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Thin-film coolers take the heat out of lasers

07 Dec 2007

Thermoelectric coolers the size of chips are helping to control the temperature of lasers used in data networks and spectroscopy applications. Tim Hayes spoke to Burkhard Habbe of Micropelt to find out more about the technology, its applications and its benefits.

Controlling the operating temperature of telecom lasers is about to become a volume business, as the demands for enhanced bandwidth in optical fibres and the use of multiple laser wavelengths in the same fibre grow. A new approach to manufacturing thermoelectrics (TEs) and thermoelectric coolers (TECs), developed by German start-up Micropelt, can produce devices combining high cooling power with small form factors.

"Micropelt specializes in thin-film thermoelectrics," explained Burkhard Habbe, vice-president of business development at the company's Freiburg, Germany, headquarters. "Our manufacturing process combines microelectronic thin-film and MEMS-like wafer technologies, which bring a scalability and performance that differentiate our thermoelectrics from standard, much larger, components."

Technology
TECs exploit the Peltier effect, in which a solid-state heat pump built from an alternating structure of n- and p-type semiconductors will transfer heat from one side of the device to the other when electrical energy is supplied. Applying a current produces a "cold side", which is cooled significantly below the ambient or hot-side temperature.

Micropelt's manufacturing method uses microelectromechanical (MEMS) micro-structuring technology to create tens of thousands of n- and p-type thermoelectric elements in a bulk process, rather than the discrete assembly steps needed to produce each conventional TEC. The use of standard semiconductor production equipment and wafer sizes is the key to Micropelt's novel approach.

Two wafers are produced, each with a layer of TE material applied by sputtering, where one is covered with p-doped material and the other with n-doped. Photomasking and selective etching are used to form the required microscopic TE structures, and a fully functional TE element is then created by bonding the two wafer sections with opposite doping together. The smallest building-block of thin-film TE made in this way has a footprint of only 36 µm and a height of 18 or 36 µm.

The Micropelt technology also allows array-bonding of multiple TE cooler elements into clusters, also called tiles, in one assembly step. These arrays serve as bulk handling units for test and characterization, and could be used as a batch assembly platform for the components to be cooled. "We believe that mounting multiple lasers or sensors onto an array of coolers formed by our tiles can help facilitate the assembly process at high precision," said Habbe. To develop this bulk approach further, the company is working on scaling up the current tile-orientated process to full-wafer scale. Finally, each tile is cut into discrete, fully functional thermoelectric elements, which can have a footprint between 0.6 and 25 mm2.

"As well as the economies of scale brought by bulk production of the very small TE elements, this technology also allows the TE structures to be shaped, even designed, according to their intended use," said Habbe. "It is now possible to truly batch-optimize TE elements to match a given purpose. A cooler can be made of just a few TE legs with maximal footprint and minimal spacing, which when combined with the very small layer thickness increases the pumping power of the device considerably compared with conventional thermoelectrics."

For example, a small Micropelt TEC can pump about 0.6 W through a cold-side surface area of 0.6 mm2, which equates to a performance level of about 100 W/cm2. This is an order of magnitude better than conventional coolers, which according to Habbe usually offer less than 10 W/cm2.

Improvements in response time, sometimes by a factor of a hundred, also stem directly from the inherently small layer dimensions and miniature size of the device, and the reduced thermal mass that results.

Taken together, these factors have enabled a Micropelt Micro-TEC to demonstrate a record-holding 60 ÂșC cooling over a 36 µm leg height, according to the company.

The Seebeck effect, effectively the counterpart to the Peltier effect, allows temperature differences to be converted into electrical energy by using TEs as energy harvesters. Their performance is also enhanced in thermogenerators made using the company's thin-film process.

"With our technology it requires little electronic effort to generate significant voltage and several milliwatts of power from a single 10 mm2 device, which can be enough to drive low-power electronics directly," said Habbe. "We have also designed DC/DC booster circuitry that, when attached to a Micropelt generator, would supply stable power up to 5 V and a few hundreds of microamps, at a temperature difference in the lower two digits. That's enough to drive wireless sensors or other stationary, low-power electronic devices."

Laser applications
Thermal management systems are used to either increase the laser's efficiency or to stabilize its wavelength in telecoms and industrial laser applications. Conventional TE modules are widely used but limited in miniaturization and cost reduction capability – a barrier to developing low-cost WDM solutions. Micropelt coolers allow packages to be miniaturized and become cheaper with growing production volumes.

"Increasing demands on the laser systems used in data networks, driven by the move from single to multiple wavelengths in the same fibre, is behind the need to use cooled lasers rather than uncooled systems," Habbe explained. "These WDM systems also have a tendency to involve lower supply voltage and ever-smaller form factors, trends that are both well supported by our devices."

Habbe also sees a growing market for cooling lasers in spectroscopy applications, where laser sources detect the presence of chemical species using their specific extinction wavelengths. "The more precisely you want to measure, the better control you need to have over the temperatures of both the emitter and the receiver used to establish the measuring system's reference." he said. "By leveraging the low thermal mass of our components, we allow customers to create thermal control systems that are much more dynamic and precise than those based on standard designs of thermoelectrics. Often, such a design has a visibly better energy balance than conventional TEC implementations."

Spin-out
Applying TECs to laser cooling applications is a direct connection back to Micropelt's origins – a research collaboration between Infineon and the Fraunhofer Institute for Physical Measurement Technologies (IPM). That project was based on combining Infineon's expertise in chip production with Fraunhofer's knowledge of TE materials. Infineon's decision to divest a multitude of its research projects was the starting point for Micropelt.

"Micropelt was spun-out from Infineon in early 2006 as a management buy out, and we're very happy to have found regional investors for our early stage Series A funding," said Habbe. The company employs 16 full-time staff, with additional part-time support from Fraunhofer IPM.

"We are currently in a preproduction phase, with a production capacity up to a few hundred thousand items depending on component size, while we continue to develop the technology and carry out research," said Habbe. "We expect to be ready for volume production in a new facility in 2009 and break even soon after."

• This article originally appeared in the December 2007 issue of Optics & Laser Europe magazine.

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