Circular Waveguide Microwave Measurement for Wafer Mobility
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Solution Overview
Problem
Existing microwave instruments for measuring sheet resistance in GaAs wafers with thin cap layers are unable to accurately separate the properties of multiple conducting layers, requiring low-temperature measurements and complex calibration, and often involve destructive techniques that damage the wafers.
Innovation Solution
A nondestructive measurement system using a circular waveguide configured to transmit only the TE11 mode of microwave power, with a magnetic field applied perpendicular to the wafer, allowing for separate detection of sheet resistance and Hall effect signals, enabling accurate measurement of mobility and sheet charge density at room temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional X-band waveguide configuration with 300 ohms characteristic impedance is used, then measurement of sheet resistance can be performed, but accurate measurement of wafers with thin cap layers is not permitted
Solution Approach 1:
The patent changes the operating frequency parameter from conventional X-band to a different frequency range, and modifies the waveguide characteristic impedance from 300 ohms to a optimized value. These parameter changes enable the measurement system to accurately characterize thin cap layer wafers while maintaining measurement precision for sheet resistance.
2Measurement precision
If conventional measurement systems are used, then sheet resistance measurement is possible, but separate measurement of multiple conducting layers cannot be performed
Solution Approach 1:
The patent applies segmentation by separating the measurement of different conducting layers through frequency-domain analysis. The system measures impedance at multiple frequencies and uses mathematical decomposition to extract the individual properties of each conducting layer, enabling separate characterization of the cap layer and channel layer in HEMT wafers.
3Measurement precision
If conventional systems are used, then measurement can be performed, but low-temperature measurements are required to increase channel-layer mobility
Solution Approach 1:
The patent changes the measurement approach by using frequency-dependent impedance analysis combined with mathematical modeling to extract mobility parameters. This eliminates the need for low-temperature measurements, as the system can accurately determine channel-layer mobility at room temperature through the relationship between measured impedance and theoretical models.
4Measurement precision
If conventional microwave instruments are used, then sheet resistance measurement is possible, but involved calibration procedures are required
Solution Approach 1:
The patent implements self-service calibration by using the measurement system itself to determine calibration parameters. The system performs automatic calibration by measuring known standards and using the measured data to establish calibration curves, eliminating the need for complex manual calibration procedures while maintaining measurement accuracy.
5Measurement precision
If destructive techniques are used, then sheet resistance and mobility can be measured, but the wafers or materials are damaged
Solution Approach 1:
The patent replaces destructive mechanical or chemical measurement techniques with non-contact microwave impedance measurement. The system uses electromagnetic fields to probe the electrical properties of the wafer without physical contact or material removal, completely eliminating wafer damage while providing accurate sheet resistance and mobility measurements.
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
Enables accurate, nondestructive measurement of mobility and sheet charge density in semiconductor wafers and flat panel displays, improving measurement precision and eliminating the need for low-temperature operations and destructive techniques.
Implementation Method 1
a circular waveguide configured to transmit microwaves received from the microwave source to a conductive sheet material
Implementation Method 2
Perpendicular to the plane of the wafer (and along the axis of the waveguide), a variable magnetic field is applied. In this configuration, a given incident TE11 wave will cause two reflected waves. One is the ordinary reflected wave in the same polarization as the incident one. This is used to measure sheet resistance. The other reflected wave is caused by the Hall effect. Its polarization is perpendicular to the former wave
Data Source
AI summary
An apparatus (10) for contactless measurement of sheet charge density and mobility includes a microwave source (16), a circular waveguide (50) for transmitting microwave power to a sample (59), such as a semiconductor wafer or panel for flat panel displays, at a measurement location, a first detector (18) for detecting the forward microwave power, a second detector (23) for detecting the microwave power reflected from the sample, and a third detector (95) for detecting the Hall effect power. An automatic positioning subsystem (700) is also provided for allowing automatic positioning of a wafer (59) within the test apparatus (10). The positioning system (700) includes a first end effector (706) and a rotator-lifter (704). The first end effector (706) can grasp a sheet element (59) and move it to a desired position within the test apparatus (10), while the rotator lifter (704) provides incremental adjustment of a theta angle of the sheet element (59) to allow automated mapping of an entire sheet element without the need for manual adjustment of the position of the sheet element. A second end effector (716) can be mounted opposite the first end effector (706) and can be used to automatically position the sheet element (59) within a sheet resistance testing module (718) located at an opposite end of the apparatus (10).