Improve Temperature Uniformity Research Articles

Temperature Uniformity in Water Films and Wet Paper Through Spectrally Selective Infrared Heating

Experimentally-derived radiant emissions from a near-greybody source, as well as a source with a narrow-band emission centered at 1.5 µm (both gas radiant burners), were used to calculate temperature distributions along the depth of water and wet paper (fiber–water suspensions) films. A grey emitter resulted in a wide temperature gradient through the film depth due to significant overlap of regions of high spectral emission with regions of high spectral absorption. The narrow-band emitting source succeeded in the goal of reducing the temperature gradient though the water film, though a significant percentage of radiant energy transmitted through the entirety of the film. Using numerical optimization, narrow-band spectral emission sources were calculated from Gaussian functions (3.8 µm for water and 1 µm for paper with 80% water) which improved temperature uniformity without high transmission radiation losses.

Improvement of temperature uniformity in microwave‐reheated rice by optimizing heat/hold cycle

AbstractA mathematical model using a finite difference method was established to predict the temperature distribution of a microwave‐heated food tray. The model was linked with an optimization algorithm to minimize temperature non‐uniformity in the microwave‐reheating of a cooked rice package. The mathematical model developed was validated by comparing the predicted and measured temperatures, and was used to obtain the optimum heat/hold cycle. Temperature non‐uniformity was predicted to increase initially with time and then decrease after the surface corner temperature reached around 100 °C. Minimized temperature non‐uniformity was thus attained with the longest heating time, satisfying the highest allowable average temperature under the appropriate number of heat/hold cycles. The control strategy may serve as a useful concept in food service, to improve the heating uniformity of microwaveable dishes.

Postcooking Temperature Changes in Beef Patties

Beef patties (86 and 143 g) formed from high-fat (20 to 29%) and low-fat (6 to 10%) ground beef obtained in eight different selections for both high and low fat content were cooked by either a gas grill or an electric griddle. Patties were cooked to either 66.1 or 68.3°C as determined in the thickest section, and internal temperatures were recorded after cooking at 1-s intervals for 180 s in both thick and thin sections of patties. Time-temperature curves (after cooking) were evaluated for compliance with regulatory requirements for classifying patties as fully cooked. For patties cooked to 66.1°C, the maximum highest temperature in the thickest patty section often did not reach 71.1°C (recommended for cooking of beef patties by consumers). Although thin sections of patties had higher temperatures than thick sections at the termination of cooking, temperature variability was greater and declines in temperature occurred sooner in thin patty sections. Failure to meet fully cooked, time-temperature requirements was greater in thin than thick sections. Thicker (143-g) patties possessed longer post-cooking times before declining in temperature than thinner (86-g) patties. Although many beef patties cooked in this study achieved regulatory time requirements for maintaining 66.1 or 68.3°C (as well as attaining 71.1°C), some patties did not meet these requirements. Because of the considerable temperature variability that can exist within patties at the conclusion of cooking, use of end point temperatures of less than 71.1°C is not recommended for consumers. Consumers should allow several minutes of holding following cooking before consumption to maximize the increases in postcooking temperature. Further research is required to establish cooking procedures that will improve temperature uniformity and eliminate “cold spots” during cooking of beef patties.

Uniform Processing of V-Shaped and Tri-Planar Composite Parts Using Microwaves

Uniform processing of V-shaped and tri-planar graphite/epoxy composite parts has been achieved in a single mode cavity using variable frequency microwave energy. A computer process control system was used to achieve uniform heating and stable curing temperature. The control parameters were microwave frequency and power. The control algorithm was mainly composed of two parts: intelligent mode selection for uniform heating, and power control for constant curing temperature. Experimental results were compared between single mode heating, mode sweeping heating, and intelligent mode switching heating. The comparison showed significant improvement of temperature uniformity using the intelligent mode switching technique.

Diamond synthesis by high-gravity d.c. plasma cvd (hgcvd) with active control of the substrate temperature

In the first part of this paper, subatmospheric CVD methods for diamond thin film synthesis under normal gravity and other conditions are reviewed, following their increased capability for large-scale manufacturing. Emphasis is given to current problems and advances in moderate pressure d.c. plasma CVD operating under normal gravity and high-gravity. In the second part, an experimental technique aimed at solving the problem of temperature uniformity on the substrate surface at high gravity is presented. This technique improved temperature uniformity during diamond deposition at various gravity conditions and allowed a more accurate assessment of the influence of gravity on diamond growth and thin-film morphology.

A rapid micro-polymerase chain reaction system for hepatitis C virus amplification

A rapid micro-polymerase chain reaction (μ-PCR) system was integrated to amplify the complementary DNA (cDNA) molecules of hepatitis C virus (HCV). This system consists of a rapid thermal cycling system and a μ-PCR chip fabricated by MEMS fabrication techniques. This rapid μ-PCR system is verified by using serum samples from patients with chronic hepatitis C. The HCV amplicon of the rapid μ-PCR system was analyzed by slab gel electrophoresis with separation of DNA marker in parallel. The μ-PCR chip was fabricated on silicon wafer and Pyrex glass using photolithography, wet etching, and anodic bonding methods. Using silicon material to fabricate the reaction well improves the temperature uniformity of sample and helps to reach the desired temperature faster. The rapid close loop thermal cycling system comprises power supplies, a thermal generator, a computer control PID controller, and a data acquisition subsystem. The thermoelectric (TE) cooler is used to work as the thermal generator and a heat sink by controlling the polarity of supplied power. The μ-PCR system was verified with traditional PCR equipment by loading the same PCR mixture with HCV cDNA and running the same cycle numbers, then comparing both HCV amplicon in slab gel electrophoresis. The HCV amplicon from the μ-PCR system shows a DNA fragment with an expected size of 145 bp. The background is lower with the μ-PCR system than that with the traditional PCR equipment. Comparing the traditional PCR equipment which needs 5.5 h for 30 cycles to gain the detectable amount of HCV amplicon in slab gel separation, this μ-PCR system takes 30 min to finish the 30 thermal cycles. This work has demonstrated that this rapid μ-PCR system can provide rapid heat generation and dissipation, improved temperature uniformity in DNA amplification.

Selection of measurement locations for the control of rapid thermal processor

Rapid thermal processing (RTP) offers a growing potential in integrated circuit manufacturing as the feature sizes in ULSI move further down into the subhalf micrometer technology. The main problem of RTP control lies in temperature uniformity. In this work, process characteristics of RTP systems are explored and difficulties in control are explained. Two important factors have to be taken into account in the RTP temperature control. They are: temperature uniformity across the wafer and the controllability of the resultant control structure. Measurement selection criteria are devised to achieve both these objectives. The conventional equal-spaced measurement selection criteria in general cannot guarantee temperature uniformity. A nonlinear measurement selection criterion is proposed to overcome this problem. An alternative procedure is also explored to achieve both objectives by varying the temperature set points. Once the control structure is determined, internal model control (IMC) principle is employed for the design of multivariable controllers. As a result of ramp inputs, this leads to a PID type of controller with double integrators: a PI 2D controller. Nonlinear compensation is also implemented to deal with a wide range of set point changes. Simulation results show that a 42% improvement in temperature uniformity can be achieved by using the proposed design procedure.

Inverse-Computation Design of a SiC Bulk Crystal Growth System

ABSTRACTInverse modeling was applied to the optimization of a crucible design for SiC sublimation growth. We found a crucible shape providing the optimal temperature distribution in terms of the powder source stability during long-term operation and of the convex crystal shape. Considerable improvement of temperature uniformity throughout the powder charge was achieved. The results obtained show selective sensitivity of the thermal field inside the crucible to modification of the crucible design. The inverse problem approach is easy-to-adapt to various optimization criteria and seems to be especially effective in the case of multi-factor optimization.

Design for control: Temperature uniformity in rapid thermal processor

It is well known that the design of the heating source imposes an inherent limitation on the performance of the rapid thermal processor (RTP). In this work, the similarities and differences between flat and angled reflectors are studied. Thediscontinuous characteristic of the angled reflector can be used to compensate for the edge heat loss of the thin wafer and, consequently, a better temperature uniformity can be achieved. A design procedure is proposed to place the lamp ring as well as the angle of the reflectors. For the control system design, the measurement selection criterion of Huang et al. is employed to find candidate measurement sets followed by a structured singular value criterion. Once the control structure is determined, multivariable temperature controllers are designed according to the internal model control (IMC) principle. From process insight, a fairly simple nonlinear compensation is also devised. Simulation results show that, while only half of the thermal budget is used, improved temperature uniformity can be obtained by using the proposed approach.

Effect of Buoyancy, Susceptor Motion, and Conjugate Transport in Chemical Vapor Deposition Systems

The fluid flow and heat transfer in the chemical vapor deposition (CVD) manufacturing process are studied numerically. Several crucial aspects such as thermal buoyancy, continuous processing, and conjugate transport are considered. For each aspect, the predicted heat transfer rate and the susceptor temperature are computed and qualitatively linked with the rate and uniformity of film deposition. It is shown that buoyancy effects in helium carrier gas commonly used in diffusion-limited CVD has a negligible effect on deposition rates. Susceptor motion is shown as a feasible alternative to improving the productivity. Conjugate heat transfer effects that arise demonstrate that reactor wall thickness and material may be judiciously chosen to improve temperature uniformity and enhance heat transfer rates, thereby improving deposition rate, film uniformity, and quality.

Element incorporation in GaInAsP for uniform large area MOMBE

In this study we investigated the material incorporation efficiencies in GaInAsP and InAsP layers grown on InP substrates for large area metalorganic molecular-beam epitaxy (MOMBE). We found an optimum growth temperature for the quaternary material ( λ G=1.55 μm) around 500–510°C, since in this range the lattice matching shows a temperature coefficient of about 80–120 ppm only. The P incorporation efficiency is improved with increasing growth temperatures (480–520°C). We found for InAsP and GaInAsP ( λ G=1.05 μm) a strong reduction of the P incorporation efficiency with increasing V/III ratio, which is accompanied by a reduced P content in the layer. Additionally with this reduced P, the Ga incorporation efficiency for the quaternary material is improved. Using the same V/III and As/P ratio for the InAsP and GaInAsP material, a higher P content in the InAsP layers is achieved. The results were used as calibration data for the development of a novel type of wafer holder with improved temperature uniformity. For GaInAsP single layers ( λ G=1.55 μm) a standard deviation in emission wavelength of |Δ λ|<1.5 nm was achieved. From the results for all material compositions a temperature distribution with Δ T<1°C can be inferred across a 2″ wafer.

Transient liquid crystal thermometry of microfabricated PCR vessel arrays

Polymerase chain reaction (PCR) using micromachined structures promises improved temperature uniformity and cycling time together with decreased reagent and sample volumes. Thermal design of these structures will benefit from measurements of the temperature distribution in the reacting liquid. We report measurements of temperature uniformity and time constant in a microfabricated 18-vessel array using encapsulated liquid crystals suspended in the liquid. Separate sets of crystals are used to image temporal and spatial temperature variations near the processing thresholds of 55/spl deg/C and 95/spl deg/C with a resolution of 0.1/spl deg/C. While the thermometry technique developed here is particularly useful for characterizing microfabricated PCR systems, it can also aid with the thermal design of a broad variety of microfluidic devices.

Characterization, modeling, and design of an electrostatic chuck with improved wafer temperature uniformity

The resulting temperature distribution of a silicon wafer held by an electrostatic chuck (ESC) in an electron-cyclotron-resonance chemical vapor deposition (ECR-CVD) reactor is characterized and modeled. The effects of the clamping voltage VESC, pressure between the ESC and wafer PHe, and the surface finish and pattern on the ESC are investigated. Heat transfer coefficients between the wafer and various ESCs are determined experimentally. A model is developed to predict the temperature distribution at the surface of the wafer, and used to explain the experimentally observed temperature variations both within wafer and between different chucks. The model is then used to aid in the design of an ESC which provides improved temperature uniformity at the wafer surface. The results of this study indicate: (a) the thermal resistances across the interface between the wafer and ESC control both the absolute wafer temperature and the wafer temperature uniformity; (b) the surface roughness of the ESC and the size of the ‘‘contact’’ regions are major design factors controlling the absolute temperature of the wafer—the temperature can be adjusted by varying the value of VESC and fine tuned by adjusting the value of PHe; (c) the nonuniform temperature distribution across the wafer surface is dictated by the surface pattern on the ESC, the variation in surface roughness, and the size of the ESC relative to the wafer; (d) wafer temperature variations from chuck to chuck are reduced by controlling the surface finish of the ESC and by ensuring that PHe is a dominant heat transfer mechanism; and (e) maximum uniformity in the temperature of the wafer is obtained when the radius of the ESC is matched as closely as possible to that of the wafer. We have shown that numerical heat transfer models can be used to optimize the geometry of the ESC to provide a uniform distribution of temperature across the surface of the wafer.

A Universal Thermal Module™ for Clusterable RTP and MOCVD Applications

AbstractA Universal Thermal Module™ (UTM™) has been developed for vacuum-integrated cluster RTP and MOCVD as well as stand-alone atmospheric RTP applications. The UTM™ architecture provides highly modular RTP and MOCVD tool configurations for various single-wafer thermal processes. The UTM™ design comprises a temperature-controlled UHV-grade process chamber as well as an ultraclean gearless wafer rotation assembly for improved temperature uniformity and enhanced built-in reliability. The UTM™ RTP employs bottom/backside wafer heating using a multi-zone axisymmetric illuminator and a reflective multi-zone gas showerhead located above the wafer. Precision RTP control is achieved using a multi-zone controller in conjunction with a multipoint sensor system with real-time multi-zone compensations for wafer emissivity and illuminator light interference effects. The RTP temperature measurement and control techniques fully compensate for any axisymmetric or non-axisymmetric wafer emissivity patterns, eliminating the need for pre-RTP wafer backside conditioning. The UTM™ showerhead provides capabilities for ultraclean multi-zone gas injection and in-situ RF plasma for MOCVD and RTCVD processes. The UTM™ MOCVD module meets the most stringent specifications and tool parameters for various metallization and high-K dielectric deposition applications. This module has been used for deposition of low-resistivity copper films with excellent void-free sub-half-micron gap fill for high-performance multilevel interconnects. The UTM™ RTP and MOCVD modules have been implemented based on design optimizations in a virtual reactor environment.

Studies on the Scale-Up of the Microwave-Assisted Nitridation and Sintering of Reaction-Bonded Silicon Nitride

AbstractStudies using laboratory test samples have shown that microwave heating produces sintered reaction-bonded silicon nitride materials with improved properties [1,2]. The final challenge for processing this material by microwave heating is the development of a technology for processing larger batch-size quantities of these materials. Initial microwave scale-up experiments were performed using powder compacts of a bucket tappet geometry. In experiments using microwave-transparent boron nitride sample crucibles, temperature gradients within some crucibles led to larger variations in the sample densities than were obtained with the conventionally processed samples. The use of a microwave-suscepter type crucible made of silicon carbide and boron nitride resulted in an improved temperature uniformity and in density variations comparable to those obtained for the control groups.

Temperature uniformity during hyperthermia: the impact of large vessels

During hyperthermia the presence of a large vessel entering the heated volume and carrying blood at the systematic temperature can be an important source of temperature non-uniformity and possible underdosage. The minimal tumour temperature near a large vessel is determined by the vessel wall temperature: a number of factors influencing the vessel wall temperature are considered-effective tissue conductivity, flow type, vessel size, entrance effects and counter-current flow. In some specific cases, especially when tissue perfusion is high, the vessel wall temperature may reach therapeutic levels when the mean blood temperature is still low. In general, well perfused tumours have a better chance of being heated uniformly. Regional heating improves temperature uniformity by reducing entrance and equilibration effects as blood is heated before entering the tumour. Raising the core temperature also reduces temperature inhomogeneity. Spatial SAR resolution should preferably be of the order of magnitude of a centimetre or better.

Microwave Sintering of Multilayer Ceramic Capacitors

ABSTRACTMultilayer ceramic capacitors of several compositions (both Z5U and NPO types) were sintered in air in both conventional and microwave furnaces. Several casketing and insulation techniques were used to improve temperature uniformity and minimize dopant losses. Zirconia and yttria are suitable packing materials as either powders or fibers. After sintering, the packing material remains friable and can be recycled. The presence of metallization layers did not interfere with the microwave process and no arcing was observed at surfaces where the internal electrodes emerged. In some runs, however, local thermal runaway was observed, which could severely decrease process yields and must be eliminated in a commercial process.

Novel slip-free rapid thermal annealing of GaAs in vacuum with excellent uniformity and reproducibility

This paper describes novel rapid thermal annealing for GaAs wafers under vacuum conditions (VRTA) using a three-zone lamp power control method. The developed RTA technology eliminates generation of crystallographic slip lines and wafer deformation due to the convection effect caused by ambient gas. A three-zone lamp power control method produced excellent uniformity in the activated layer, presenting the best data ever attained by RTA. Also, numerical simulation demonstrates improved temperature uniformity achieved by a three-zone lamp power control method which reduces the edge radiation effect. Moreover, we have found that VRTA technology is particularly effective for annealing large-size GaAs wafers, which are more easily deformed or slip-lined than 2-in. wafers. We have applied VRTA to fabricating ion-implanted n+ contact regions for self-aligned 0.5-μm-gate doped-channel hetero-metal-insulator-semiconductor field-effect transistors with a lightly doped drain, and have obtained excellent Vt uniformity, σVt=19 mV, on a 2-in.-diam wafer. These features, together with a simple wafer-supporting method, using several quartz pins, cause the improved VRTA technology to provide high throughput and production yield for high-performance short-gate GaAs integrated circuits.

Improvement of Temperature Uniformity in Rapid Thermal Processing Systems Using Multivariable Control

AbstractDuring rapid thermal processing (RTP) of a semiconductor wafer, maintenance of nearuniform wafer temperature distribution is necessary. This paper addresses the problem of insuring temperature uniformity in a cylindrical RTP system with multiple concentric circular lamps.A numerical technique is presented for optimizing steady-state temperature distribution by independently varying the power radiated by each lamp. It is shown for a simulated system, over a wide range of temperature setpoints, that the temperature uniformity achievable with multivariable (“multiple knob”) control of lamp powers is significantly better than that achievable with scalar (“single knob”) control.The difficulties of using scalar control in RTP are more severe in the case of temperature trajectory design than in the case of steady-state temperature maintenance. For example, with scalar control the rate of temperature increase during ramping is limited because temperature nonuniformity can cause slip defects in the wafer. A numerical technique is presented for designing multivariable lamp power trajectories to obtain near-optimal temperature uniformity while wafer temperature tracks a specified ramp, resulting in slip-free ramp rates much faster than those achievable with scalar control.

Temperature Uniformity Optimization Using Three-Zone Lamp and Dynamic Control in Rapid Thermal Multiprocessor

AbstractRapid thermal processing (RTP) can play an important role in in situ single-wafer thermal multiprocessing, since it allows for a rapid wafer throughput rate. Conventional dedicated RTP equipment, where temperature uniformity is achieved by optimized reflector and chamber geometries for a specific process, typically cannot provide uniformity for different processes, or for a range of processing conditions. In this work we present a new flexible lamp system, in which tungsten-halogen lamps are configured in three concentric rings that are independently and dynamically controlled. The resultant circularly symmetric flux, which can be varied and controlled both temporally and spatially, offers significantly improved temperature uniformity. This is demonstrated using thermocouples as well as actual processes such as implant annealing, thermal oxidation and chemical vapor deposition of silicon. Through added flexibility and more precise control, this approach offers a powerful tool for multiprocessing and rapid process prototyping.