Wafer Temperature Measurement Research Articles

Research on the Dynamic Range of Silicon Photodiodes for Optical Pyrometry Applications

Optical pyrometry is one of the main non-contact methods for precise temperature measurement of semiconductor wafers for vapour-phase epitaxy from metal-organic compounds (MOCVD). The requirements for the photocell of the pyrometer are due to the peculiarity of the process. In the pyrometer, the silicon photodiode operates in a mode that is characterized by a small bias voltage value, high sensitivity to weak light radiation, and low noise level. The main temperatures used in vapour-phase epitaxy technology depend on the semiconductor material being grown and the process parameters. Typically, process temperatures range from 500 to 1200 °C. A study of the dynamic range of a silicon photodiode for use in optical pyrometry was conducted. It was established that the minimum value of the dark current and the maximum value of the spectral sensitivity are key to obtaining the desired characteristics, namely, sensitivity to thermal radiation at a temperature of 450 °C. The peculiarities of the manufacturing technology of the planar-diffusion structure of the photodiode to achieve the necessary characteristics that ensure the production of photodiode structures with improved parameters are also considered.

(Invited) Plasma Processing with Feedback Control of Wafer Temperature By Non-Contact Temperature Measurement System

Plasma processes such as deposition and etching, trimming, are required for nanoscale devices which are Fin-field effect transistors (FETs), nanowires, and others 3D devices. Control plasma processes according to the plasma internal parameters, which define the process characteristics directly, are effective because the parameters can vary over process time even if external parameters are fixed. Significant one of the parameters is the wafer temperature for the precise etching process because the etched profile is sensitive to the temperature [1]. Additionally, Increasing the wafer temperature by heat flux from a plasma are considerable to be one of the most important disturbances in the etching process because the recombination coefficients and sticking probability of active species on the surface depend significantly on surface temperature. The temperature monitoring system with high precision had been required to develop the process characteristics. Many methods of measuring substrate temperatures during plasma processes were proposed. However, there remain difficulties such as their poor tolerance to mechanical disturbances, resulting in limited resolution and temperature ranges when monitoring silicon wafers. We developed a real-time wafer temperature measurement system by an autocorrelation-type frequency-domain low-coherence interferometry (ACT-FD-LCI) system [2-4]. Since the wafer temperature is rapidly increased by energetic ion bombardments and radiative heat-fluxes from the plasma during processes, the surface reaction mechanism also varies with process time. To suppress the temperature variation during processes, a feedback control system of the wafer temperature was constructed. Controlling the plasma excitation and switching the biasing power “ON” and “OFF” enabled the actual temperature to be maintained within a range of ±1.2 °C from the set temperature.[5,6] This system revealed that the profile evolutions of anisotropic organic pattern etching by a H2/N2 plasma were temporally separated into vertical and lateral directions. Subsequently, trimming process of etched organic were performed without supplying bias power, while the substrate temperature was precisely controlled within ±0.7 °C of a set temperature due to lower heat flux from the plasma. The free-standing feature of organic material, with a width narrower than 12 nm, was fabricated in a self-limiting manner. [7] We expect that this self-limiting process could be applied to various advanced processes. References; [1] H. Yamamoto, H. Kuroda, M. Ito, T. Ohta. Hori, Jpn J Appl Phys, 51, 016202 (2012). [2] T. Tsutsumi, T. Ohta, K. Ishikawa, K. Takeda, H. Kondo, M. Sekine, M. Hori, M. Ito, Appl. Phys. Lett., 103, 182102 (2013). [3] T. Tsutsumi, T. Ohta, K. Ishikawa, K. Takeda, H. Kondo, M. Sekine, M. Hori, M. Ito, Jpn J Appl. Phys., 54, 01AB03 (2015). [4] T. Tsutsumi, T. Ohta, K. Ishikawa, K. Takeda, H. Kondo, M. Sekine, M. Hori, M. Ito, Appl. Opt., 54, 7088 (2015). [5] T. Tsutsumi, Y. Fukunaga, K. Ishikawa, K. Takeda, H. Kondo, T. Ohta, M. Ito, M. Sekine, M. Hori, IEEE Trans. Semicond. Manufact., 28, 515 (2015). [6] Y. Fukunaga, T. Tsutsumi, K. Takeda, H. Kondo, K. Ishikawa, M. Sekine, M. Hori, Jpn. J. Appl. Phys. 56, 076202 (2017). [7] Y. Fukunaga, T. Tsutsumi, H. Kondo, K. Ishikawa, M. Sekine, M. Hori, Jpn. J. Appl. Phys. 58, 020906 (2019).

In Situ Solar Wafer Temperature Measurement during Firing Process via Inline IR Thermography

Herein, an inline IR thermography system as an innovative application for real‐time contactless temperature measurement of wafers—both metallized and nonmetallized—during the firing process is successfully realized in an industrial firing furnace as proof of concept and example for a thermography system in a conveyor furnace. As observed by the new system, thermocouples (TCs) seem to measure lower temperature on wafers—especially in combination with TC frames—than wafers exhibit at standard firing conditions (here up to ΔT ≈ 40 K). Furthermore, highly resolved spatial temperature distribution can be successfully measured on the wafer.

Rapid and low power laser actuation of sputter-deposited NiTi shape memory alloy (SMA) MEMS thermal bimorph actuators

NiTi SMA thermal bimorph actuators have potential as high-force, high-displacement MEMS actuators. Historically, even microscale SMA actuation response has been limited to a maximum of ˜100 Hz. As NiTi film and device dimensions are scaled down into the micro and nano scales, heat transfer, and thus device cycling speeds can be significantly improved upon. We have used an in-situ annealed nickel-titanium (NiTi) shape memory alloy (SMA) sputter deposition process to sputter equiatomic NiTi films at 600 °C. We characterized our thin film (270 nm NiTi – 1.6 μm NiTi) material and verified its reversible shape memory effects (SME) using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and wafer bow versus temperature measurements. Upon release, the NiTi material exhibited a reversible phase change around 60 °C with a hysteresis of ˜3 °C. For the substrate confined case (i.e. NiTi on Si or Pt), hysteresis was much larger (i.e. ˜40 °C) with the phase change completed at ˜80 °C. In addition to SMA material characterization, we fabricated NiTi/Pt bimorph actuators at several (100 nm to 1.2 μm) NiTi and Pt thicknesses. Free-standing bimorph actuators were produced via a dry etch release, and temperature dependent curvature of these cantilevers was investigated. To address the low power, and high response time aspects, we performed a dynamic characterization using a 440 mW, 532 nm “green” laser to irradiate devices from 2 to 24 W/cm2 while measuring actuation response times that varied from 3 ms at the highest irradiation fluxes to 240 ms at the lowest. Our results showed decreased actuation powers and faster heating or actuation times compared to past works with NiTi microactuators. The NiTi films with 600 nm thickness on top of 20 nm Pt films exhibited the greatest change in curvature from 200 μm to flat states, and actuated in under 3 ms due to the very small volume of SMA requiring to be heated. These results suggest that NiTi/Pt bimorphs have potential applications as lower-power, faster-response, laser-activated micro shutters or thermal switches without needing a traditional wired power source.

Advanced in-situ control for III-nitride RF power device epitaxy

In this contribution, the latest improvements regarding wafer temperature measurement on 4H-SiC substrates and, based on this, of film thickness and composition control of GaN and AlGaN layers in power electronic device structures are presented. Simultaneous pyrometry at different wavelengths (950 nm and 405 nm) reveal the advantages and limits of the different temperature measurement approaches. Near-UV pyrometry gives a very stable wafer temperature signal without oscillations during GaN growth since the semi-insulating 4H-SiC substrate material becomes opaque at temperatures above 550 °C at the wavelength of 405 nm. A flat wafer temperature profile across the 100 mm substrate diameter is demonstrated despite a convex wafer shape at AlGaN growth conditions. Based on the precise assignment of wafer temperature during MOVPE we were able to improve the accuracy of the high-temperature n-k database for the materials involved. Consequently, the measurement accuracy of all film thicknesses grown under fixed temperature conditions improved. Comparison of in situ and ex situ determined layer thicknessess indicate an unintended etching of the topmost layer during cool-down. The details and limitations of real-time composition analysis for lower Al-content AlGaN barrier layers during transistor device epitaxy are shown.

Measurement and thermal modeling of sapphire substrate temperature at III-Nitride MOVPE conditions

Growth rates and alloy composition of AlGaN grown by MOVPE is often very temperature dependent due to the presence of gas-phase parasitic chemical processes. These processes make wafer temperature measurement highly important, but in fact such measurements are very difficult because of substrate transparency in the near-IR (~900nm) where conventional pyrometers detect radiation. The transparency problem can be solved by using a mid-IR pyrometer operating at a wavelength (~7500nm) where sapphire is opaque. We employ a mid-IR pyrometer to measure the sapphire wafer temperature and simultaneously a near-IR pyrometer to measure wafer pocket temperature, while varying reactor pressure in both a N2 and H2 ambient. Near 1300°C, as the reactor pressure is lowered from 300Torr to 10Torr the wafer temperature drops dramatically, and the ∆T between the pocket and wafer increases from ~20°C to ~250°C. Without the mid-IR pyrometer the large wafer temperature change with pressure would not have been noted. In order to explain this behavior we have developed a quasi-2D thermal model that includes a proper accounting of the pressure-dependent thermal contact resistance, and also accounts for sapphire optical transmission. The model and experimental results demonstrate that at most growth conditions the majority of the heat is transported from the wafer pocket to the wafer via gas conduction, in the free molecular flow limit. In this limit gas conductivity is independent of gap size but first order in pressure, and can quantitatively explain results from 20 to 300Torr. Further analysis yields a measure of the thermal accommodation coefficients; α(H2) =0.23, α(N2) =0.50, which are in the range typically measured.

Wavelength dependence for silicon-wafer temperature measurement by autocorrelation-type frequency-domain low-coherence interferometry.

We investigate silicon wafer temperature measurement characteristics based on optical low-coherence interferometry by altering the light source wavelength. Variations in Si wafer optical thickness with temperature are expressed by thermal expansion and the refractive index. The optical characteristics determine the measurement precision and range. In this study, the measurement precision and the measurable temperature range were evaluated for three wavelengths: 1040, 1310, and 1550nm. The maximum measurable temperature at 1040nm was the lowest because of signal light absorption caused by fundamental interband absorption. The measurement precision at 1040nm was the highest at 0.020°C because optical thickness changes per degree C increase with decreasing wavelength.

Non-contact temperature measurement of silicon wafers based on the combined use of transmittance and radiance

We propose a non-contact temperature measurement method that combines the temperature dependence of transmittance below 600°C and radiation thermometry above 600°C. The combined method uses a polarization technique and the Brewster angle between air and a dielectric film such as SiO2 or Si3N4 grown on silicon wafers. A prominent feature of this method is that both measurements of transmittance and radiance are performed with the same geometrical arrangement.For a semitransparent wafer, the measurement of p-polarized transmittance at the wavelengths of 1.1, 1.2 and 1.3μm enables temperature measurement in the range from room temperature to 600°C. For an opaque wafer above 600°C, the p-polarized radiation thermometry at the wavelength of 4.5μm allows the temperature measurement without the emissivity problem. The combined method with the use of transmittance and radiance is valid in the entire temperature range irrespective of variations of film thickness and resistivity.

2″‐4″ diameter GaN‐on‐sapphire substrates free of wafer bow at all temperatures

AbstractA heteroepitaxial GaN‐on‐sapphire template growth strategy is presented that eliminates wafer bow in the final structure at both room temperature and at high crystal growth temperatures. The technology is demonstrated here on 2″ and 4″ substrate sizes with GaN epitaxy thicknesses up to 150 µm with a sub‐nm, epi‐ready, final surface roughness. Wafer bow vs. temperature measurements for (a) 15 µm of GaN on 4″ 650 µm thick sapphire showing 280 µm bow shift, (b) 10 µm GaN film on 2″ 430µm thick sapphire showing 120 µm bow shift, and (c) and (d), 15 µm of FLAAT GaN on 2″ and 4″ sapphire, respectively, each showing less than 30 µm of bow shift. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature mapping using single wavelength pyrometry during epitaxial growth

Epitaxial substrate temperature remains one of the most important parameters to control during deposition for the production of heterostructures with a high level of quality and uniformity. To this end, full wafer temperature measurement offers a powerful way to obtain information on the epitaxial process. This article presents a simple method to obtain in situ temperature mappings over a 4 in. wafer using a low-cost commercially available CCD camera as a single wavelength pyrometer in the near-infrared wavelength range. By correlating this pyrometric measurement with an independent temperature measurement, the emissivity at a single point can be extracted during epitaxial growth and is then used to correct temperature mappings under certain conditions.

マイクロ温度センサを用いたIn-Situウェハ温度計測制御 : 第2報,MEMS技術を応用したセンサの試作(機械力学,計測,自動制御)

The precise in-situ measurement of wafer temperature has not been achieved in the semiconductor processes of 200[℃] or less such as photolithography and dry etching. The post exposure bake (PEB) sensitivity is well known as one of the main factors to critical dimension (CD) non-uniformity in photolithography. For that reason, we proposed an in-situ measurement and control method of wafer temperature based on micro area contact to a processing wafer using some micro sensors. The 1st report showed the effectiveness of the proposed method, in-situ measurement accuracy 0.1[℃] etc. from experimental results using a large model of the micro sensor. This paper shows the fabrication of 650×650×280[μm^3] sensor chip based on MEMS technology and its characteristics. The process flow of the fabrication consists of thermal insulation structure formulation using permanent photoresist, half cut dicing and polishing of Si substrate to 10[μm] or less in thickness, through-vias formulation into thermal insulation structure using Ag nano paste, and so on. The measurement accuracy of the sensor chip is 0.03[℃], and it is high enough as a resistance temperature detector. In addition, the mounting method of the micro sensor in a plate surface using the nanomembrane of 200[nm] in thickness is proposed for the purpose of its protection against cleaning of the plate surface at maintenance, suppression of particle generation from it, and so on. The proposed mounting method will be able to improve resistance to trouble without degradation of in-situ measurement accuracy.

Wafer heating mechanisms in a molecular gas, inductively coupled plasma: in situ, real time wafer surface measurements and three-dimensional thermal modeling

The authors report measurements and modeling of wafer heating mechanisms in an Ar∕O2 inductively coupled plasma (ICP). The authors employed a commercially available on-wafer sensor system (PlasmaTemp™ developed by KLA-Tencor) consisting of an on-board electronics module housing battery power and data storage with 30 temperature sensors embedded onto the wafer at different radial positions. This system allows for real time, in situ wafer temperature measurements. Wafer heating mechanisms were investigated by combining temperature measurements from the PlasmaTemp™ sensor wafer with a three-dimensional heat transfer model of the wafer and a model of the ICP. Comparisons between pure Ar and Ar∕O2 discharges demonstrated that two additional wafer heating mechanisms can be important in molecular gas plasmas compared to atomic gas discharges. The two mechanisms are heating from the gas phase and O-atom surface recombination. These mechanisms were shown to contribute as much as 60% to wafer heating under conditions of low bias power. This study demonstrated how the “on-wafer” temperature sensor not only yields a temperature profile distribution across the wafer, but can be used to help determine plasma characteristics, such as ion flux profiles or plasma processing temperatures.

In-Situ Measurement and Control of Wafer Temperature Using Micro Sensor (1st Report, An Experimental Study by Using Large Model)

In-Situ Measurement and Control of Wafer Temperature Using Micro Sensor (1st Report, An Experimental Study by Using Large Model)

Optical measurements of silicon wafer temperature

An optical technique for precise, non-contact, and real time measurement of silicon wafer temperature that uses the polarized reflectivity ratio R p / R s is described. The proposed method is based on temperature dependence of the optical functions of silicon. Expected strong temperature sensitivity is obtained near band gap. Simultaneous monitoring of temperature and oxide layer thickness is discussed using measurements at four wavelength 365 nm, 405 nm, 546 nm, and 820 nm.

Full Field Temperature Measurement of Specular Wafers During Rapid Thermal Processing

Many of the processes involved in the creation of semiconductor devices involve high-temperature processing of silicon wafers. The benefits of reduced thermal budget and faster cycle time make rapid thermal processing (RTP) a possible key technology for semiconductor manufacturing. However, the problem of nonuniform wafer temperature has prevented it from further spread among the industry. The first step in developing controls to maintain a uniform wafer temperature is accurate temperature measurement during processing. In this paper, a system was developed to exploit the specular reflectivity of silicon wafers and obtain a measurement of the wafer temperature profile. The spectral reflectivity is determined by measuring the intensity of an incident beam and the beam reflected from the wafer surface. With this measured reflectivity value the spectral-directional wafer emissivity was determined using Kirchhoff's law. The obtained emissivity then was used to calculate the wafer temperature profile from an image obtained with an infrared camera. An experimental study of the transmittance of an undoped silicon calibration wafer at an elevated temperature is also discussed

Measurement and modeling of time- and spatial-resolved wafer surface temperature in inductively coupled plasmas

The transient temperature profile across a commercial wafer temperature sensor device in an inductively coupled Ar plasma is reported. The measured temperatures are compared to model predictions, based on a coupled plasma-wafer model. The radial temperature profile is the result of the radial profile in the ion energy flux. The ion energy flux profile is obtained by combining the Langmuir probe measurement, the ion wall flux probe measurement, and a plasma model. A methodology to estimate the ion flux profile using the sensor measurements has been validated by combining the plasma measurements, the wafer temperature measurements, and the plasma-wafer model. It is shown that with minimal heat transfer between the wafer and the chuck, the initial transient wafer temperature profile after plasma ignition can be used to estimate the ion energy flux profile across the wafer.

In situ measurement of wafer temperature using two sensors with different dynamical properties

As more and more features are packed onto smaller silicon chips, there is a need to optimize wafer fabrication process conditions to the fullest. Closed-loop control is one approach for meeting the rising need for more stringent control over the absolute and spatial temperature of wafers undergoing the post-exposure bake (PEB) process. For an automatic control loop to be implemented successfully, the feedback signals must be accurate and reliable. However, in situ temperature measurement systems based on either contact or non-contact sensors cannot achieve the required accuracy. Another challenge is to measure the wafer temperature without disrupting the manufacturing process. In this paper, a non-intrusive method that uses the readings from two contact sensors and the recursive least-squares algorithm to estimate the sensor model is described. When the sensor model is identified, the inverse model method is applied to achieve fast and accurate temperature measurements. Experimental results demonstrate that the algorithm is able to provide accurate real-time temperature measurement, which may be used as the feedback signal to achieve closed-loop control.

Investigation of Lightpipe Volumetric Radiation Effects in RTP Thermometry

A major obstacle to the widespread implementation of rapid thermal processing (RTP) is the challenge of wafer temperature measurement. Frequently, lightpipe radiation thermometers are used to measure wafer temperatures in RTP reactors. While the lightpipe distorts the wafer temperature profile less than temperature measurement techniques which require physical contact, the presence of the lightpipe influences the wafer temperature profile. This paper presents the results of a theoretical study exploring that influence for an idealized RTP reactor in which the wafer is treated as a nonconducting, opaque, constant-heat-flux surface imaged by the lightpipe. The coupled radiation/conduction transport in the lightpipe measurement enclosure is solved numerically. Radiation transfer in the system is modeled with varying levels of rigor, ranging from a simple volumetrically nonparticipating treatment to a full spectral solution of the radiative transfer equation. The results reveal a rather significant effect of the lightpipe on the wafer temperature, which depends on the separation between the lightpipe tip and the wafer. The study illustrates clearly the need to model the lightpipe as a volumetrically participating, semitransparent medium, and further, the importance of accounting for spectral variation of the lightpipe properties in the prediction of the radiative transfer. Finally, two primary mechanisms are identified by which the lightpipe affects the wafer temperature distribution.

Use of SiC band gap temperature dependence for absolute calibration of emissivity corrected pyrometers in III-nitride MOVPE

In this paper we will demonstrate a new method for temperature calibration by using the in-situ measured band-gap shift of SiC in conjunction with real-time emissivity corrected pyrometry. The complete procedure for temperature calibration and real-time wafer temperature measurement on transparent substrates will be presented.

Automated emissivity corrected wafer-temperature measurement in Aixtrons planetary reactors

A procedure and set-up for high-precision determination of the true surface temperature of wafers during growth by metalorganic vapour phase epitaxy is described. The reflectance of the surface measured by an EpiRAS ® sensor is used for correcting the signal of a pyrometer for changes in emissivity. This allows for determination of the true surface temperature with a precision of ±1°C even for multilayer structures. This high precision allows to determine differences in the temperature of wafers of different size or rotating at different speed in an Aixtron Planetary Reactor ®.