Measuring Objects with High Aspect Ratios
Ever since semiconductor line widths have been becoming narrower, making Moore’s Law no longer applicable, a range of manufacturing methods for stacking semiconductors vertically have been introduced and manufactured. With this introduction of convoluted manufacturing process technologies, maintaining production yields is of utmost concern to manufacturers. As manufacturing processes shift, production yields must keep up—installation of high-performance inspection equipment is therefore vital.
As an example, numerous Through Silicon Via (TSV) holes are used to connect electrical circuits between each layer. These holes are tall, narrow in diameter, and have a large aspect ratio of up to 1 to several tens of times. 3D measurements are therefore essential to determine whether a TSV hole in a structure is completely open, and if not, at what depth it is blocked. To inspect TSV holes in 3D, an objective lens with high lateral resolution and depth of field is required; however, to increase lateral resolution, numerical aperture of the objective lens must also be increased, resulting in an inevitable low depth of field. With this, it becomes impossible to capture and measure the depth of a long hole with a single image. Although depth measurement is feasible to an extent by refocusing using an AI algorithm, to secure a dataset, plenty of images must be acquired and processed by moving and shooting while focusing in the direction of the optical axis.
Still, the depth of field of an objective lens with a lateral resolution of several hundred nanometres is around several hundred nanometres, similar to the lateral resolution. For instance, an objective lens with a lateral resolution of 300mn has a depth of field of about 300mn and a working distance of about 1,300nm. When the hole of the measurement object is 2,000nm, due to working distance limitations of the objective lens, it is only possible to focus down to about 1,200nm of the hole depth and imaging is impossible below that focal plane. Due to this barrier, it is impossible to acquire images of depth or height beyond the working distance of the objective lens, or to create a dataset for AI learning.
Our technology and products use objective lenses with high numerical aperture, low depth of field and working distance for objects with high aspect ratios, and provide three-dimensional depth with just a single-shot image capture.
Lateral Resolution vs. Depth of Field
As previously mentioned, the trade-off between numerical aperture and depth of field has been a persistent challenge in optical systems.
With recent strides in nanomaterial processing and manufacturing technologies, companies are tackling this challenge by developing meta-lenses. Still, because the issue remains unresolved, the process of reprocessing captured images with software is imperative.
A method for increasing digital/numerical lateral resolution of captured images by adopting AI based on various advanced neural networks is also being introduced; nevertheless, AI alone cannot fix the optical lateral resolution issue. Although it is ideal to acquire an image with high lateral resolution using an objective lens with high numerical aperture, and thereafter increase the digital resolution using AI technology, optical resolution is still limited to the numerical aperture of the objective lens. Immersion types of micro-objectives have higher numerical aperture than non-immersion types but cannot be applied to opaque target samples. It cannot exceed approximately 0.95, which is the maximum numerical aperture of a non-immersion type objective lens. Even when an objective lens with a numerical aperture of around 0.95 is used, the depth of field is low, making it difficult to image objects with large height or depth in 3D.
Angstrom Vision’s exclusive technology and products increase the system optics’ depth of field by tens of orders of magnitude, regardless of how high the numerical aperture of the objective lens is. As a result, our 3D microscopes are widely applicable to diverse applications. Technology based on extended depth of field allows for flexibility in objective lens selection and solution provision. Moreover, because the auto-focus function is not necessary within the extended system depth range, purchase costs are cut while scanning speed levels up.
Insensitivity to Vibration, Acoustic Noise, and Air Turbulence
The physical environment of manufacturing industries is starkly different from laboratory environments. Most measurement equipment including high-resolution microscopes, scanning electron microscopes, and atomic force microscopes are used in labs without the disturbance of vibration or noise. In contrast, various equipment installed on semiconductor and display manufacturing lines are operated by motors and pneumatic actuators, which also constantly operate in various transport devices. When these motors or actuators operate, low (lower than 10Hz) or high (over 10Hz to kHz) frequency vibration is set off and acoustic noise is generated. Moreover, complex manufacturing environments producing air currents, like dust-free, humid, or temperature-controlled environments, result in air turbulence.
For the purpose of preventing vibration, high-precision measurement equipment is installed on large, heavy, and flat granite surfaces. To prevent air turbulence, the system is double sealed within the housing to minimize airflow within the measurement equipment. Naturally, equipment design costs are added and manufacturing costs rise. Even if complex factors such as vibration, acoustic noise, and air turbulence are eliminated as much as possible, high-precision 3D measurement microscopes that require measurement precision in the nano- and sub-nanometre units are still extremely sensitive.
The robustness and resilience of our technology and products against environmental stressors are unparalleled. We provide a solution that is immune to low and high frequency vibrational noise via an advanced optical structure design with noise-free concept. This noise-free optical system is applied across products that provide a high measurement range using multiple light sources. Minimal noise that still remains after optical noise removal is eradicated within the error range with our innovative numerical noise processing algorithm and AI de-noiser.
Optimizing Measurement Speed
While microscopes like scanning electron microscope and atomic force microscope have been used for measuring microscopic linewidths in semiconductors, their shortcomings like slow speed and risk of product damage limit realistic applicability in in-line inspection. As such, these microscopes have mainly been used for reviewing purposes of target samples. Meanwhile, automatic optical inspection microscopes have widely been used for in-line inspection of various product manufacturing processes as it offers fast scanning speed. Despite its advantages, the automatic optical inspection microscope also faces drawbacks as a quality inspector for semiconductor manufacturing processes, as semiconductor manufacturing methods are changing to vertical stacking.
In the recent years, leading semiconductor manufacturing companies are competing to reduce the semiconductor line width beyond 2 and 1.5 nanometres to the angstrom level. When inspecting semiconductor wafers manufactured in ultra-fine processes, the field of view is reduced to approximately 1mm x 1mm or several hundred micrometres in horizontal and vertical directions. When inspecting the entire surface of a wafer with a diameter of 300mm, the 3D inspection machine must move at a high speed and scan the entire area to be inspected. Assuming that scanning is performed while shooting 30 images per second, the measurement time takes at least 30 minutes.
To clearly inspect features in the captured field of view, it is essential to implement an optical system that maximizes not only the optical resolution, but also the digital resolution. If the height or depth of the feature to be measured is greater than half of the wavelength of the light source used, an optical system that applies multiple light sources must be used. To satisfy these two conditions, a 3D inspection equipment has no choice but to use a camera with a high number of pixels.
If the surface features of a measured object have a high aspect ratio, the existing AOI system has no choice but to scan while repeatedly stopping using an auto-focusing device. Considering these overall system constraints, inspecting a single wafer measuring 300mm in diameter can take several hours.
By realizing a large optical system depth of field, our products do not require auto focusing nor lose optical and digital resolution. Our systematic parallel processing algorithm enables real-time scanning and our proprietary optical system structure produces minimal noise, cutting noise removal time.