Laser Pump-Probe 3D Thermal Imaging for Directional Conductivity
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Solution Overview
Problem
Traditional non-contact laser pump-probe techniques like steady-state thermoreflectance (SSTR) can only measure harmonic average thermal conductivity and are limited to a single spot, failing to evaluate directional thermal conductivity of structures such as thermal vias and heat spreaders, with a small measured signal (ΔR/R ~ 10E-3).
Innovation Solution
The method extends SSTR for 3D mapping by measuring in-plane and out-of-plane thermal conductivity at specific locations with high spatial resolution, using a thermal measurement system with a pump and probe laser, transducer layers, and anti-reflection coatings to minimize sample damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional SSTR technique is used to measure thermal conductivity, then thermal conductivity can be measured non-contactly, but only harmonic average thermal conductivity is obtained and directional thermal conductivity cannot be evaluated
Solution Approach 1:
The patent transitions from traditional single-spot SSTR measurement to a two-dimensional scanning measurement system. The laser beam is scanned across the sample surface in x and y directions, enabling measurement of thermal conductivity in different spatial directions (in-plane and out-of-plane) and generating two-dimensional thermal conductivity distribution maps, thus resolving the limitation of only obtaining harmonic average values.
Solution Approach 2:
The measurement process is divided into multiple discrete measurement points across the sample surface. By scanning the laser beam to different locations and performing separate measurements at each point, the system obtains directional thermal conductivity data at multiple positions, which are then compiled into comprehensive two-dimensional thermal maps.
2Ease of operation
If traditional SSTR measures single spot thermal conductivity, then measurement is simple, but spatial resolution and 3D thermal imaging capability are insufficient
Solution Approach 1:
The system adds spatial dimensionality to the measurement by implementing two-dimensional scanning of the laser beam across the sample surface. This transforms the measurement from a single-point technique to a multi-point mapping technique, achieving high spatial resolution thermal conductivity distribution and enabling three-dimensional thermal imaging capabilities while maintaining operational simplicity through automated scanning.
3Measurement precision
If pump laser power is increased to improve signal strength, then thermal reflectance change detection improves, but sample damage risk increases
Solution Approach 1:
The patent employs continuous wave (CW) pump laser instead of pulsed laser to provide continuous heating. This allows the system to accumulate thermal effect over time, improving the thermal reflectance signal strength without requiring high peak power that would cause sample damage. The continuous action maintains a steady thermal state suitable for precise measurement.
Solution Approach 2:
The system uses modulated pump laser with specific frequency to induce periodic thermal effects. By operating at optimized modulation frequencies, the system enhances the detectable thermal reflectance signal while controlling the thermal load on the sample, thereby improving signal strength without proportionally increasing damage risk.
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 non-destructive 3D thermal imaging with improved resolution, allowing for precise characterization of heat dissipation in transistors and chip bonding structures, particularly in the back-end-of-line (BEOL) of fabrication.
Implementation Method 1
heating a designated spot of the sample using a pump laser
Implementation Method 2
using a probe laser for generating a reflectance signal of the sample, and determining a thermal reflectance change over time from the reflectance signal
Data Source
AI summary
High resolution 3D thermal imaging can be obtained by using enhanced non-destructive heat transducer designs. A thermal property measurement method includes providing a sample for thermal property measurement, and bonding a transducer layer on the sample through a temporary bonding layer. Thermal measurement processes are performed along the X-Y, X-Z and Y-Z planes of the sample, wherein the X-Y plane is parallel to a top surface of the sample, and the X-Z plane and Y-Z plane are perpendicular to the top surface of the sample. Each thermal measurement processes include heating a designated region of the sample covered with the transducer layer using a pump laser, and using a probe laser for generating a reflectance signal of the sample, and determining a thermal conductivity in the designated region of the sample from the reflectance signal. Furthermore, the transducer layer is removed along with the temporary bonding layer from the sample.