Peter Hoffrogge, Product Manager of PVA TePla Analytical Systems
The demand for equipment that can perform non-destructive imaging and materials analysis has increased significantly for silicon ingots, wafers, integrated circuits (ICs), MEMS and other electronic packages produced by the billions in automated production environments. Today, however, continuously evolving high-volume production requirements are rapidly migrating beyond the products recently considered standardised in the industry.
New device designs, packaging methods, shrinking dimensions and bonded wafer interfaces as well as increased product yields are driving market demand for significantly improved manufacturing equipment capabilities. This includes higher levels of automation integration for device handling, improved clean room performance and scanning of ever smaller devices and interface connections. Scanning acoustic microscopy (SAM) technology therefore continues to advance and is rapidly becoming the inspection technique of choice.
SAM utilises ultrasound waves to non-destructively examine internal structures, interfaces and surfaces of opaque substrates. The resulting acoustic signatures can be constructed into 3D images that are analysed to detect and characterise device flaws such as cracks, delamination, inclusions and voids in bonding interfaces, as well as to evaluate soldering and other interface connections.
SAM 302 HD² scanning acoustic microscope.
Ultrasound provides a clear advantage in ensuring good adhesion and mechanical integrity of devices. Unlike alternative techniques such as X-ray, which are used to evaluate volumes and densities, ultrasound focuses on interfaces. Take, for example, a sinter connection on a power device, where the gaps measure only a few nanometres. Using X-ray, there is no contrast so one is unable to tell if the die has adhesion through the interlay, but using ultrasound, it is clear to see.
The challenge today is to 100 percent inspect, identify and remove devices that do not meet quality requirements at extremely high throughput. Defects often occur in different layers of a device, necessitating advanced equipment that simultaneously inspects several layers—often on multiple channels—scanning multiple samples in handling trays in an automated fashion.
SAM Auto Tray scanning acoustic microscope.
Increasing throughput requirements has traditionally required sacrificing image resolution, however today’s advanced inspection equipment can overcome these limitations. The majority of equipment is custom designed for complete integration into high-volume manufacturing systems. This includes equipment specifically designed for inspection of items such as crystal ingots, wafers and electronics packages in a range of standard sizes. For items that have more unique geometries or sizes, equipment can be semi-customised to meet the requirements of the application, based on established standard components.
Acoustic scanning
The unique characteristic of acoustic microscopy is its ability to image the interaction of acoustic waves with the elastic properties of a specimen. In this way, the acoustic microscope is used to image the interior of an opaque material.
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 (scattered at 180°) or transmitted (scattered at 0°). 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.
Pre-developed integrated systems
Today, there is SAM equipment that has been pre-developed to handle standardised items, for example, bonded wafer inspection of MEMS, complementary metal-oxide semiconductor image sensors, etc. This equipment tests for inclusions or delaminated areas in the bonding interfaces and other defects. Damage inspection is typically performed during late-stage production, prior to dicing, to ensure the device is 100 percent error free.
PVA TePla Analytical Systems’ line of Auto Wafer scanning acoustic microscopes is for the inspection of bonded wafers. It affords four transducers and automated handling for high-throughput.
Automation features of the Auto Wafer 300 include: cassette loading systems (open load port, standard mechanical interface (SMIF) to facilitate quick loading, front opening unified pod (FOUP) or customised input and output cassettes), a robot for handling 5–12 in. wafers, and integrated scanners for wafer tracking.
Auto Wafer scanning acoustic microscope.
PVA TePla Analytical Systems’ Auto Ingot scanning acoustic microscope is for the inspection of single crystal ingots, for example, silicon (Si), germanium (Ge) and gallium arsenide (GaAs). It employs multiple transducers (four heads) to estimate the 3D location of defects inside the crystal volume and is thus capable of analysing voids and inclusions, effectively judging their depth and size simultaneously.
The Auto Ingot is able to inspect 5–12 in. Si ingots up to 400 mm in thickness and 75 kg in weight. The defect resolution may reach down to 100 μm voids in silicon.
Customised equipment
PVA TePla Analytical Systems’ portfolio of standard core components can be used or modified to accommodate applications involving unique geometries and sizes.
Transducers can be designed and produced according to specific application, device or inspection requirements. They are manufactured in a very wide frequency range—from 3 to 2,000 MHz—using a proprietary thin-film technology that PVA TePla Analytical Systems has spent over three years developing.
Up to four transducers can simultaneously scan, depending on the level of throughput required. Multiple transducers can be used to scan a single substrate and then the images stitched together, or multiple transducers can be used to simultaneously scan multiple substrates.
Throughput can also be increased by incorporating ultra-fast single or dual gantry scanning systems or six-axis robots. Other possible add-ons include rotation axes (flipping), vacuum chucks and customised water tanks.
PVA TePla Analytical Systems