Ultrasonic AFM Sub-Surface Wafer Imaging
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Solution Overview
Problem
Current sub-surface metrology techniques are limited by destructive sample processing, low resolution, and inability to efficiently image large samples, particularly in the semiconductor industry, where high throughput and minimal contamination are required for nanoscale features buried deep beneath the surface.
Innovation Solution
A scanning probe microscope system that uses a cantilever with a tip to couple ultrasonic energy into samples, allowing for non-destructive imaging with high resolution and minimal contamination, featuring a translation stage for rapid positioning of the ultrasonic source and a beat signal detector to measure ultrasonic energy, and optionally incorporating a piezoelectric transducer for GHz frequency operation and Fresnel lens focusing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If transmission electron microscopy is used for sub-surface imaging, then sub-surface details can be revealed, but the sample requires destructive and time-consuming thinning
Solution Approach 1:
The patent replaces the mechanical thinning process with acoustic wave propagation through the intact sample. Ultrasonic waves can penetrate and reflect off sub-surface features without requiring physical sample preparation, thereby maintaining measurement precision while eliminating destructive sample preparation
Solution Approach 2:
The patent introduces an ultrasonic transducer as an intermediary that generates acoustic waves to probe sub-surface features. This intermediary enables non-destructive imaging by using sound wave reflection and transmission characteristics to reveal sub-surface details without direct mechanical contact or sample thinning
2Adaptability or versatility
If acoustic microscopy is used for sub-surface metrology, then a wide range of applications can be covered, but lateral resolution is limited to millimeters to microns
Solution Approach 1:
The patent employs high-frequency mechanical vibrations (ultrasonic waves) to achieve finer resolution. By increasing the frequency of the acoustic waves beyond conventional acoustic microscopy ranges, the wavelength decreases, enabling lateral resolution at the nanoscale while maintaining broad applicability across different materials and applications
Solution Approach 2:
The patent changes key parameters of the acoustic imaging system, specifically operating at higher frequencies and using specialized transducer designs, to achieve nanoscale resolution. This parameter change transforms the system from conventional micron-scale acoustic microscopy to a high-resolution nanoscale imaging tool while preserving versatility
3Measurement precision
If projection x-ray systems are used for sub-surface microscopy, then sub-surface imaging can be achieved, but samples must be diced and thinned, causing damage
Solution Approach 1:
The patent replaces ionizing x-ray radiation with mechanical acoustic waves for sub-surface imaging. This substitution eliminates the harmful effects of high-energy radiation and physical sample damage while achieving comparable or superior sub-surface imaging capability through non-destructive ultrasonic wave propagation and reflection
Solution Approach 2:
The patent converts the potential harm of high-energy x-rays into a beneficial non-destructive imaging method by using acoustic waves. The acoustic waves interact with sub-surface features through reflection and transmission without causing ionization damage, crosslinking, or structural degradation to the sample
4Ease of manufacture
If standard AFM is used for surface imaging, then non-destructive nanoscale imaging can be achieved, but only surface features or features a few nanometers below can be detected
Solution Approach 1:
The patent uses high-frequency ultrasonic vibrations to extend the detection capability of AFM beyond the surface. The vibrational energy penetrates deeper into the sample, allowing the probe to detect acoustic wave reflections from features buried tens to hundreds of nanometers below the surface while maintaining non-destructive imaging
Solution Approach 2:
The patent merges the advantages of AFM (non-destructive imaging, nanoscale resolution) with acoustic microscopy (deeper penetration capability). By combining the sharp probe of AFM with ultrasonic wave generation and detection, the system achieves both non-destructive operation and enhanced detection depth for sub-surface features
5Measurement precision
If SNFUH technique is used for sub-surface imaging, then nanoscale resolution can be achieved, but the technique is not scalable to wafer-scale samples
Solution Approach 1:
The patent segments the large wafer into smaller regions that can be imaged with nanoscale resolution. By systematically scanning and stitching together multiple high-resolution images of smaller areas, the system achieves wafer-scale coverage while maintaining nanoscale resolution throughout the entire sample area
Solution Approach 2:
The patent introduces dynamic scanning capabilities to the imaging system, allowing the probe and/or sample to move relative to each other. This dynamic operation enables coverage of large wafer areas while maintaining nanoscale resolution by acquiring and assembling multiple high-resolution images from different positions
6Measurement precision
If SNFUH technique is used for sub-surface imaging, then high resolution can be achieved, but bonding ultrasonic actuators to each sample is required, reducing throughput
Solution Approach 1:
The patent extracts the ultrasonic actuation function from the sample-specific domain and implements it in the instrument domain. By placing the ultrasonic transducer in the measurement instrument rather than bonding it to each sample, the system eliminates the time-consuming bonding and debonding processes while maintaining high imaging resolution and achieving industrial throughput
7Measurement precision
If SNFUH technique is used for sub-surface imaging, then nanoscale features can be detected, but backside particle contamination exceeds industry limits
Solution Approach 1:
The patent replaces mechanical contact between the sample holder and wafer with non-contact acoustic wave generation. By using acoustic waves to probe sub-surface features without physical contact, the system achieves feature detection capability while eliminating particle contamination from mechanical handling and contact
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 imaging of sub-surface features with resolution exceeding 20 nm laterally and 1 nm vertically, facilitating imaging over arbitrary sample sizes with improved throughput and minimal contamination, overcoming the limitations of existing techniques.
Implementation Method 1
incorporating a piezoelectric transducer for GHz frequency operation
Implementation Method 2
a cantilever with a tip to couple ultrasonic energy into samples
Implementation Method 3
Fresnel lens focusing
Implementation Method 4
a beat signal detector to measure ultrasonic energy
Data Source
AI summary
A method, and corresponding apparatus, of imaging sub-surface features at a plurality of locations on a sample includes coupling an ultrasonic wave into a sample at a first lateral position. The method then measures the amplitude and phase of ultrasonic energy near the sample with a tip of an atomic force microscope. Next, the method couples an ultrasonic wave into a sample at a second lateral position and the measuring step is repeated for the second lateral position. Overall, the present system and methods achieve high resolution sub-surface mapping of a wide range of samples, including silicon wafers. It is notable that when imaging wafers, backside contamination is minimized.