Hari Polu, president, OKOS
An increasing number of semiconductor fabricators are using scanning acoustic microscopy (SAM), an advanced quality inspection tool, to detect miniscule defects in the multiple material layers of the sintered electrostatic chucks (ESCs) used to hold wafers during processes such as vapour deposition and etching, thus improving reliability.
ESCs utilise a platen with embedded electrodes that are energised with high voltage to establish an electrostatic holding force for gripping very delicate items such as wafers, foils or films. ESCs are built in layers, with a layer of screen-printed electrodes sandwiched between layers of insulative ceramic material. The entire structure is then sintered to create a single chuck.
A significant challenge has been the lack of high-resolution tools capable of inspecting multiple layers of sintered material to validate the integrity of the package. ESCs can display fluctuations or non-uniformities in the thickness of the dielectric layer as well as extremely small cracks and pores that can adversely affect their ability to electrostatically secure items.
Since ESCs must be constructed and perform flawlessly to maintain the extreme precision required in semiconductor wafer production, SAM has become an important new tool for the industry to validate ESC integrity during manufacture.
SAM is a non-invasive, non-destructive ultrasonic testing method. It is already the industry standard for 100 percent inspection of semiconductor components to identify defects such as voids, cracks and delamination of different layers within microelectronic devices. Now, the same rigor of quality testing and failure analysis is being applied to ensure ESC integrity.
The OKOS MACROVUE-P 600 scanning acoustic microscopy (SAM) system.
SAM can not only detect fluctuations in the thickness of the dielectric layer but also the presence of extremely small cracks and pores that could compromise the ESC’s ability to electrostatically secure the substrate to the chucking surface. It is essentially the equivalent of an X-ray inside the part, so is a comprehensive test method to ensure quality.
The many benefits of SAM for ESC inspection
For validating the integrity of electrostatic chucks, advanced, phased array SAM works by directing focused sound from a transducer at a small point on a target object. The sound hitting the object is either scattered, absorbed, reflected or transmitted. By detecting the direction of scattered pulses as well as the time of flight, the presence of a boundary or object can be determined as well as its distance.
To produce an image, samples are scanned point by point and line by line. Scanning modes range from single layer views to tray scans and cross-sections. Multi-layer scans can include up to 50 independent layers. Depth-specific information can be extracted and applied to create two- and three-dimensional images without the need for time-consuming tomographic scan procedures and more costly X-rays. The images are then analysed to detect and characterise flaws such as any irregularities in the thickness of the ESC’s dielectric layer as well as miniscule cracks and pores.
OKOS has leveraged the lessons and the tight specifications from semiconductor fabrication and adapted its SAM scanning systems to provide solutions specifically for ESC quality inspection. This type of testing allows for the inspection of materials at a level one to two orders of magnitude better than conventional options to discover defects such as cracks, voids and the delamination of different layers as small as 50 µm that were previously undetected. Companies such as OKOS offer a range of SAM products from compact, tabletop units to fully automated production line systems.
When high throughput is required for 100 percent inspection, ultra-fast single or dual gantry scanning systems are utilised along with 128 sensors for phased array scanning. Multiple transducers can also be used to simultaneously scan for higher throughput.
As important as the physical and mechanical aspects of conducting a scan, the software is critical to improving the resolution and analysing the information to produce detailed scans. Multi-axis scan options enable A, B and C scans, contour following, off-line analysis and virtual rescanning for ESCs, resulting in highly accurate internal and external inspection for defects via the inspection software.
Various software modes can be simple and user friendly, advanced for detailed analysis or automated for production scanning. An off-line analysis mode is also available for virtual scanning.
OKOS decided early on to deliver a software-driven, ecosystem-based solution. The company’s ODIS acoustic microscopy software supports a wide range of transducer frequencies from 2.25 to 230 MHz. It is estimated that this software-driven approach drives down SAM testing costs while delivering the same quality of inspection results. As a result, this type of equipment is well within reach of even modest testing labs, R&D centres and material research groups.
Summary
A growing number of semiconductor fabricators are switching from traditional options to SAM testing for ESC quality control because the levels of failure detection and analysis afforded are far superior. Increased accuracy, reliability and yield as well as decreased cost and time involved are the factors driving this change.
OKOS