε(n) – εo(k)|| – Dcal]*Cpt(k) – Wres, Figure 1. System layout, top view. Proximity sensor triggers data acquisition as eddy probe passes carrier ring. Slip ring passes serialized eddy and optical data to controller. Dual-platen polishers, such as Ebara’s FREX-300™, may have CW rotation tables; such setups have proximity flags before the reflectometer.
In typical operation, the host computer downloads recipe data to the endpoint controller (Fiure 1). After a delay for polish process stabilization, data acquisition from the eddy current probe and the multi-angle reflectometer begins. A proximity sensor, or, alternatively, the indication of the metal carrier ring from the eddy current device, triggers acquisition. Sensors report data on every platen revolution. The electronics serializes the data and passes it through the slip ring assembly to the computer. Before the next trigger, the controller’s algorithm software analyzes the data for characteristic endpoint features. When the algorithms detect the recipe’s prescribed endpoint feature, the endpointer notifies the host, which stops the current polish step.
(2)
where ε(n) is the complex EMF; εo(k) is open coil EMF; Scal and Dcal are calibration scale and offset, respectively; Cpt(k) is the pad and temperature compensation, and Wres is an optional offset for a low resistivity wafer substrate. The multi-angle reflectometer can be understood through modeling the reflectance and transmission through the optical objective to the wafer.4 The wafer
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Principles of operation: eddy current probe and multi-angle reflectometer
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Eddy current probe principles are well-known for testing and metrology.2 By Faraday’s law, the EMF ε (volts) produced across the sense coil of N turns in MKS units is
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Figure 2. Evolution of in-phase eddy sensor signal: 200 mm 931
(1)
patterned test wafer; 27-Jan-2001; 300 samples/channel; vertical lines are wafer edges; platen RPM: 70; cycles 50, 100, 150, 200.
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consists of isotropic media M0, M1, …, Mm+1; with complex refractive indices N0, N1, …, Nm+1; where M0 is the semi-infinite ambient (e.g., DIW); Mm+1 is the semi-infinite wafer substrate; Mi has thickness di, 1 ≤ i ≤ m; the angle of incidence is φ0; and the angle of refraction in Mi is φi, 1 ≤ i ≤ m+1. The 2×2 scattering matrix is the product S = I01L1I12…LmIm,m+1/(t01t12…tm,m+1),
(3)
Wafer areas do not clear uniformly, of course. To spatially resolve endpoint declarations, the system either (1) divides the wafer into simple annular zones or (2) computes the actual sensor path. The sensor electronics samples at uniformly timed rates so, in either case, the sensor spots do not cover the wafer uniformly. The carrier and platen (Figure 1) rotate at Rc and Rp RPM, respectively, so their angular orientations are:
ω(t) = 2tπRc/60
where Li and Ii,i+1 are the layer and interface matrices,
[
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Ii,i+1 =
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βi = [2πdiNicos(φi)]/λ is the layer phase thickness; λ is the wavelength; ti,i+1 is either the p- or s-polarization Fresnel transmission coefficient ti,i+1, p =
ti,i+1, s =
2Nicos(φi) Ni+1cos(φ i) + Nicos(φ i+1)
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2Nicos(φi) ; Ni cos(φ i) + Ni+1cos(φ i+1)
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ri,i+1, p =
Ni+1cos(φ i) – Nicos(φ i+1) Ni+1cos(φ i) + Nicos(φ i+1)
N cos(φ i) – Ni+1cos(φ i+1) ri,i+1, s = i . Ni cos(φ i) + Ni+1cos(φ i+1)
Pxy (t) = (rs cos(
2tπRp 2tπRp ),rs sin( )). 60 60
2tπRp 2tπRp )–rs, rs sin( )). 60 60
Now, if a point has coordinates (a, b) in the carrier (u, v) system, and the carrier rotates by an angle ω, then the
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Thus, via p- or s-polarization values, the wafer transmission coefficient is t = (S11)-1, the reflection coefficient is r = S21/S11, and so the reflectance is R = |r|2. Varying di at Mi’s polish rate and computing R at each step produces a model of the CMP process. The model (Figure 3) establishes threshold and time delay parameters for endpoint detection algorithms.
(9)
where rs is the sensor path radius.
(6a)
(6b)
(8)
In (u, v) coordinates, with the carrier not rotating, this point path would be Puv (t) = (rs cos(
and ri,i+1 is either the p- or s-polarization Fresnel reflection coefficient
(7)
If the machine- and carrier-relative coordinate systems are (x, y) and (u, v), respectively, then the sensor path relative to (x, y) is
Polish stack: Cµ[2000A]/Ta[200A]/SiO2[300A]/substrate
Li =
φ(t) = 2tπRp/60.
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Endpoint detection algorithms
Endpoints of interest to the Cu CMP process engineer are: bulk copper to a specified thickness, copper clear, and barrier clear. Endpoint on eddy probe values (2) is as simple as specifying the absolute target thickness. Thin Cu endpoints rely on reflectance values whose endpoint behavior is governed by the thin film model (3)-(6). 56
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Thickness removed (Å) Figure 3. Reflectance model for a Cu (2000Å) wafer, 200Å tantalum barrier, and 300Å of oxide overpolish. Vertical axes: reflectance. Horizontal axes: material thickness removed (Å).
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new coordinates of the point will be (10) This means that after rotation by ω(t), the coordinates of the sensor over the wafer, relative to the (u, v) coordinate system will be
(11) Figure 4. Reflectance data from an edge-slow polish of a 200 mm
Once thick Cu clears locally, the system relies on the optical sensors to find remaining thin patches. The algorithm, a conventional blob analysis5, is as follows: (1) Cu reflectance values are combined to get a composite optical trace (Figure 3), (2) the algorithm sets a reflectance baseline Rb across the wafer. At this time, Cu is still fairly thick, and the reflectance should be near its theoretical average (slurry effects and process step changes can harm the Rb calculation, but standard process trend and control chart techniques greatly mitigate the risk6); (3) Points where Cu remains are found by comparing reflectance to this baseline value. Thus, if T is the threshold, and Rm is the measured reflectance, then Cu is present at sample n if Cu (n) = 1 ⇔ Rm (n) ≥ T * Rb (n) •
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(iv) Median filtering the Cu array fills narrow gaps between high R regions. (v) The width of the contiguous Cu regions is determined, and where the width exceeds a percentage of a zone width, a Cu blob is declared for the zone. If W is the zone width, Tw is the threshold, and Blob(m,n) means there is a Cu blob in the region [m, n], then we have Blob (m,n) ⇔ Min {Cu (k) | m ≤ k ≤ n} = 1
&Cu(m – 1) = 0&Cu(n + 1) = 0&n – m ≥ W*Tw
(13)
The percentage of blob points that intersect a zone is calculated from the blob array. Though the system acquires data at fixed time intervals, the wafer samples are not uniformly spaced. However, the sensor path (11) gives the spatial extent of Blob(m,n). Precession calculations using the carrier and platen RPM provide the relative location between successive wafer sweeps. (4) Finally, Cu clear endpoint occurs when the percentage of blob points in a zone is less than a threshold. A time
patterned Cu wafer.
delay, proportional to the sensor path (11) coverage of wafer zones, reduces false positives. Figure 4 shows reflectometer data for a 200 mm Sematech 931 patterned Cu test wafer. The Cu thickness and reflectance values can be fused. One technique fits a regression line to thickness values to predict clearing. The reflective patch analysis (12)(13) confirms Cu clearing. Another method is to optimize a local thin film model using the measured reflectivity and thickness values. Conclusion
The eddy current sensor measures impedance vectors as opposed to simple scalar measurements offered by competing eddy current devices. Absolute measurement makes CMP process recipes easy to establish, reliable, and portable. Absolute thickness capability also allows tools to be dynamically controlled for within-wafer uniformity. After bulk metal clear, the application software merges the eddy current and reflectometry sensors’ data streams within their overlapping ranges; a crisp and repeatable soft landing to the copper-clear endpoint results. Multiple angle reflectometry advances CMP monitoring and diagnostics by giving process engineers more repeatable and flexible control over endpoints. Software can combine these reflectance signals, improving signal-to-noise ratios, or analyze them separately, for increased robustness of endpoint control on certain thin metal films. Depending on the process and film stack properties, some angles may give a stronger endpoint signal than others (Figure 3). Real-time wafer mapping Winter 2002
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of metal thickness, non-uniformity, and copper patches allows for dynamic feedback control during CMP and results in a sharp and reliable copper-clear endpoint. The in-situ endpoint system shortens process development cycle, eliminates underpolishing, and reduces overpolishing to increase yield. Multi-angle reflectometry allows the endpointer to support other CMP applications: shallow trench isolation (STI) and tungsten processes. Single angle reflectometry has been tried for STI, but the reflectance signal feature that signifies endpoint is not unique. Such endpointers depend critically on timing parameters and knowledge of incoming film thickness. Any problem upstream of the CMP process affects endpoint timing accuracy. In positive contrast, the PRECICE system manipulates its angular spectrum reflectometry data to extract a signal feature unique to STI polish endpoint. Thus, it supports a CMP process independent of layer thickness variation from prior deposition steps.
References 1. C.L. Mallory, W. Johnson, and K. Lehman, “Eddy current test method and apparatus,� U.S. Patent No. 5,552,704, September 3, 1996. 2. R.C. McMaster, P. McIntire, and M.L. Mester eds. Nondestructive Testing Handbook: Electromagnetic Testing, 2nd edn., American Society for Nondestructive Testing, 1986. 3. J.D. Jackson. Classical Electrodynamics, 3rd edn., New York: Wiley, 1998. 4. R.M.A Azzam and N.M. Bashara. Ellipsometry and Polarized Light, Amsterdam: North-Holland, 1992. 5. D. H. Ballard and C.M. Brown. Computer Vision, Englewood Cliffs, NJ: Prentice-Hall, 1982. 6. M. Basseville and I.V. Nikoforov. Detection of Abrupt Changes: Theory and Application, Englewood Cliffs, NJ: Prentice-Hall, 1993. A version of this article was originally published in the 2001 ISSM/IEEE proceedings International Symposium of Semiconductor Manufacturing Conference, October 8-10, 2001, San Jose, California, USA.
KLA-Tencor Trade Show Calendar January 30, 2002
YMS China, Shanghai, China
February 5-7
SEMICON Korea, Seoul, Korea
February 11-15
AVS, Santa Clara, California
March 3
Lithography Users Group Meeting, Santa Clara, California
March 5-6
SPIE/Microlithography, Santa Clara, California
March 26
SEMICON China, Shanghai, China
April 10-12
ACE/APC, Dresden, Germany
April 16-18
SEMICON Europa, Munich, Germany
April 17
YMS Europa, Munich, Germany
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