Hybrid Control System for Semiconductor Substrate Heating
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Solution Overview
Problem
Current methods for heating semiconductor substrates face challenges in accurately controlling temperature due to limitations in contact thermocouples, such as time lag and temperature offset, and inaccuracies with pyrometers caused by changing emissivity, making it difficult to efficiently heat substrates to a desired temperature without requiring expensive calibration processes.
Innovation Solution
A hybrid control system that employs open loop control to rapidly heat a substrate to a threshold temperature, followed by closed loop control to maintain the target temperature, using a combination of contact thermocouples and pyrometers, with calibration techniques to correct for temperature offset and time lag, and the use of inverse low pass filters to compensate for sensor delays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If contact thermocouples are used to measure workpiece temperature, then temperature measurement is possible, but time lag and temperature offset reduce measurement precision and control accuracy
Solution Approach 1:
The system performs preliminary heating in open-loop mode to bring the workpiece close to the target temperature before switching to closed-loop control. This preliminary action reduces the temperature differential that causes time lag in thermocouple measurements, allowing the sensor to track temperature changes more accurately during the critical maintenance phase.
Solution Approach 2:
The control system dynamically switches between open-loop and closed-loop modes based on the heating stage. During rapid heating, open-loop control is used to minimize time loss. When approaching the target temperature, the system transitions to closed-loop control to compensate for thermocouple time lag and offset, optimizing both speed and precision at different phases of the heating process.
2Measurement precision
If pyrometer is used to measure workpiece temperature, then remote measurement is possible, but changing emissivity of silicon causes inaccurate readings over wide temperature ranges
Solution Approach 1:
The system uses a calibration workpiece with known emissivity properties as an intermediary to establish a reference relationship between pyrometer readings and actual temperatures. This calibration creates a mapping that compensates for the workpiece's temperature-dependent emissivity changes, allowing accurate temperature measurement across wide temperature ranges without requiring the pyrometer to directly measure the actual workpiece emissivity at each temperature.
Solution Approach 2:
The control system changes the operating parameters of the pyrometer based on the measured temperature range. By adjusting the pyrometer's emissivity setting according to the calibration data for different temperature ranges, the system maintains measurement accuracy across the full heating range from room temperature to elevated temperatures.
3Measurement precision
If calibration workpiece with permanently attached thermocouples is used for every new workpiece type, then accurate temperature readings are achieved, but the process becomes expensive and time consuming
Solution Approach 1:
The system uses a universal calibration workpiece that can be used across multiple workpiece types and coating combinations. The calibration process measures the thermal properties of the calibration workpiece once, and these calibration parameters are then applied universally to subsequent measurements, eliminating the need to perform expensive and time-consuming calibration for every new workpiece type while maintaining accurate temperature readings.
Solution Approach 2:
Instead of physically attaching thermocouples to each new workpiece, the system creates a thermal model or lookup table based on calibration measurements. This copied thermal characteristic data is then used to interpret pyrometer readings for various workpiece types, replacing the need for physical thermocouple attachment while preserving measurement accuracy.
4Speed
If open loop control is used to heat workpiece rapidly, then heating speed is improved, but temperature control precision deteriorates near target temperature
Solution Approach 1:
The control system dynamically transitions from open-loop to closed-loop control as the workpiece temperature approaches the target. During the majority of the heating process, open-loop control maintains high heating speed. When the temperature nears the target range, the system switches to closed-loop control to provide precise temperature maintenance, optimizing both heating speed and temperature control precision at different phases.
Solution Approach 2:
The system performs preliminary heating using open-loop control to rapidly bring the workpiece close to the target temperature. This preliminary action covers the majority of the temperature range where high heating speed is beneficial. Once the workpiece is near the target temperature, the system transitions to closed-loop control for precise maintenance, ensuring that the transition occurs at the optimal point where speed can be sacrificed for precision.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for faster and more accurate temperature control of semiconductor substrates, reducing heating time and improving throughput by minimizing the impact of temperature sensor limitations, enabling precise temperature maintenance and reducing the need for costly calibration processes.
Implementation Method 1
a heating element; and a controller, in communication with the temperature sensor and the heating element
Implementation Method 2
The first technique is to use of contact thermocouples that are pressed against the workpiece. The thermocouple measures the temperature of the workpiece
Implementation Method 3
The second technique is to use a pyrometer or other infrared sensor to measure the temperature of the workpiece remotely
Data Source
AI summary
A system and method of heating a workpiece to a desired temperature is disclosed. This system and method consider the physical limitations of the temperature device, such as time lag, temperature offset, and calibration, in creating a hybrid approach that heats the workpiece more efficiently. First, the workpiece is heated using open loop control to heat the workpiece to a threshold temperature. After the threshold temperature is reach, a closed loop maintenance mode is utilized. In certain embodiments, an open loop maintenance mode is employed between the open loop warmup mode and the closed loop maintenance mode. Additionally, a method of calibrating a pyrometer using a contact thermocouple is also disclosed.