Photonic device package design, assembly and encapsulation.

Transcription

1 Photonic device package design, assembly and encapsulation. Abstract. A.Bos, E. Boschman Advanced Packaging Center. Duiven, The Netherlands Photonic devices like Optical transceivers, Solar cells, LED s (Light Emitting Diodes) etc, have been developed for high speed data communication, power generation and light sources. Photonics include, among others, emission, transmission, switching and sensing of light. GaAs and GaNi are examples of semiconductor materials used to create diodes, able to generate light. Different types of diodes can be made to generate light in the spectrum ranging from Ultra Violet via visible to Infra Red light. VCSEL s (Vertical Cavity Surface Emitting Laser diodes) can be used for data transmission via thin glass fiber cables and operate mainly at wave lengths around 850 nm. Photo diodes are used as light sensors and are able to receive data in the form of light pulses or modulated light. A photo diode is sensitive for photons striking the diode surface also known as the photo electric effect. When used for environment lightening, LED s can generate light in a variety of colors in the visible spectrum. Solar cells are light sensitive diodes used in a Photo Voltaic mode and able to convert sun light into electric energy. In most of these applications the diodes require a lens because the light is diffracted and a lens must be used to collimate the light. Due to their application and operating environment these diodes require a different assembly and packaging approach compared to conventional semiconductor devices. The focus of APC is on package design, process development for assembly and encapsulation of these types of devices and the generation of samples and low volume production quantities. The choice of package concept, lens parameters, assembly and encapsulation materials are of utmost importance for the functioning and reliability of the devices. For example the design of optical transceivers requires the package design to be such that no cross talk can take place between transmitter and receiver and the glass fiber cables can be connected in a reliable way. To limit transmission losses, the assembly process requires high precision lens shapes and accurate placement of transmitter / receiver chips towards their lens centers. Encapsulation material properties like adhesion, transmittance, refractive index, CTE should be taken into account. Photonic devices are often subjected to extreme environments of radiation and heat and have to full fill a life time expectancy of 10 to 0 years. 1. Introduction. Photonics is derived from the Greek word photos which means light. When combined with electronics the expression optoelectronics is more relevant because this technology combines electrical and optical functions to create light emitting and receiving devices. Packaging of photonic devices, like CPV Solar cells, Led, and Optical transceivers etc, require special attention in relation to aspects like package and lens design, material choices and production technologies. 1.1 CPV Solar cells. CPV Solar cells can be packaged in two versions, with or without molded lens. See Fig.1. In case of a non molded lens, an external lens, usually made of glass, will be attached to the CPV chip after molding to concentrate the sun light. The active CPV chip area, which will be subjected to sun light, is exposed and the remainder surrounded by molding compound. For exposed chip surfaces it is important that it is kept very clean and not polluted by molding compound residue. For molded lens versions the molding materials are usually clear Epoxy or Silicone based compounds, 1

2 exhibiting excellent optical properties and must be capable to withstand the harsh environment of radiation and high temperatures generated by the concentrated sun light. Fig.1 Molded and external lens examples 1. LED. Lenses are also used for high brightness Led applications. These lenses are in most cases molded lenses integrated in the package. See Fig.. The molding material is clear compound. For packaging and lens molding, the process is comparable with CPV devices. avoid so called cross talk between these two chips when light generated by the VCSEL can reach the Photo Diode through the package instead of via the glass fiber cables. See Fig.4. Because of the relatively small light beam and cable diameters, the combined position accuracy of cable, lens and active chip area for individual transmitter and receiver is very important to avoid loss of light due to mismatch. Besides accurate positioning the accuracy of the required lens shape is crucial to avoid light loss. The molding process consists of two steps. First the transmitter and receiver chips are molded with integrated lenses using clear compound. The second molding step will be with black compound which forms the outer package including the required light barrier to avoid cross talk and also has the mounting connections for fixing the glass fiber cables. Fig.3a Optical transceiver molding step 1 with clear compound. Fig. LED package with molded lens. 1.3 Optical transceivers. Packages used for Optical transceivers are usually Multi Chip Packages (MCP) consisting of a transmitter chip (VCSEL), a receiver chip (Photo Diode), and possibly driver chips together with some decoupling capacitors. See Fig.3. These packages must have a means to connect Glass Fiber cables required for data transport. Transmitter and receiver chips must be optically separated to Fig.3b Optical transceiver molding step with black compound. Fig.3c Two package Optical transceiver with molded lenses.

3 Lens 1 Transmitter Lens Receiver Fig.4 Optical separation by black compound.. Film Assisted Molding (FAM). Film Assisted Molding (FAM) technology is used for the encapsulation of Photonic devices together with a mold tool and high quality optical lens mold parts. Both mold halves are completely covered with a film layer which will be formed into the mold cavities and lens domes. As such, the molding compound stays only in contact with the film and the devices to be encapsulated. It does not touch any mold steel. Further more the film protects package leads and exposed die pads from being polluted by compound residue called resin bleed. In the case of exposed die molding the film will not touch the active chip area but only the area outside the active area with a pre-selected film pressure. The film will keep the active chip surface perfectly clean. can be used in the top and bottom mold in case of double sided package types. The Polyester film is much stiffer and can be used for single sided package types like QFN, DFN etc. In this case the film will be put on the flat bottom mold while the Teflon film is used in the top mold where the mold cavities are placed. Both films cover the complete surfaces of the top and bottom mold. In this way the film prevents the mold from being polluted by the molding compound residue build-up in time. This means the mold does not need frequent cleaning as required in the conventional molding process without film. Due to the film the molded packages release very easily from of the mold cavities and as such prevents against mold sticking, chipping and delaminated products. The FAM process can handle compound without release agent required in the conventional molding process to overcome sticking problems in the mold..1 Film properties and use. Two different film types can be used for the molding of Photonic devices. One film is a Teflon based film and the second a Polyester carrier film with an adhesive layer on one side. See Fig. 5. Both films have a melting temperature of about 60 C. and are able to withstand the molding process temperature which is dependent on the molding compound type used and usually can vary between 10 to 180 C. The Teflon based film has a high elongation factor and can be vacuum formed into the deeper mold cavities with the lens domes. This film Fig.5 Film used in FAM process.. FAM Mold design for Photonic lens devices. FAM mold design for Photonic devices deviate from conventional molds because of the use of film and the required lens shapes in the mold cavities. The mold is equipped with two separate 3

4 vacuum systems. One vacuum system is used for film fixation and stretching of the film, in order to overcome film wrinkling, and the second for vacuum forming the film into the package cavities and lens domes. The mold can be equipped with specific insert types. An insert with integrated lens shape or an insert for exposed chip/external lens. See Fig.8 and 9. An array of lens shapes can be fabricated in the mold. See Fig.6 and 7. The mold lens domes are fine polished in order to obtain high quality molded lenses. Due to the elongation of the film during vacuum forming into the lens domes, the shape of the lens can deviate from the specification. The degree of deviation is dependent on the dome depth to diameter ratio. For high precision lenses this deviation is difficult to predict or calculate because the film deforms due to non-linear elongation and slip-stick effects between film and mold insert steel. When high precision molded lenses are required, the lens inserts will be designed and manufactured for nominal lens shape corrected for the film thickness. The first lenses, molded in a small test mold with a limited amount of lens inserts, will be accurately measured and deviations from the nominal lens shape will be corrected in the final inserts. Fig.7 Mold insert with lens array. Fig.8 Lens insert. Fig.9 Exposed chip insert..3 Molded lens shape calculation. The required lens shape for a specific photonic device is dependent on the application and can be calculated. See formula 1. Typical lens parameters like radius and conical constant are lens parameters which are required for a given lens. The formula calculates the values for the lens height (z) as a function the different positions along the lens length (h). The radius (R) determines the shape of an ideal sphere and the conical constant (k) determines the required deviation from the ideal sphere. For k = 0 the lens shape is defined as perfectly spherical, while for k = -1 the lens shape is parabolic. Fig.6 Sample mold cavity strip. z f ( h) R1 Formula 1. h 1 (1 k) h R A4h 4 A h

5 the steel insert can be corrected for the measured deviation caused by the film deformation..4 Molded lens shape measurement. Molded lenses can be very accurately measured with confocal microscopy technique. It can measure steep angles and has a high lateral resolution. Surface irregularities or roughness can be measured from 1nm up to very rough surfaces in the range of 10 to 100 mm. Furthermore the system has a high Z- scan range of several mm. See Fig. 10 to 1. Due to the fact that clear compound is used, the molded lenses must be coated before they can be measured. Fig.11 Confocal microscopy. Fig.1 Graphical lens representation..5 Molding materials for lenses. Commonly used clear compound types for lens molding by transfer molding process are Epoxy based and Silicone based compounds. It is available in liquid, pelletized and powder form. See Fig.13 Fig.10 Confocal measurement principle. The lenses are measured in 3D and the measurement data can be imported in the CAD system where de molded lens shape can be compared with the specified lens shape. The lens shape in Fig.13 Liquid, pelletized and powder compounds. 5

6 .5.1 Clear Epoxy compound. Epoxy based compound for lens molding consists mainly out of epoxy resin and hardener. It is a heat curable compound and available in pelletized and casted cylindrical shape. These clear compound types contain usually no Silica filler like the black variants and therefore exhibit a higher CTE which often leads to a higher warp and stress factor. The refractive index varies from roughly from 1.50 to 1.58 depending on the wave length of the light source. The transmission coefficient is usually > 90%. In general, epoxy compounds are more susceptible to influences of UV radiation and heat. This shows up as discoloration (yellowing) of the compound in time and as such lowers the transmission coefficient during life time. Clear Epoxy compound is commonly used for LED, transceiver end CPV packaging..5. Clear Silicone compound. Silicone based compounds for transfer molding are usually part heat curable compound types and exhibit very good optical properties. It must be accurately mixed in the proper ratio and de-aired after mixing. The transmission coefficient can be close to 100% over a wave length ranging from 50 to 1550 nm. Silicone compounds are available in two types, Methyl based and Phenyl based silicones. They differ in refractive index, which is about 1.4 for the Methyl and slightly over 1.5 for the Phenyl type. Silicone based compounds show excellent resistance against UV radiation and heat. The material hardness is in the Shore A range which is much softer compared to Epoxy compounds. This is an advantage for high power LED and CPV devices because this lowers the stress in the encapsulated devices. Silicone compounds can have a rather low viscosity causing difficulties in controlling bleed in non-fam molds..6 FAM molding process. The FAM molding process sequence is shown in Fig. 14 The FAM molding system can be equipped with one or two film handlers. One film handler can be used for the top mold and the second for the bottom mold. Film roles are mounted on each film handler en can be easily exchanged or renewed. Fresh film is loaded after each completed molding cycle. A vacuum system in both top and bottom mold will vacuum form the films into the mold cavities. Mold inserts for lens or exposed chip molding can be placed in top and/or bottom mold. For exposed chip molding the inserts covered with the film land on the chip surface outside the optical sensing areas when the mold is closed and put a pre defined force locally on the chip via the film in order to keep the active chip surface free of molding compound. 1 3 Fig.14 FAM molding process steps

7 The FAM mold and molding system can handle pelletized as well as liquid compound types due to a patented plunger and compound reservoir system. See Fig.15 and 16. TOP FILM BOTTOM FILM 13 Fig.16 Fully Automatic FAM molding system. 13 PELLET ROW Fig.15 FAM mold for pelletized and liquid compound. 7