Laser Crystallization Technology Breaking Chip Processing

Recently, Germany's Fraunhofer Institute of Laser Technology (ILT) announced a key project jointly developed with the Institute of Silicon Technology (ISIT) and the Institute of Surface Engineering and Thin Films (IST) - laser deposition and crystallization technology compatible with semiconductor chips. It is reported that this technology will adopt a micro structure connection mode completely different from the traditional welding connection to avoid introducing too much redundant space between the micro structures on the chip, thus realizing the optimization of the actual size of MEMS and further improving the integration of the chip. In addition, this technical solution will be first applied to the fields related to sensors to achieve the design of high-performance sensors.

MEMS miniaturization development dilemma

MEMS originated from microelectronic technology and has been steadily developed under the promotion of semiconductor technology. As an emerging frontier technology with multi-disciplinary intersection, MEMS has achieved good application results in information communication, aerospace, biomedical and other fields, and has a strong role in promoting the development of world science and technology and economy. To take the simplest example, automobile manufacturing cannot be separated from the guarantee of safety and control systems, and sensors designed based on MEMS can monitor key data during driving to control the start of airbag and other stabilization programs; In addition, MEMS based inertial sensors are also used in the design of smart phones, watches, unmanned aerial vehicles and other consumer goods. According to relevant data, China's MEMS market will break through the 100 billion mark this year.

Fig. 1 Morphology of Common MEMS

(Source: 51hei.com)

The main goal of MEMS is to realize the organic combination of micro mechanical structure and integrated circuit (IC), including three parts: micro sensors, micro actuators and related components. Micro machining technology is the most relevant to the development and application of MEMS. Micro machining is the core of MEMS, which determines the minimum size and optimal performance of each component. For the preparation method of traditional MEMS sensor chip, the sensor unit needs to be combined with the special integrated circuit on the silicon chip. However, since the integrated circuit cannot withstand the high temperature above 450 ℃, it is necessary to first prepare the silicon based MEMS sensor module, and then use wires to connect it with the integrated circuit by welding or crystal bonding.

"The traditional connection method needs more reserved space, which makes the MEMS sensor chip unable to continue to develop towards miniaturization", commented Florian Fuchs, research assistant of the ILT film processing team. It is precisely for this reason that the MEMS sensor unit made of monocrystalline silicon cannot be directly connected with the ASIC. Under the background of the era driven by the huge market of MEMS, it has become the general trend to create a new micro machining technology.

To break the situation by surprise, laser crystallization technology

In order to explore the advanced processing technology of MEMS, ILT researchers have conducted in-depth exchanges and cooperation with ISIT and IST colleagues. A new laser crystallization process has been developed. The biggest difference between this process and the traditional processing process is that it avoids the need for wires and solder joints in the traditional processing scheme, thereby saving more space on the chip, and better realizing the miniaturization design concept. In addition, the perfect compatibility of this technology with complementary metal oxide semiconductor (COMS) has also become the key to high-quality MEMS monolithic fabrication.

Fig. 2 MEMS sensor chip designed based on laser crystallization technology

(Source: Fraunhofer ILT)

Specifically, this research mainly relies on high-energy laser to deposit monocrystalline silicon. ILT researchers use selective laser crystallization to directly couple the silicon based MEMS sensing structure designed by ISIT in advance to the temperature sensitive integrated circuit (similar to the coupling between optical fiber and silicon based micro cavity). In this way, MEMS micro sensing structure and integrated circuit without the intervention of wires, This completes a good connection and greatly reduces the occupation of chip space. In addition, during the research, the researchers also got an unexpected surprise: using laser crystallization to grow the structure on the integrated circuit can not only avoid the ablation of the integrated circuit due to excessive temperature, but also skillfully use high-energy laser to activate the doped elements in the silicon based materials, so that MEMS can maintain a proper conductivity.

Fig. 3 Detailed observation on array structure prepared by ILT selective laser crystallization

(Source: Fraunhofer ILT)

Guarantee of yield: 3D efficient heat dissipation

In this study, the researchers used a beam of light with a diameter of 10 μ M laser carries out silicon based crystallization, and scans the entire processing surface under the guidance of the mirror to ensure the processing quality. It should be noted that when the high-energy laser irradiates and heats the silicon substrate in a high temperature environment lower than its melting point, it will promote a selective rapid crystallization process (millisecond scale) in its spatial structure. This manufacturing method effectively reduces the mechanical stress in the layered material, and does not damage the sensitive electronic components on the underlying integrated circuit.

Fig. 4 Schematic Diagram of Laser Crystallization Process

(Source: Fraunhofer ILT)

In the above-mentioned spatial selectivity process, the heat brought by processing is effectively dissipated in three spatial dimensions. In an interview, Dr. Christian Vedder, the head of the ILT thin film processing group, commented: "Because the energy brought by the high-energy laser is absorbed by the tiny silicon crystal unit, the maximum temperature generated on the MEMS structure during processing exceeds the damage threshold of the integrated circuit. However, due to the extremely fast processing speed and effective heat dissipation guarantee, the integrated circuit is not damaged."

This new processing method has also successfully reduced the resistivity of silicon based MEMS structures by four orders of magnitude, as low as 0.05 Ω· cm. Based on the thin film structure with low resistivity, capacitive MEMS acceleration sensors with typical finger structure can be successfully fabricated.

MEMS processing is at your feet

When asked whether the technology will be suitable for large-scale application, Florian Fuchs added: "Because of the excellent compatibility between silicon and CMOS, we can develop a highly integrated MEMS IC chip without changing the CMOS manufacturing process." This answer comprehensively and profoundly expresses the technical advantages of the new processing method. In addition to a high degree of integration, this research can also achieve lower interference from current and electromagnetic field noise than expected. This method has a very positive impact on the high-quality signal transmission of the sensor.

In general, the mainstream development direction of MEMS IC chips is still miniaturization and intelligence. Laser crystallization technology has a good processing effect, which can meet the market's technical requirements for the miniaturization of sensor chips, while ensuring the characteristics of environment-friendly. For MEMS micro processing, it is a well deserved open road.

Source: Yangtze River Delta Laser Alliance