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FEATURE OPTICAL METROLOGY g


available, Pichot explained that this is dependent on the stage of the production process. ‘There are basically three main stages,’ he said. ‘The first is the front end, where the wafer is prepared for manufacturing, so the functions are built up into the chip. Then, at some point the wafer is divided into small components, and then there is the packaging stage. That becomes a critical component, so they need very different, but always robust, sensors that can be integrated into each stage of their production processes.’ In each stage there are many smaller


steps. At the front-end of production, for instance, comes photolithography, which allows the specification to be provided to the chip, which is then polished and brought to the correct thickness. This is a crucial point when it comes to optical metrology. ‘The thickness of the wafer needs to be monitored in-process,’ he said. To save production time and prevent


errors, ideally this needs to be an automated process, Pichot added. ‘If an instrument does not have sensors that can tell you at any time the thickness of the wafer, then the process will involve grinding, with breaks to check from time to time,’ he said. Without in-system programming,


the manufacturer might miss the target thickness they need to reach. ‘By using sensors like ours, grinding can be much more efficient because they can tell, at any time, the thickness. Only in the last minutes of the process does the grinding speed need to be reduced to get the right value,’ Pichot said. This can help to ensure that


‘Instrument manufacture requires an understanding of materials’ properties and the physics involved in making the measurement’


the product quality is more accurate and repeatable, but also offers the additional benefit of reduced material waste. During photolithography, micrometre-


accurate alignment of the mask and wafer is essential. One method is to use two or three chromatic confocal and interferometric non-contact detector probes to change the levelling of the wafer to ensure that it is the correct flatness before the photolithography process. ‘Specialist probes can measure the very thin layers on top of the wafer, to find the position of the wafer surface and separate the top surface to the wafer surface. It can be achieved down to one or two microns,’ explained Pichot. A further challenge for semiconductor manufacturers when it comes to thickness


22 Electro Optics June 2021


Semiconductor manufacturers benefit from completely non-contact optical metrology equipment to reduce the chance of a sample being damaged


verification is an increased market demand for wafers with ever-lower total thickness variation (TTV) or defined structured surface. The challenges lie in varying wafer thickness ranges, differing prime wafer materials, as well as what can often be a harsh inline process environment – for example, if there is no clear view of the wafer because of grinding sludge. ‘With TTV, a wafer, after being


processed, might see some variation of signals from one side to the other,’ Pichot said. ‘With a sensor, you can measure the signal just from one side with super-high accuracy and map signal distribution. It can tell you “it is thinner here, but thicker here”, because it can check that the thickness everywhere is inside the specifications. ‘We also developed a powerful scanner


for extremely fast measurement of this thickness variation, because the wafer does not need to be moved underneath the sensor. Mapping can be achieved accurately, and provides thickness, but also information about what we call the bow and warp – the shape of the wafer in one single pass. It’s time-saving for the customer.’


A polished finish Advanced optical metrology equipment can also be used to provide preventive maintenance during the polishing process, noted Pichot, as it can help determine the degree of surface wear of the polishing pads, caused by what can be quite a harsh process. ‘The polishing pads are in contact with the wafer during the polishing process and after hours of activity, those pads wear down until they need changing.


By incorporating a sensor directly inside the brushing machine the depth can be measured of the segments in between the pads, so the instrument can light up to tell the user the pad needs to be replaced.’ A specifically-designed quality inspection service can be very useful when it comes to defect detection such as chips or cracks that can occur when the die is separated from a wafer of semiconductor (dicing). Dicing is usually performed by scribing, breaking, mechanical sawing or laser cutting. ‘For fast and accurate solution,’ said Pichot, ‘optical sensors can be used to check the roughness of the wafer nanometric scale. We can measure the depth after the use of the laser. For example we can detect with advanced microscopy problems like cracks in the wafer. Sensors can also check any photo-resistant coatings that may have been applied to a wafer, have been removed without causing damage.’ With so many steps involved in the


manufacture of semiconductors, and with new materials being introduced all the time, there is an almost non-exhaustive list of applications for which optical metrology is essential. Other applications include the measurement and inspection of wafer- level and solder; die and wafer bonding; the inspection of probe cards and even measurement of the tiny wire bonding that connects the chips to the package. ‘In the semiconductor industry, we have a lot of customers, and therefore a lot of interesting applications,’ said Pichot. ‘Once we get started, it really is quite fascinating, the number of things we can achieve.’ EO


@electrooptics | www.electrooptics.com


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