Temperature calibration - Laytec

Temperature calibration 

Dr. Kolja Haberland 

Head of customer support 

18 October 2009

Outline 

• motivation 

• pyrometry 

• pre-calibration 

• viewports 

• calibration 

• solutions 

18 October 2009 Temperature calibration 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 

18 October 2009 Temperature calibration 

3

Motivation: temperature effects 

Vertical temperature profile of a III-N reactor 

2...20K 

20...50K 

>300K 

temperature 

vertical position in reactor 

reactor upper boundary 

gas 

wafer 

satellite susceptor 

main susceptor 

heater 


4

Motivation: temperature effects 

Temperature access in III-N processes 

EpiCurve, Pyro400 (T wafer) 

EpiTT, EpiCurveTT (T pocket) 

thermo couple, 

light pipe 

reactor upper boundary 

wafer 

satellite susceptor 

main susceptor 

heater 

gas 


5

Motivation 


• 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 


• pyrometry 


• viewports 


• solutions 


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 

emissivity corrected temperature 

conservation of energy: 

α + r + t = 1 

opaque semiconductor: 

α + r = 1 

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 


• pyrometry 


• viewports 


• solutions 


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


Black body calibration 

• various temperature steps between 

400°C and 950°C are measured 

assuring linearity in the whole 

temperature range 

calibration report 


12



provides correct absolute 

temperature without any 

additional on-site calibration for 

the following MOVCD reactors: 

• AIXTRON planetary G3 

• AIXTRON planetary G4 

• AIXTRON 200 series 

example: 

measurement in AIX G3 with BB cal 

1060°C 


13



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 


• pyrometry 


• viewports 


• solutions 



How is the situation for CCS/CRIUS systems? 

AIXTRON planetary G3/G4 AIXTRON CCS/CRIUS 

viewport is bigger than beam: 

- no clipping 

narrow tube 

in showerhead 

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 


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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 


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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 


• pyrometry 


• viewports 


• solutions 


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



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



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



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 


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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 


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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

Some gems 

need a little 

extra help to 

sparkle 

www.laytec.de

Temperature calibration - Laytec

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