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Journal Electronic Materials, Vol. 27, No. 12,Temperature 1998 WaferofEmissivity Independent

Measurements

Special Issue1323 Paper

Wafer Emissivity Independent Temperature Measurements 1 S. ABEDRABBO, F.M. TONG,1 N.M. RAVINDRA,1 J. GELPEY,2 S. MARCUS,2 and A.T. FIORY 3

1.—New Jersey Institute of Technology, Newark, NJ. 2.—Steag-AST Electronik USA Inc., Tempe, AZ. 3.—Bell Laboratories, Lucent Technologies Inc., Murray Hill, NJ

A study on the techniques to yield wafer emissivity independent temperature measurements in rapid thermal processing has been presented. This study focuses on the Steag-AST Electronik approach to enhance wafer emissivity by using the Hotliner*. The Hotliner comprises of a heavily doped p-Si substrate sandwiched with Si3N 4/SiO2 from both sides. Experimental measurements on the optical properties of the Hotliner using a spectral emissometer operating in the wavelength range of 1–20 µm are presented here. Results of the simulation of the experimental data using the MIT/SEMATECH Multi-Rad model are discussed. Key words: Emissivity, Hotliner, pyrometer, Si

INTRODUCTION As we approach the 21st century, it appears that single wafer and cluster-based tools will be the manufacturing approach adopted by the silicon device industry. This trend is coupled with the miniaturization of devices. These factors necessitate a rigid, reliable, and reproducible process control and have thus led to novel processes like rapid thermal processing (RTP). For RTP, pyrometers are the instruments of choice for in-situ temperature measurements. Pyrometers measure the amount of radiation emitted, usually, from the backside of a wafer. The ratio of the wafer emitted radiation to that of a blackbody under the same conditions of temperature, wavelength, angle of incidence, and direction of polarization is referred to as emissivity. Emissivity of silicon is a complicated function of both temperature and wavelength.1,2 It is also a function of surface roughness.3,4 This emissivity is referred to as intrinsic emissivity.5 The emission that the pyrometer detects, however, is a function of layers on top of the substrate, the surroundings, and chamber components. This is referred to as effective (Received February 16, 1998; accepted August 10, 1998) * Hotliner is a trademark of Steag-AST Electronik, patent pending.

emissivity. In this paper, a materials based approach that leads to emissivity independent temperature measurements is described. This approach is compared with other similar methodologies that are being developed in the silicon industry. EXPERIMENTAL DETAILS In this study, a spectral emissometer operating in the wavelength range of 1–20 µm and capable of the simultaneous measurement of reflection, transmission, and emission of samples under investigation has been utilized. The methodology deployed by this novel tool has been described in detail in previous papers.6,7 The schematic of the spectral emissometer is presented in Fig. 1. This tool utilizes the reversion theorem or the reciprocity theorem of Helmholtz8 which states that a certain point source p0 as in Fig. 2 will produce at another point p the same effect as a point source of equal intensity placed at p will produce at p0. Considering the schematic of the emissometer as in Fig.1, the source blackbody (BB) represents p0, and the parabolic mirror accounting for the reflection and the transmission paths, individually, constitutes the point p. The emissometer utilizes an extended BB source. Basic optics9 of the ellipsoidal mirror allows the first focus point, where BB is situated, to be considered as p0. The application of the reciprocity 1323

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theorem to the spectral emissometer establishes the validity of this technique with accuracy >99%.

this study. The composition of the Hotliner is as follows: 57 nm Si3N4 /25 nm SiO2 /700 µm p-Si/25 nm

RESULTS AND DISCUSSION The Hotliner, is the sample of choice considered in

Fig. 1. Schematic of bench top emissometer showing components and optical paths for radiance, reflectivity, and transmissivity.

Fig. 2. Illustration of the Helmholtz reciprocity theorem.

Fig. 4. Measured radiative properties of low doped n-silicon at room temperature.

Fig. 3. Experimental optical properties of front side of the Hotliner at three different temperatures: (1) 155, (b) 565, and (c) 954°C.

Wafer Emissivity Independent Temperature Measurements

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Fig. 5. Experimental optical properties of the backside of the Hotliner at three different temperatures: (a) 142, (b) 563, and (c) 942°C.

SiO2/57 nm Si3N4. The substrate is heavily boron doped CZ-Si, > 1 × 1019 cm–3, with resistivity ρ = 0.001Ω.cm. The oxide is grown thermally and the nitride is deposited using standard low-pressure chemical vapor deposition (LPCVD) technique. The radiative properties of this wafer have been studied using the spectral emissometer at temperatures ranging from 155–954°C from the front and backside incidence. Figures 3a–3c show the apparent reflectivity, apparent transmissivity, and emissivity of the Hotliner at three different temperatures of measurements taken from the front side. As can be seen in these figures, the emissivity of the Hotliner is high in the entire spectrum considered, i.e., 1–20 µm, with an average value >0.71. This is typical of an ideal gray body.10 It is not, however, typical for ordinary singleside-polished intrinsic silicon substrate (roughness ≈1 µm) at this low temperature of 155°C. Figure 4 shows the optical properties of a lightly doped silicon sample at room temperature. It may be noted here that in Fig. 3a, there was no heating of the sample except for the exposure to BB source also utilized in obtaining Fig. 4. The high emissivity of Hotliner can be explained as follows. As is well known, in a closed system in thermal equilibrium, conservation of energy states that the emitted and absorbed fluxes are equal. In addition to this, the radiation field in this system is isotropic, implying that the directional spectral emissivity and the directional spectral absorptance must be equal11 as given by Kirchhoff’s law: ε(λ;θ,φ) = a(λ;θ,φ)

(1)

The total absorption coefficient consists of several contributions. It is given by αtotal(λ,T),

Fig. 6. A view of the AST process chamber showing the pyrometer (at the bottom) lamps, quartz tube, wafer, and Hotliner (beneath the wafer).

αtotal(λ,T) = αag(λ,T) + αbg(λ,T) + αfc(λ,T) + αphonon (λ,T)

(2)

where αag(λ,T) is the above fundamental edge absorption, αbg(λ,T) is the fundamental edge absorption, αfc(λ,T) is the free carrier absorption, and αphonon(λ,T) is the phonon absorption. This is valid for a long range of wavelengths, i.e., from 0.4 to 20 µm. The wavelength range of interest in this study is from 1–20 µm.12 In this range of wavelengths, the effect of αag(λ,T) is not seen. The free carrier absorption coefficient in semiconductors, in MKS units, is given by:13 α = (q3λ2p)/(4π2εoc3nm*2 µ)

(3)

where λ is the wavelength, p is the density of free cariers, n is the refractive index, m* is the conductivity effective mass, and µ is the mobility. This absorption is the cause of high emissivity and negligible transmissivity14 observed at low or room temperatures in the silicon substrate. Considering the backside of the Hotliner, as in Fig. 5, a slight increase in emissivity over that when the incidence is on the smooth side is noticed. This is due to the backside

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a

b

c

d

Fig. 7. Simulated radiative properties as function of wavelength (0.4–20 µm) of (a) heavily doped 700 µm thick p-silicon, 1.3 × 1019 cm–3, (b) 25 nm SiO2/700 µm p-silicon, (c) Hotliner - 57 nm Si3N4/25 nm SiO2/700 µm p-silicon, and (d) 77 nm Si3N4/35 nm SiO2/700 µm p-silicon, at 155°C.

roughness.15,16 The high doping in the substrate enhances the absorption mechanism at three regions. These regions comprise of: (a) the band gap by causing degenerate states17 and shrinking the gap, (b) the impurity region from 1.13–1.5 µm, and (c) the free carrier region from 1.5–20 µm. It can be seen from Fig. 3 that the strong effect of the high concentration of free carriers on emissivity suppresses the phonon absorption which usually starts showing at wavelengths longer than 5 µm. The effect of coatings on the heavily doped silicon in the Hotliner is very minimal. It is expected to enhance the emissivity by a very small fraction. This is because of the smaller refractive index for both the oxide and the nitride as compared with silicon. The presence of thin layers leads to decrease in reflectivity at the surface of the two dielectrics and increases the absorption in the bulk of the silicon substrate. The functionality of coatings is to form a buffer layer and a protective shield for the silicon substrate when placed inside the Steag-AST Electronik RTP chamber. Indeed, the Hotliner is meant to enable a wafer emissivity independent temperature measurement. It can also enhance the emissivity of the process wafer in the RTP chamber by being situated at a very short distance (0.99) at a very narrow band at λ = 0.85 µm. This technique utilizes the effective emissivity principle through enhancing the emissivity using a BB type cavity.5 In this configuration, as the wafer is irradiated with photon flux from lamps, it will absorb and emit a radiance equivalent to: M(T) = σεT 4

(4)

where σ is the Stefan-Boltzmann constant, ε is the emissivity of the wafer, and T is the temperature of the wafer at that instance. After the first reflection from the perfect reflector situated beneath the wafer, the wafer radiance becomes: M(T) = σε(1–a)T4

(5)

where a is the amount of light absorbed by the wafer. After n cycles of this reflection and radiation process, and applying Kirchoff’s law as in Eq. (1), the light intensity emitted from the backside of the wafer becomes: M(T) = σε[∑n=0→∞(1–a)n]Τ4 = σε[∑n=0→∞(1–ε)nΤ4 = σεεeffΤ4

(6)

As n approaches infinity, the effective emissivity becomes: εeff = ε/[1 – (1 – ε)] = ε/ε = 1

(7)

Thus, a very high and constant emissivity is achieved independent of the emissivity of the process wafer. AGI has reported that in a range of 0.65 < εeff < 0.95, the temperature reading is within ±1°C. This is indeed a great achievement. AGI has reported that they have calibrated their methodology using TC wafers up to 750°C for RTCVD processes where very high temperatures may not be needed. The AMAT approach of emissivity independent temperature technique utilizes a combination of simple yet effective

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method. A reflector, made from film stacks compatible with silicon processing, of high reflectivity (>0.9) is situated below the wafer. Again, this methodology utilizes the effective emissivity principle. AMAT utilizes several pyrometers situated in small holes in the reflector and the process wafer is rotated to eliminate any unevenly distributed radiative effects on the pyrometer. One concern about all the above techniques is the approach to inclusion of the edges of the process wafer. For device manufacturing purposes, the edge of the wafer contributes to waste material. Despite this, AST and AMD have reported that these effects known as photon box, during ramping up and edge effect during steady state heating, were factored in their calibration technique and the results were very promising. CONCLUSIONS A spectral emissometer operating in a long range of wavelengths has been utilized to measure the optical properties of a 57 nm Si3 N4/25 nm SiO2/700 µm p-Si/ 25 nm SiO2/57 nm Si3 N4. The substrate is a heavily doped CZ-Si with doping concentration >1019 cm–3. The experimental data of the radiative properties shows consistency and repeatability as function of temperature. This agrees with the simulated data obtained by utilizing the MIT/SEMATECH MULTIRAD model. The Hotliner is intended to be an emissivity independent approach to temperature measurements in the RTP chamber. Studies performed at AMD/Steag-AST have shown that, using the Hotliner, repeatability of temperature uniformity from wafer to wafer have shown great improvements. Similar approaches aimed at wafer emissivity independent temperature measurement techniques have been discussed. ACKNOWLEDGMENTS The authors thank DARPA and SEMATECH for their partial financial support.

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