微透镜压印光刻技术趋势

1. Introduction  

In 1994, about 20 years after the invention of the digital  still camera and 10 years after first mobile phones  appeared on the market, the first digital consumer  camera, the “Apple Quick Take”, was introduced to the  market. It took another 8 years until digital cameras were  integrated into mobile phones. In the beginning, a mobile  phone camera was just another gimmick with limited  value for the mobile phone user. Memory was expensive,  and the image quality was poor. Today, it is difficult to  find a mobile phone without digital camera. Most mobile  phones are now equipped with two cameras, a primary  Mega-Pixel (MP) camera for photography and a  secondary CIF or VGA camera for video calls. Far more  than a billion mobile phone cameras were sold in 2007,  and the numbers are still increasing rapidly.

2. Trends for Mobile Phone Cameras  

It is difficult to predict long term developments in the  field of mobile phone technology. Regarding mobile  phone cameras, the situation is much easier, they will get  better, they will get cheaper – but they remain to be an  indispensable part of a mobile phone. Today, two main  trends are observed:  a) High resolution cameras, providing autofocus, optical  zoom, mechanical shutter, stabilization and Xenon flash.  Mobile phones equipped with high resolution cameras  are in direct competition with digital still cameras.  b) Wafer-Level Cameras, providing a high potential for  cost-reduction in the low end camera market. The  camera should be “reflowable”, i.e. it must survive  standard SMT reflow soldering process at about 260°C.  Both trends will certainly have a tremendous impact on  the current optics and digital camera industry.

3. Wafer-Level Camera (WLC)  T

oday’s mobile phone cameras consist of some 10 to 20  different components like plastic or glass molded lenses, pupils, baffles, actuators, lens holders, barrel, filters and  the image sensor. These components are manufactured  and assembled by different sub-suppliers.  The Wafer-Level Camera approach is rather simple: All  components are manufactured on 8’’ wafer, the optowafers are mounted together with the CMOS wafer, and  the wafer stack is diced into the individual camera  modules. The complete mobile phone camera, including  the optics, is manufactured and packaged on wafer-level  using standard semiconductor technology.

Fig. 1. Wafer-Level Camera (WLC) concept. Optowafers and CMOS wafer are mounted together (left) and  diced into individual camera modules (right).

Wafer-level imaging systems for mobile phones were  already proposed more than 10 years ago [ 1 ]. First  prototypes were shown about 5 years ago [2]. Since then,  WLC technology was developed by research institutes  like the Fraunhofer IOF and small companies like  Heptagon, Anteryon and DOC/Tessera. It is worth to  mention that most WLC research and development was  carried out by the micro-optics experts, and not by  manufacturers of mobile phone cameras.  Recently, major mobile phone companies promoted  WLCs as being the ultimate technology for next  generation low-cost mobile phone cameras. Despite  WLC technology is still not mature; all major camera  suppliers are now investigating WLC solutions. The simplicity of the WLC concept often leads to the  conclusion that CMOS manufacturers just have to use their highly developed semiconductor technology to  manufacture the 8’’ opto-wafers.

                                     Fig.2. Wafer-Level Camera built in WALORI EU-IST  Project [1]. Backside illumination through thinned  CMOS image sensor and ball grid for bonding.

Compared to the 150 different process steps to manufacture a CMOS imager, the opto-wafers with some bulky  lenses should be rather simple to do. In semiconductor  industry “everything is possible” and further miniaturization is only a question of time. However, these assumptions are not valid for wafer-level optics:  

             Standard semiconductor technology is not suitable for the manufacturing of bi-convex aspherical lenses on 8’’ wafer level.  • Well established materials from semiconductor  industry can’t be used for wafer optics.  • Most materials used for plastic optics do not  survive reflow processes at 260°C.  • In WLC the CMOS sensor is covered by the  glass opto-wafers. Electric contact pads have to  be place on the backside of the CMOS chip and  Through Silicon Via (TSV) technology or backside thinning of the CMOS are required.  • Fundamental physical laws limit a scaling down  to ultra small cameras with high resolution.  The WLC approach requires a close co-operation of  optics and semiconductor industry with equipment  suppliers. Novel technology has to be developed and  existing production tools have to be modified.

4. Microlens Imprint Lithography  8’’ wafer technology for refractive and diffractive microoptics was first implemented by SUSS MicroOptics in  1999. Standard manufacturing technology from semiconductor industry, like resist coating, lithography,  reactive ion etching (RIE), deposition, sputtering, and  lift-off were optimized to manufacture high-quality  micro-optics in 8’’ Fused Silica, Silicon and Borofloat  wafers. Unfortunately, the well-established 8’’ wafer  technology is not suitable for WLC. Resist melting and  RIE transfer do not allow manufacturing of microlenses  with more than 100 µm lens sag. RIE speed of somemicrons per minute does not allow cost-efficient etching  of bi-convex lenses on wafer level.  The most promising technology is imprinting or UVembossing of polymer lens structures on glass wafers by  soft PDMS stamps. First experiments to imprint microoptics on wafer level using a modified SUSS Mask  Aligner MA6 have been carried out at the CSEM about  10 years agoCSEM’s microlens imprint technology has been  transferred to Heptagon, also based in the Zurich area in  Switzerland. Heptagon developed proprietary processes  for reflowable micro-optics and is regarded as one of the  pioneers in WLC technology. Recently Heptagon opened  a WLC manufacturing facility in Singapore. Another  pioneer in this field is the Fraunhofer IOF in Jena,  Germany. They successfully developed replication and  imprint technologies for low-cost wafer-level optics.  Fully operable prototypes of ultra-flat cameras, endoscopes cameras and WLC systems have been presented.  Microlens Imprint Lithography allows the manufacturing  of lens arrays with a lateral accuracy of ± 1 µm on 8’’  wafer level.

                                                        Fig. 4. Microlens Imprint Lithography using a soft mold  to imprint double-sided lens arrays onto opto-wafers.