On March 14, 2016, the European Space Agency (ESA) — in cooperation with the Russian Federal Space Agency Roscosmos — launched the ExoMars mission, sending a Trace Gas Orbiter and an Entry, Descent and Landing Demonstrator Module (EDM) called Schiaparelli to Mars via Proton rocket. Mission objectives include seeking evidence of life on Mars, past or present, investigating variations in water and the geochemical environment, and the study of Martian atmospheric trace gases and their sources. The Finnish Meteorological Institute (FMI) contributed sensing instrumentation for the environmental observation station on board the Schiaparelli.
The station, designed and tested by ESA, uses a suite of sensors to monitor systems on the module and gather data on environmental conditions, such as wind speed and direction, humidity, pressure, surface temperatures, atmospheric electrification, and other surface variables. Sensors used to gather data include humidity and pressure sensors designed and manufactured by Finnish manufacturer Vaisala.
From Earth’s cleanroom to interstellar study
Typically used for industrial or meteorological applications to measure a range of parameters, sensors measure dew point, humidity, pressure, temperature, and CO2 concentration. The pressure and humidity sensors currently used in space exploration can function under the harsh conditions in large part because of their stability.
Along with sensor design and calibration, the most important factor to ensuring sensor stability is the quality of the cleanroom facilities in which the sensors are manufactured. Like all cleanrooms, the efficacy of Vaisala’s cleanroom depends heavily on careful design, process controls, and the expertise of the cleanroom personnel.
Cleanroom design aided by in-house equipment
There are several factors in the cleanroom that contribute to sensor robustness and stability. Cleanroom design is the first, most obvious fact. However, unlike most cleanroom facilities, and by virtue of the sensors it manufactures, Vaisala can use its own devices to measure their cleanroom conditions. Other cleanroom equipment is also designed fit for purpose, and tightly specified for how it is used. A feedback control system ensures the environment is reliably controlled.
Along with measuring cleanroom conditions with its own measurement devices, low volume is used to customize sensors or space exploration. Vaisala manufactures about 1.5 million chips annually. The production equipment is intended for high-mix/low volume manufacturing, meaning smaller wafer size and smaller batches than volume-based cleanroom facilities. Thus, small changes and variations can be applied to standard processes and products to tailor sensors for very limited use-purposes.
Manufacturing low volumes in small batches also allows staff to continually learn from processes, making tools like Statistical Process Control more meaningful. In the case of the capacitive thin-film polymer humidity sensors, manufacturing the chips takes a few weeks, whereas other sensors can take up to several months. Statistical Process Control ensures high quality over long lead times, and sensor materials and structures undergo dozens of tests.
The key: Longstanding, expert cleanroom operators
Highly motivated and well-trained people are probably the single most important factor to cleanroom effectiveness. With many different cleanroom technologies available, the human element can be forgotten. But expertise is critical to creating pressure and humidity sensors that can withstand the conditions of space travel and exploration.
The majority of the processes have been automated. However, as these sensors are manufactured in small batches, there are also several manual processes as part of production. For instance, operators manually load wafers into a machine, or perform time-based etchings — a skill that takes time to learn and refine.
This sort of manual work in combination with long experience has endowed staff with instincts that allow them to spot even minute changes in output. Experienced operators train new staff on the “silent knowledge” of their cleanroom processes. With the human element, the cleanroom work is in many ways as much craftsmanship as science.
Today’s cleanrooms lead to tomorrow’s interplanetary discoveries
The recent ExoMars mission is only the latest space exploration application for Vaisala’s sensors. The Finnish Meteorological Institute has collaborated with Vaisala in space exploration projects since 1988. ExoMars is the fifth venture in which FMI is using Vaisala sensors.
Other applications include NASA’s Curiosity Rover, where the sensors are gathering surface measurements near the Martian equator. This technology will be used to compile time series on pressure and humidity for the duration of the entire Mars Science Laboratory program lasting one Mars year, or 686 Earth days.
Since 1992 scientists at the University of Colorado have also been using carbon dioxide, humidity, and temperature sensors to control life science experiments both on board Space Shuttle flights and at the International Space Station (ISS).
Thanks to the stability and accuracy of sensors — a consequence of stellar cleanroom design and expert staff — their use is sure to be included in the future of space exploration.
Tomi Salo is Principal Scientist in Vaisala’s Controlled Environment division. Tomi has a diploma in physics from the Helsinki University of Technology, and he has also completed his Doctorate Technical degree in electrical engineering from the Swiss Federal Institute of Technology, Zurich. Over his 17 years with Vaisala, Tomi has worked on the design and development of various micro-sensor technologies, including polymer thin-film sensors, bulk silicon pressure sensors, as well as optical gas sensors. tomi.salo@vaisala.com; www.vaisala.com
This article appeared in the May/June 2016 issue of Controlled Environments.
