FEATURE CALIBRATION
The makers of high transistor-count semiconductor devices cannot afford to take chances when it comes to ensuring accuracy and repeatability in their processes. Stuart Knight, director of HORIBA UK, discusses the importance of like-for-like compatibility
Fukuchiyama Technology Centre, in Japan, where results produced by the system have ISO/IEC 17025 accreditation; accredited by NIST’s National Voluntary Laboratory Program (NVLAP Lab Code 600160-0). The system records weight change over
GAMBLING CHIPS? HORIBA STEC’s
Gravimetric System is in a temperature-controlled chamber and includes an electronic balance that is not susceptible to the effects of vibration
T
he semiconductor industry has always pushed technology boundaries to keep
pace with Moore’s Law, and to place increasing numbers of transistors into integrated circuits (ICs). For instance, in 2017, Intel announced its 10nm process was enabling the company to pack more than 100 million transistors into each square millimetre of silicon. With its previous process (14nm, which came into play in 2014) the figure was 37.5 million transistors per square millimetre. Understandably, to fabricate high gate- count devices, process accuracy is essential. But so too is repeatability because many manufacturers have multiple fabs and want to be able to send chip designs to any of their plants around the world and get exactly the same results. However, semiconductor manufacturing process chambers are not identical and process engineers need to fine tune critical parameters – such as chamber pressure, temperature and gas flow – to achieve the desired outcome for each and every process step. No two supposedly identical pieces of
equipment – used to control or measure a process – will ever be exactly the same though. There will always be a degree of uncertainty. For example, while the accuracy and repeatability of an instrument will be declared on its paperwork, what was the accuracy and repeatability of the equipment used to establish those figures? And how was that equipment calibrated? Indeed, at what point does uncertainty enter the equation, what can we do to reduce it and how do we provide traceability back to absolute standards? For example, Mass Flow Controllers (MFCs) are used extensively in the
16 JUNE 2018 | INSTRUMENTATION
semiconductor industry to control the stoichiometry (‘recipe’) needed for fabrication; with the ‘ingredients’ either adding to the structure (e.g. deposition) or assisting with the removal of material (e.g. etching). MFCs work by controlling the flow of a mass of gas or liquid over time. But what is mass and what is time? The base unit of mass is the kilogram,
the reference for which is a 1kg cylinder of platinum-iridium alloy called the International Prototype of the Kilogram (the IPK). As for time, one second is 9,192,631,770 cycles of a Caesium atomic clock. The kilogram and the second are Système International (SI) units.
ACCREDITATIONS As for the accuracy to which mass can be measured, one widely accepted starting point is the National Institute of Standards and Technology (NIST). Its services include accrediting the calibrating processes and equipment used by laboratories and manufactures against ISO/IEC 17025:2005. A metric used is the Calibration and
Measurement Capability (CMC). It is a measure of how closely the calibration method represents absolute accuracy. Or, to put it another way, the CMC is the ‘uncertainty’ figure. No calibration method, or equipment employed therein, can deliver absolute accuracy, so CMC will always be greater than zero. Where HORIBA-STEC’s MFCs are concerned, production units are calibrated in-line against ‘master units’ (or working standards) which will themselves be calibrated using a Gravimetric System, scheduled to come on line before the end of 2018. Developed in-house, the system is located at the company’s Kyoto
time in a cylinder supplying gas to a master MFC to derive a flow rate. Measurements are taken in a temperature-controlled room to eliminate any bias due to air buoyancy. Also, the Gravimetric System includes an electronic balance that is not susceptible to the effects of vibration. In other words, the recording of the core parameters cannot be influenced by others. Calibration capability using HORIBA
STEC’s Gravimetric System has a CMC of between 0.08 per cent to 0.20 per cent, depending on flow rate, and calibration capability using the working standard has an expanded uncertainty ranging from 0.12 per cent to 0.30 per cent (again depending on flow rate). Another thing to consider is that
calibration needs to be against reference (inert) gases, such as N2
, Ar, CO2 , N2 O or
even air, plus the semiconductor manufacturing process gases. Accordingly, to assure accuracy and repeatability in manufacturing scenarios, an MFC requires two calibrations. In HORIBA STEC’s case they will be: • Primary Standard (using the
Gravimetric System) using inert gases; • Secondary Standard (using
traditional dynamic and static PVTt methods) using process gases. Both are compared to national standards but of these, the Primary Standard is fundamental because only two base units of measurement are involved.
SUMMARY To adhere to Moore’s Law, accuracy and repeatability for smaller process geometries must be maintained. This puts immense pressure on critical parts of the process equipment because the ‘total measure of uncertainty’ needs to be as low as possible. Moreover, if a process engineer wishes to be assured of out-of-the-box, like-for- likeness then it is important to understand what the CMCs really mean, and how they trace all the way back to a single kilogram of platinum-iridium and time derived from atomic clocks.
HORIBA UK
www.horiba.com
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