In-Situ Measurements for Chemical Mechanical Polishing James Vlahakis Caprice Gray CMP-MIC February 20, 2006

Outline Slurry layer imaging using Dual Emission Laser Induced Fluorescence (DELIF) –Why DELIF works –How to take data –Ways to interpret data In-Situ Force measurements –Experimental Equipment/Setup –Coefficient of Friction –Downforce frequency analysis

In Situ Slurry Layer Image Acquisition in Chemical Mechanical Planarization Caprice Gray PhD. Candidate Tufts University 20 February

Dual Emission Laser Induced Fluorescence (DELIF) Types of Measurements –Dye choice –pH, temperature, fluid layer thickness How to make DELIF measurements –time averaged (no laser) –2 Dye System –1 Dye System DELIF Images and Analysis –Qualitative examination of data –Calibration procedure for quantitative –Types of quantitative measurements

Is a DELIF system hard to set up? YES

Types of Measurements Time averaged Low Resolution Instantaneous High Resolution Instantaneous 1” 1 cm 1mm

Why Examine 2 Emissions? Pad Slurry Division of 2 images cancels variations in image source intensity = Ratio

DELIF Light Paths Laser Shot Camera Mirror Laser Mirror Camera 1 Camera 2 Fluorescent Light Optics Box BK7 Optical Glass Wafer

Optics Box Calcein Camera Pad Camera

DELIF Spectral Analysis (1 dye)

Spectral Filter Regions

How DELIF works Variables –I 1f, I 2f = Fluorophore fluorescent intensity –I L = laser intensity –I cam = light collected by camera –  = quantum efficiency –A = absorption – = wavelength –z = passive scalar –a, b, C, D = constants

Sources of Error Measurements are independent of viewing geometry Short wavelength absorber/emitter: –Independent of the passive scalar (type 3 conflict * ) –Does not absorb its own emission –Does not absorb the long wavelength absorber’s emission Long wavelength absorber/emitter: –Does not absorb its own emission –Does not absorb Laser emission –Absorption must be a linear function of z (true for thin films) Choose a short wavelength filter band outside the absorption region of the long wavelength absorber/emitter (type 2 conflict * ) Choose filter bands where emissions do not overlap (type 1 conflict * ) * Spectral conflict types as identified by Coppeta and Rogers Experiments in Fluids, 25 (1998) 1-15.

Choosing Dyes QuantityDye Candidates pHFluorescien(5-8), HPTS(6-9), Rhodamine B(<6), LDS 698(<6), 1-4 DHPN(6-9) pH IndependentLucifer Yellow, Sulforhodamine, Kiton Red, Phloxine B TemperatureFluorescien, HPTS, Rhodamine B, Kiton Red, Phloxine B, LDS 698, 1-4 DHPN Temperature Independent Sulforhodamine ThicknessCalcein, Pyrromethene 650 Thickness Independent Coumarin 4, Pyrromethene 567 References: J. Copetta, C. Rogers. Experiments in Fluids, 25 (1998) C. Hidrovo, D. Hart. Measurement Science and Technology, 12 (2001) J. Lu, et. al. Journal of The Electrochemical Society, 151 (4) (2004)

Time Averaged DELIF Slurry Layer Pad z UV Lamp Light Dye 1Dye 2 NO LASER, uses UV lamp –Mostly qualitative – global slurry behavior –Quantitative- average fluid layer thickness, average pH, or average temperature Must suppress the natural fluorescence of the pad with carbon black Need 2 dyes

2 Dye System Slurry Layer Pad z Laser Shot 355 nm Dye 1 Dye 2 Laser allows instantaneous measurements Need laser beam expander for low resolution images Must suppress the natural fluorescence of the pad with carbon black

1 Dye System Slurry Layer Pad z Laser Shot 355 nm Dye Particle Laser allows instantaneous measurements Must use pads with natural fluorescence and spectra that fit the model –Polyurethane based pads work well –Surface coatings are not good enough Depending on laser power, may need beam expander to prevent pad burning

Calibration Methods 1mm = 232 pixels X-Y Calibration Capture image of millimeter ruler and measure pixels/mm 2-Dye Calibration Inject dyed slurry between 2 microscope slide shimmed on 1 side 1-Dye Calibration Flat wafer shimmed by microscope slide Need very flat pad Must image near wafer edge (difficult)

Most Recent Calibration Method Etch wells into glass wafer to different depths –Depth must be greater than pad surface roughness Relative calibration –Need wafers with 2 different well depths –Can acquire pixel to pixel slurry depth differences Absolute calibration –Need wafers with at least 3 different well depths –Know slurry thickness under every pixel

Pad Images (Low R a Flat) (a) Flat wafer (b) 14 um deep 1mm 2 square well We can see striations in the pad due to the motion of the conditioner. Striations direction is along the dotted line. Slurry flow direction is indicated by arrows

Air Pockets Low Resolution High Resolution Air pockets get trapped in features Grooves help to transport air pockets under the wafer features

Histogram Analysis The asperity size distribution is the same shape as the fluid layer thickness distribution. When force is applied to the wafer, the distribution changes both shape and location. Standard deviation comparison → pad compression Peak location → fluid layer thickness. Compression factor: 

Histogram Data Red line = 10psi, White Line = 0psi Histograms of 2mm 2 regions at different points on the pad.

Pad Shape Near Wafer Features

DELIF Summary How and why DELIF works –Must be very careful in choosing dyes/fluorophores and filter regions Different ways to employ DELIF Calibration methods Types of image analysis –Qualitative: slurry starvation, air travel under the wafer –Quantitative: Pad compression using sub-region histograms, Pad shape near wafer features

Pad Images (Low R a Grooved) Fruedenberg FX9 K-grooved Fruedenberg FX9 K-grooved Rodel IC1000

Pad Images (High R a ) Flat Experimental Pad xy-grooved experimental Pad Thin-grooved Experimental Pad

Polishing Setups Old Setup New Setup