Temperature calibration Dr. Kolja Haberland Head of customer support 18 October 2009
Outline motivation pyrometry pre-calibration viewports calibration solutions 2
Motivation Heteroepitaxy of III-Nitride semiconductors Wafer temperature has influence on: material quality composition of ternary compounds evolving film stress substrate/wafer susceptor Bow Δz ~ ΔT Stress due to lattice mismatch results in wafer bowing which leads to: Change in temperature uniformity across the wafer Inhomogeneity of material properties 3
Motivation: temperature effects Vertical temperature profile of a III-N reactor reactor upper boundary >300K 2...20K 20...50K temperature vertical position in reactor gas wafer satellite susceptor main susceptor heater 4
Motivation: temperature effects Temperature access in III-N processes reactor upper boundary EpiCurve, Pyro400 (T wafer ) gas EpiTT, EpiCurveTT (T pocket ) wafer satellite susceptor thermo couple, light pipe main susceptor heater 5
Motivation Temperature calibration temperature is most important growth parameter themocouple or light pipe temperature used to control reactor temperature in recipe have high offsets to pocket and wafer temperature temperature measurement might be influenced by viewport geometry and window transparency proper and accurate calibration of the EpiTT/EpiCurve pyrometer is essential 6
Outline motivation pyrometry pre-calibration viewports calibration solutions 7
Pyrometry Principle of pyrometry infra red visual ultra violet Planck's equation: intensity of emission or incandescence from heated black body is correlated to its temperature But emission of real body (wafer) is different from black body, so emissivity ε has to be determined M. Planck, Verh. Dtsch. phys. Ges. Berlin, 2 (1900) 202 and 2 (1900) 237. 8
Pyrometry Emissivity Corrected Pyrometry thermal emission and reflectance measured simultaneously at 950nm conservation of energy: α + r + t = 1 opaque semiconductor: α + r = 1 emissivity corrected temperature Kirchhoffs Law α (λ,t) = ε (λ,t) ε = 1 r measure r and correct for changes of ε 9 H. Grothe and F.G. Böbel, J. Cryst. Growth 127 (1993) 1010.
Outline motivation pyrometry pre-calibration viewports calibration solutions 10
Pre-calibration Factory pre-calibration at LayTec Every pyrometer is pre-calibrated using a certified black body radiation source: original viewport and window are used calibration is accurate if viewports and windows on-site are identical (mind coated windows!) calibration results in an accurate temperature measurement on black bodies emissivity correction ensures accurate temperature during growth on all materials Important: keep reactor windows clean! 11
Pre-calibration Black body calibration calibration report various temperature steps between 400 C and 950 C are measured assuring linearity in the whole temperature range 12
Pre-calibration Black body calibration provides correct absolute temperature without any additional on-site calibration for the following MOVCD reactors: AIXTRON planetary G3 AIXTRON planetary G4 AIXTRON 200 series 1060 C example: measurement in AIX G3 with BB cal 13
Pre-calibration Black body calibration experience of major LED manufacturer: "black body calibration of several EpiTT and EpiCurveTT systems minimized reactor-toreactor variations to less than 5K in AIXTRON G3/G4 systems" residual temperature variations might even be real important: keep windows clean window cleaning is standard after any hardware equipment change (susceptor / ceiling) 14
Outline motivation pyrometry pre-calibration viewports calibration solutions 15
Pre-calibration How is the situation for CCS/CRIUS systems? AIXTRON planetary G3/G4 AIXTRON CCS/CRIUS narrow tube in showerhead viewport is bigger than beam: - no clipping viewport is clipping beam: - intensity depends on specific viewport 16
Viewports CCS/CRIUS viewports "new" "old" there are different viewport types in the field different aperture positions and sizes providing different intensities during black body calibration type of viewport might be unknown 17
Viewports CCS/CRIUS viewports "new" additional effect: nominal identical viewports have different aperture sizes manufacturing tolerances of diameter is 2.2 mm ± 0.1 mm = 10% variation possible this leeds to significant temperature variations individual on-site calibration needed! re-calibration needed when changing showerhead 18
Viewports CCS/CRIUS viewports experiment: using a black body calibrated EpiTT measuring identical temperature ramps through 5 nominal identical viewports of "new type" comparing to one "old type" viewport without in-situ calibration about 40K variation! 19
Outline motivation pyrometry pre-calibration viewports calibration solutions 20
Solutions LayTec's new solutions for T calibration In-situ on-site calibration is necessary for certain reactor types reference light source for in-situ viewport characterization allows in-situ transmission measurement of each individual viewport easy and quick solution to avoid ring-to-ring and reactor-to-reactor variations 21
Solutions patent pending Reference light source 925 C hot susceptor surface is mimicked by this handheld "cool" reference light source 950nm light source uniform emitting area of 1 cm² equals a temperature of 925 C (black body calibrated) put below showerhead can measure viewport-toviewport variations on-site (ring-to-ring and reactor-toreactor) takes just a few minutes 22
Solutions Reference light source in field test example: AIXTRON planetary reactor 23
Results of field test New method was used in Taiwan In 08/2009 all MOCVD systems in one fab were re-calibrated (all reactors running same process, i.e. produced LEDs have same PL emission wavelength) Planetary MOCVD systems (preliminary results) For most reactors the black body calibration was confirmed within ± 1 K For other reactors (due to coated viewports or inappropriate vertical alignment) the calibration was corrected Result after calibration: true temperature variation between the reactors during MQW growth was 2K 24
Results of field test New method was used in Taiwan In 08/2009 all MOCVD systems in one fab were re-calibrated (all reactors running same process, i.e. produced LEDs have same PL emission wavelength) Planetary MOCVD systems (preliminary results) For most reactors the black body calibration was confirmed within ± 1 K For other reactors (due to coated viewports or inappropriate vertical alignment) the calibration was corrected Result after calibration: true temperature variation between the reactors during MQW growth was 2K 25
Results of field test New method was used in Taiwan CCS MOCVD systems (preliminary results) Reactor-to-reactor variation during MQW growth before calibration: 40K after calibration: 3K NB: For EpiTT/EpiTwinTT/EpiCurveTT and EpiCurveTwinTT systems with halogen light source, an additional heating step to 1000 C is needed. Calibration time per reactor ~ 1.5 hours For newer systems with LED light source the calibrationcan be done at room temperature. Calibration time per reactor ~ 0.5 hours 26
Results of field test New method was used in Taiwan Result of six EpiTT heads residual variations might be real reactors are optimized for uniform MQW temperature growth is optimized for wafer temperature EpiTT measures pocket temperature 1200 1180 1160 1140 1120 1100 1080 1060 1040 1020 1000 980 960 940 920 900 880 860 840 820 800 780 760 740 720 700 680 buffer barrier MQW 1 2 3 4 5 6 27
Temperature calibration New calibration recommendation The reference lightsource will be recommended as standard calibration procedure as soon as additonal devices have been manufactured (work in progress) to equip field service Eutectic run will be obsolete for all GaN systems from then on EpiR DA systems (using 980nm light instead of 950nm) will require a different version of light source Not applicable for AIXTRON 200 series systems, as reactor cannot be opened 28
outline Summary black body calibration is sufficient for some reactors (e.g. AIXTRON planetary G3/G4) reason for variation of reactor-to-reactor and ring-to-ring T variations in CCS / CRIUS was found (viewport tolerances) additional in-situ / on-site calibration needed for CCS / CRIUS eutectic not precise enough new reference light source will allow quick and realiable on-site calibration in almost any type of reactor 29
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