ECE 6450 - Dr. Alan DoolittleGeorgia TechLecture 6Rapid Thermal ProcessingReading: Chapter 6ECE 6450 - Dr. Alan DoolittleGeorgia TechRapid Thermal Processing (RTP) (Chapter 6)Categories:Rapid Thermal Anneal (RTA)Rapid Thermal Oxidation (RTO)Rapid Thermal Nitridation (RTN) (and oxynitrides)Rapid Thermal Diffusion (RTD)Rapid Thermal Chemical Vapor Deposition (RTCVD)Silicides and Contact formationAdvantages:1.) Single wafer processing produces the best uniformity, especially for large wafer sizes.2.) Minimize redistribution of dopants, minimal sqrt(Dt) with maximal D (high Temperature) allows repair of damage from ion implantation.3.) Cold walls allow multiple processes to occur without cross contamination.4.) Photochemistry can be exploited.Disadvantages:1.) Absolute temperatures are almost never known.2.) Nonthermal-equilibrium conditions make modeling and predicting difficult.3.) Uniform heating is more critical than traditional furnace processing due to high ramp rates and the resulting stress.Definitions:stress: force per area => units are the same as pressureECE 6450 - Dr. Alan DoolittleGeorgia TechRTP PhysicsHeat Flow Mechanisms can be related to temperature rise by:Where Cpis the specific heat (a measure of how much energy a material can absorb before it manifests in a temperature rise), r is the gram/cm3density, and q-dot is the heat flow density (W/cm2) Note your book is inconsistent on how it uses q-dot.Temperature ramp rate can be enormous!!!!!)()()()(thicknessxxCTqdtdTpρ•=Rapid Thermal Processing (RTP)22)(cmSecondJoulescmWattsTq ==•ECE 6450 - Dr. Alan DoolittleGeorgia TechTypes of RTP1.) Adiabatic: Excimer laser pulses (<uS) anneal the thin skin of material.=>huge vertical temperature gradients2.) Thermal flux: rastering a focused beam (electron or laser) across a wafer. =>huge vertical and lateral temperature gradients3.) Isothermal: Broad area optical illumination. => minimal temperature gradients.RTP Physics3 types of Heat Flow Mechanisms:1.) Conduction: Flow of heat between two bodies in intimate contact.Heat flow per unit area in a solid is expressed in terms of a solids thermal conductivity, k(T), as,Where k(T) has units of Watts/(cm-K) and x is the thickness measured between the two temperatures. Note this is different from your book.2.) Convection: Flow of heat between two bodies through an intermediate medium (a gas in our case)For a gas with effective heat transfer coefficient, h with units (Watts/cm2-K) is,Notice that both of these expressions are linear in temperaturexTTkTq∆=•)()()()(∞•−= TThTqwaferRapid Thermal Processing (RTP)ECE 6450 - Dr. Alan DoolittleGeorgia Techsec/7.1)(07.0)/(33.2)/(75.0/19.0)()()()30(/19.0)100030(42)()()(322CcmcmgmCgmJcmWdtdTthicknessCTqdtdTcmWeTqTThTqpwafero=−====−−=−=••∞•ρRapid Thermal Processing (RTP)Example: Assuming constant power delivery, what is the initial temperature rise rate for a 700 um thick Si wafer in a furnace heated from T=30 C to T=1000 C in a gas with a effective heat transfer coefficient of 2e-4 W/(cm2-C)?Power density needed to get to 1000 degrees CECE 6450 - Dr. Alan DoolittleGeorgia Tech3.) Radiation: Flow of heat between two bodies through radiation and absorption of light.We can use the spectral radiant exitance= the radiated power per area per unit wavelength,⎟⎟⎠⎞⎜⎜⎝⎛−=⎟⎠⎞⎜⎝⎛1)()(251TcecTMλλλλε.emissivity dependant h wavelengt theis ε(λ) andK m1.4388x10c,mW3.7142x10c where222161−=−=−−exitance, total thearea,unit per radiatedpower total then theλ, oft independen is ε(λ) If4)()( TTqTMσε==&constant.Boltzmann -Stefan theis W/m5.6697x10 where428K=σNOTE:1). The unit change to meters and 2) The radiated power depends on temperature to the forth while conduction and convection depend on temperature linearly. Thus, radiation is the dominate mechanism at high temperature while conduction and convection dominate heat flow at lower temperatures.The emissivity is related to the absorbance by Kirchoff’s law of conversation of power which states that in steady state at (constant temperature and absorbed and emitted power), the power absorbed by a wafer must be equal to the power emitted.Rapid Thermal Processing (RTP)ECE 6450 - Dr. Alan DoolittleGeorgia TechThe net heat flow between 2 hot bodies is,2114224111221)(AtoAtotoFATTqqεεσ−=−••21AtoAFwhere212211)cos()cos(12121dAdArAFAAAtoAπββ∫∫=is the view factorIn for most real life surfaces (even flat wafers have finite thicknesses) this equation is not very useful unless computer calculations or simplifying assumptions are used.Rapid Thermal Processing (RTP)ECE 6450 - Dr. Alan DoolittleGeorgia TechHardware for RTPBulbs can be classified by their ‘color temperature”,TKcm−=2898.0λmax spectral radiant exitanceThe more power per unit area emitted by a bulb, the higher the color temperature (peak of exitance moves to lower wavelengths).Rapid Thermal Processing (RTP)Tungsten Halogen Bulbs: Moderate color temperatures~moderate output power density. As tungsten filament gets hot, the W evaporates and begins to coat the glass. The halogen species forms volatile (gases with high vapor pressures) W-halogen compounds that diffuse back to the hot filament, break apart and redeposit the W. Thus, longer bulb life is obtained.Arc Noble Gas Discharge Lamps: A fused silica tube containing a noble gas (or mixtures) is ignited with a high voltage pulse to ionize the gas. Once ionized the bulb can carry a huge DC current. The effect is a very intense light source with very high color temperature and additional discrete gas line spectra superimposed on the radiant exitance. Low melting point metals such as Hg are also used to increase output power in certain desired wavelengths.ECE 6450 - Dr. Alan DoolittleGeorgia TechUniformity Issues:Multizone bulb arrangements are used to supply more power to wafer edges to compensate for increased radiated power loss and lower optical “view factors”. Loss of uniformity results in inconsistent dopant activation, inconsistent dielectric properties, inconsistent stress resulting in defect generation and many other problems.Temperature Measurement:Most often in today’s RTP systems a combination of pyrometry, acoustic and in some rare cases thermocouples in a susceptor are used.Rapid Thermal Processing (RTP)1.) Pyrometry measures the intensity of light within a certain operating bandwidth emitted from a wafer and relates