4Why low k? The RC delay problem Upper Metal LayerR=2rL/PTC=2(CLL+CV)= 2ke0(2LT/P+LP/2T)RC=2rke0(4L2/P2+L2/T2)WSCLLTInterconnectPCV=CLGThe time delay in the interconnection of ICs is a performance limiting factor when feture size is scaled down below 0.25um.This is a typical example of a multilevel interconnect system where P=W+S ans W=S. The metal thickness T and the dielectric thickness above and below are equal. CLLis the lateral line-to-line capacitance, CV is the vertical layer to layer capacitance andCLG is the line to ground capacitance.The RC time delac is dependent on two material factors: the resistivity of the interconnecting metal, and the dielectric constant of the insulator.This simple first order model was used by Bohr to estimate the interconnect RC delayWhen the interconnect spacing is less than 0.3um, the interlayer capacitance(CV or CLG) is very small compared withthe total capacitance. The total capacitance is dominated by line to line capacitance. Almost 90% of the total capcaitance comes from the line to line capacitance at sub-0.25um feature sizes.Therefore reducing Line to line capacitance is one of the most critical issues for sub-0.25um IC devices.Another advantage of lower total capacitance is that crosstalk and power dissipation are reduced.Lower Metal LayerM. Bohr, Tech. Digest IEEE Int. Electron. Devices Meeting 1995 p. 241
5Why Low k?As the dimensions of ULSI devices scale to smaller features, the resistance-capacitance (RC) delay of the metal interconect becomes performance limiting.The gate delay is important only for device features larger than 1um.When the feature size shrinks to less than 0.5um, the interconnect RC delay becomes dominant.You can see the quadratic nature of the RC delay...lets take a closer look at how it originates.
6MetalsExchanging Cu for Al reduces delay by about 35% although many problems exist for integration.
7Roadmap (1999 ITRS, International SEMATECH) Interconnect Technology Requirements - Near Term
8A range of dielectrics are becoming commercially available At the April 98 MRS meeting on Low k materials there were 5 papers on ultra low k materials (k<2.3) - four were on porus silica and one was on porous MSQ. There were 23 papers at the 1999 MRS Low Dielectric Constant Symposium, ranging from porous SiO2(7), MSQ (2), parylene (2), SiON(1), HSQ(1), a-CF(6), airgaps (2) & porous polymers (2)HSQ = Hydrogen silsesquioxanes e.g. Dow Corning‘s Fox = Flowable Oxideused in TI‘s0.5um process and Philips latest BiCMOS process QuBiC3MSQ = Methyl silsesquioxanesHOSP is a siloxane polymer inorganic / organic hybrid for spin on from ALLIED SIGNALCVD low k such as APPLIED MATERIALS BLACK DIAMONDFLARE AND Nanoglass are also from ALLIED SIGNALMorey & Asthana, Solid State Technology, Vol. 2, No. 6, June 1999 p.71,
9Introduction of pores to lower k By making SiO2 porous, the effective k valueis reducedAn overview of our work on porous silica:Why porous silica?How xerogel films are madechemistryProperties of the films in comparison to commercially available mazerialIntegration IssuesOutlookAs the RC interconnect delay time becomes dominant at featauresizes in the submicron range, the switch to lower resistivity copper conductors and low k intermatal dielectrics is already underway.This diagram shows how the dielectric constant of SiO2 is expected to decrease with the increasing porosity.At 0% porosity, bulk k = 4, at 100% porosity k= 1 (air)Comprimise in the middle between low k and mechanical stability
10Sol Gel ProcessingSol Gel Science, Brinker & Scherrer, Academic Press, 1990, p. 1
11General Sol-Gel Process Flow for Porous SiO2 How do we make a porous film?Start with spinning on a SOL, a liquid containing solvents and chains of SiO2.These grow and link and form a solid network of SiO2 with solvent containing pores. This is called a gel.The film must then be dried by removing the solvents without collapsing the pore structure. Therein lies the problem!
12Mesoporous xerogel low-k film production Spin onPost-spin on gelationDrying / Annealing 450°CHydrophobisation with HMDSPre-spin on gelationPrecursor & catalyst mixingPrecursor = TEOS, water, solvent, catalysthydrolysis / condensation reactionsn-type 150 mm Si wafersSi-O-Si porous network forms after spin-onReplace hydrophylic surface-Si-OH groups withhydrophobic -Si-O-Si(CH3)3Porosity controlled by changing the time allowed for gelation reactions before and after spin onWe made 5 samples with range of porositiesRound robin to each characterisation method, which I will now describe5 process variations = range of dielectric constants
13Film properties Nanoglass is a trademark of Allied Signal / Honeywell This table shows the properties of the Nanoglass and TUC materials.They are quite similar even though the TUC material does not require a solvent exchange step.Note the higher modulus of elasticity of the TUC materialN/a = not available (no data found)Nanoglass is a trademark of Allied Signal / Honeywell
14Film Characterisation TEM of Pt imprintof xerogel filmAtomic Force Microscopyvery smooth film surfacemaximum height Rmax < 8 nmmean roughness Ra < 1 nm
16Ellipsometric Porosimetry (EP) pumpPCPadsorptivelasersampledetectorEllipsometric Porosimetry (EP)n porosityAND, toluene absorption pore size and distribution,pore interconnectednessOP=55%FP=54%CSA=260 m2 cm-3Multi-angle single frequency (=632.8 nm) EP measures the film refractive index to calculate the ‘full’porosity using the Lorentz-Lorenz equation. TOP DIAGRAMIt also measures the change in ellipsometric characteristics and during desorption of toluene vapor and calculates the ‘open’ porosity, i.e. that portion of the pore network available to toluene penetration. BOTTOM DIAGRAM LEFTIf the full and open porosities are equal, all the pores in the film are said to be interconnected. Here is such a film BOTTOM DIAGRAM LEFTThe pore size distribution (PSD) is also calculated from ellipsometric optical constant changes during toluene desorption using the Kelvin and BET equations. The cumulative surface area is calculated by integrating the PSD data for a cylindrical pore model. BOTTOM DIAGRAM RIGHTThe accuracy is estimated to be 3 %.
17Laser Induced Surface Acoustic Waves (LSAW) Laser Pulse SAW Piezoelectric pickupVelocity (m s-1)Frequency (MHz)V film thickness, density, elastic modulus and Poisson‘s rationew surface acoustic wave technique, LSAW, measures the velocity dispersion of laser-generated wideband surface acoustic waves as a function of frequency as they travel through the film and substrate . DIAGRAMdispersion depends on the properties of the layer (thickness, elastic modulus, Poisson’s ratio and average film density). The large mismatch between the acoustic properties of the Si wafer substrate and the porous silica film makes this method well suited to this task. TEXT & DIAGRAMUsing the measured film thickness and assuming a constant Poisson’s Ratio (v = 0.21 in all cases), absolute values of average film density and elastic modulus can be determined by curve fitting.
18Nanoindentation Can measure Can’t measure !! Careful elastic modulushardness Can’t measureporosity!! Carefulsample and substrate effects for films < 1µmA known force is applied to a geometrically-shaped indenter which is driven into the film sample to a measurable depth .DIAGRAMUsing analytic or finite element modelling, some of the material mechanical properties can be derived from the indentation load-displacement data. LOAD CURVEPorosity information is not available using nanoindentation.The accuracy depends on film thickness, film structure and the mismatch between film and substrate elastic constants.Despite the relative complexity involved in interpreting the results, this method is becoming widely used in the industry.UNKNOWN - the reaction of porous materials!!!!!!!!!!!!!!!
19Porosity Results LSAW & RBS agree to within experimental error EP up to 20% lower than RBSHow realistic is the reference density?Film thickness, dielectric constant values and porosity values are shown here for each of the 5 process variations, labelled A to Ek clearly decreases with increasing porosity as expected.Good correlation between measurement techniquesFull & open porosity match - pores all interconnectedHowever the porosity values themselves vary between the three techniques. LSAW values are about about 5% higher than RBS, and EP about 12-20% less than RBS.The LSAW porosities calculated using a skeletal density value of tridymite, i.e g cm-3, This value was confirmed by RBS data on a 490nm thermal SiO2 bulk film - however the fractal nature of these films, and the inclusion of F and C in the skeleton due to processing may mean that the value is too small, which would bring the LSAW and RBS porosity down.But is EP really correct??* Film reference density used for LSAW and RBS porosity = 2.26 g cm-3RBS measured thermally grown dense SiO2 = 2.24 0.1 g cm-3
20Combine LSAW or RBS mean film density with EP open porosity Skeletal DensityCombine LSAW or RBS mean film density with EP open porosity skeletal densityskeleton = g cm-3
21Surface Area & Pore size r(mean) increases and pore surface area decreases as porosity increases i.e. fewer, larger poresElastic Modulus & HardnessModulus and hardness correlate well with porosity and each otherOverestimation of nanoindentation modulus by >20% due to substrate effectsRefinements neededTOP: shows surface area in m3 cm-3 and mean pore radiusPore radius increases as porosity increases, but the cumulative surface are decreases. So, larger pores but fewer of them. This is typical for such filmsBOTTOM: Elastic modulus and hardness results low - former results were higher - process dependent.Nanoindentation modulus higher than LSAW - possible due to uncertancies regarding film crushing or pile-up. This is a recognised problem for soft films on hard substrates.More work to be done on to refine accuracy
22Patterning of xerogel thin films using PECVD SiO2 cap layer CuTiNSiO2XerogelTrench etch: ICP using CF4 / CHF3trench depth: 500 nmcap: 50 nm PECVD SiOprofile angles > 85°no undercut of cap layerbowing was observed at large open areasTrench after MOCVD TiN depositionSixerogelEtched 0.2 µm trenchescapxerogelSiMOCVD TiN
23Davies & Corbett Solid State Tech., OutlookPorous silica materials posess lowest k valuesMany integration issues remain to be resolvedDavies & Corbett Solid State Tech.,Vol. 43, Issue 4, April, 2000
25Ellipsometric Porosimetry Can measurefilm thicknessrefractive index“full” and “open” porositypore size distributionpore connectivitycumulative surface area Can’t measureelastic modulus!! Careful“full” porosity calculated using reference refractive index value of dense SiO2toluene cannot penetrate pores of radius < 0.3 nmAPPARATUS PHOTOCan obtain lots of informationMechanical characteristics such as elastic modulus are not available using EP.This method is also non-destructive, fast and can be applied to multilayers with known optical characteristics.Non-destructiveSimple vacuum equipmentNon-hazardous adsorptive
26Laser Induced Surface Acoustic Waves Can measureaverage film densityelastic modulus Can‘t measurePore size distributionsurface area!! CarefulNeed a film skeletal density value for porosity calculations e.g. dense SiO2Porosity can then be calculated using a value of film skeletal density – the simplest case being that of bulk SiO2.The inherent accuracy of the technique is related to film thickness and average density – for these samples of the order of 3 % for porosity and 0.5 GPa for elastic modulus.results are not point-specific.the only method which allows porosity information and mechanical properties to be gained at the same time.special patterned oscillating sensor substrates are not required - better adapted to serve in volume production.non-destructive, requiring relatively simple setups and results can be achieved in minutes.application of the method to multilayer systems is more complexNon-destructive / non-invasiveNo patterned substrates or vacuum chamber requiredOnly technique which provides density/porosity and mechanical characteristics
27Measurement Capabilities ComparingPOROSITY = RBS, LASW, EPElastic Modulus = LSAW, Nanoindentation Proven Possible Proven for dense films
28Conclusions Can´t use skeleton = SiO2 Porosity depends on choice of reference values used in calculations Can´t use skeleton = SiO2? Can we still use nskeleton = nSiO2EP shows pores are interconnectedLSAW mean film density correlates with RBSLSAW or RBS mean film density combined with EP open porosity gives film skeletal density skeleton = g cm-3Elastic modulus values low, hard to measure with accuracyTechniques should refine themselves as the sample database grows