Kimmo Keränen

LTCC-based differential photo acoustic cell for ppm gas sensing

Silicon MEMS cantilever-based photoacoustic technology allows for the sensing of ultra low gas concentrations with very wide dynamic range. The sensitivity enhancement is achieved with a cantilever microphone system in which the cantilever displacement is probed with an optical interferometer providing a pico-meter resolution. In the gas sensor, the silicon cantilever microphone is placed in a two-chamber differential gas cell. By monitoring differential pressure changes between the two chambers, the differential cell operates as a differential infra-red detector for optical absorption signals through a measurement and reference path. The differential pressure signal is proportional to gas concentration in the optical measurement path. We have designed, implemented and tested a differential photo acoustic gas cell based on Low Temperature Co-fired Ceramic (LTCC) multilayer substrate technology. Standard LTCC technology enables implementation of 2.5D structures including holes, cavities and channels into the electronic substrate. The implemented differential photoacoustic gas cell structure includes two 10 mm long cylindrical cells, diameter of 2.4 mm. Reflectance measurements of the cell showed that reflectivity of the substrate material can be improved by a factor 15 - 90 in the 3 - 8 μm spectral region using gold or silver paste coatings. A transparent window is required in the differential gas cell structure in order to probe the displacement of the silicon cantilever. The transparent sapphire window was sealed to the LTCC substrate using two methods: screen printed Au80/Sn20 solder paste and pre-attached glass solder paste (Diemat DM2700P/H848). Both methods were shown to provide hermetic sealing of sapphire windows to LTCC substrate. The measured He-leak rate for the 10 sealed test samples implemented using glass paste were less than 2.0 ×10-9 atm×cm3/s, which meets the requirement for the leak rate according to MIL-STD 883. The achieved hermetic level suggests that the proof-of-principle packaging demonstrator paves the way for implementing a novel differential photoacoustic gas cell for a future miniature gas sensor module. The future module consisting of a sample gas cell and immersion lens IR-LEDs together with interferometric probing of the cantilever microphone is expected to be capable of measuring ultra low concentrations of a wide range of gases with their fundamental absorption bands at 3 - 7 μm wavelength, such as CO, CO2 and CH4.

Hot Laminated Multilayer Polymer Illumination Structure Based on Embedded LED Chips

The dominant technology for manufacturing backlight illumination structure (BLIS) is typically based on the use of individually packaged surface mount device light emitting diodes (LEDs) and special light guide plate (LGP) and diffuser films. The prevailing BLIS package, however, contains several separate diffuser films, which results in a thick and costly structure. In addition, the light coupling from LED to the LGP is sensitive to alignment errors causing nonuniform and inefficient illumination. We have demonstrated a novel hot laminated packaging structure for backlighting solutions, which is based on inorganic LED chips and multilayer polymer structure. The main advantages of the implemented system compared to the traditional light guiding system are easy optical coupling with high efficiency in an integrated and thin package. The performed designs of 3×3, 5×5, and 5×7 LED chip matrices, verified by test structure implementations and characterizations, showed that the final thickness of the BLIS depends on the required uniformity of illumination, allowed LED device pitch and efficiency of the diffuser. The final BLIS demonstrator size was 50×75 mm2 consisting of six 25×25 mm2 modules. Each module consisting 5×5 LED devices resulting in total number of 150 LED devices with 5-mm pitch. The measured key characteristics of the demonstrator were as follows: average brightness 11.600 cd/m2 (ILED = 2 mA), luminous efficiency 22 lm/W, color temperature 5550 K, commission on illumination values (x = 0.331, y = 0.411), Color Rendering Index ≥ 70, and total power conversion efficiency of 6.3%. The combination of the developed Matlab performance simulation tool and cost-of-ownership cost evaluation tool enables us to estimate the manufacturing cost of a specific BLIS element against the required performance, assisting decision-making in different applications and specific individual customer cases.

Hermetic Fiber Pigtailed Laser Module Utilizing Passive Device Alignment on an LTCC Substrate

A hermetic fiber pigtailed laser module utilizing passive device alignment on a low-temperature cofired ceramics (LTCC) substrate is demonstrated. The 3-D shape of the laminated and cofired ceramic substrate provides the necessary alignment structures, including grooves and cavities, for the laser-to-fiber coupling. When the laser diode chip and component tolerances are tight enough, the passive alignment allows high coupling efficiency realizations of multimode fiber pigtailed laser modules. The ceramic substrate is intrinsically hermetic and it opens up the possibility to use the substrate as an integrated part of the hermetic module package. In our concept hermetic sealing is produced by utilizing a Kovar frame, which is soldered to an LTCC substrate. The Kovar frame has a hole for a fiber feed-through and a hermetic glass-metal seal between fiber and frame is processed using a glass preform. The module can be used as a transmitter in a laser pulse time-of-flight distance sensor and in this application it can be overdriven by a factor of 10. This means that the peak optical power in the pulses can be several dozen watts. The laser chip allows this kind of overdriving, due to the fact that the duty factor in the operation is only 0.0% at 2 kHz pulsing frequency, which leads to an average power of several milliwatts. The simulated nominal coupling efficiency between the 210 mum times 1 mum stripe laser and the 200/220 mum step index fiber (NA = 0.22) was 0.65. The measured coupling efficiency of the hermetically sealed prototypes varied from 0.14 to 0.64, where the average was 0.39. A leak rate of 1 times 10-7 . .. 8 times 10-7 [atm times cm3/s] was measured in the helium leak tests of the final operational prototypes, when the modules were tested according to MIL-STD-883F method 1014.9 specification. The rather high leak rate is mainly due to the helium absorbed by the fiber polymer buffer layer and rubber guard tube in the pressurization process. The leak rate for the dummy modules using a buffer stripped fiber without a rubber guard tube was 3 times 10-9 . .. 1 times 10-8 [atm times cm3/s]. The maximum allowed leak rate for this size of hermetic module is 1 times 10-7 [atm times cm3/s]. The background helium level before and after the tests was less than 3 times 10-10 [atm times cm3/s]. Measurements proved that the manufacturing procedure is capable of producing hermetic fiber pigtailed laser modules.

Inmould integration of a microscope add-on system to a 1.3 Mpix camera phone

A microscope add-on device to a 1.3 Mpix camera phone was selected as a demonstrator system for testing inmould integration of electronic substrates and plastic optics. Optical design of the device was quite challenging due to the fact that illumination system needed to be integrated with a double aspheric singlet lens structure as a single optical piece. The designed imaging lens resolution was adequate to resolve 10 &mgr;m features with a mobile phone camera. In the illumination optics the light from LEDs embedded into the plastic structure was collected and guided to the surface that was imaged. Illumination was designed to be uniform and adequately bright to achieve high resolution images with the camera phone. Lens mould design was tested by using injection moulding simulation software. The critical mould optical surfaces were designed as separate insert parts. Final shapes producing lens surfaces were tooled by diamond turning on nickel coatings. Electronic circuit board inserts with bonded bare LED chips and packaged SMD LEDs were assembled to the mould and then overmoulded with optical grade PMMA. Experiences proved that inmould integration of electronic substrates, bare LED chips and high resolution imaging optics in injection-compression moulding process is feasible. The yield of embedded packaged and also bare chip components was close to 100% after the right injection moulding process parameters were found. Prototype add-on system was characterized by testing the imaging properties of the device with a camera phone.

Cost-effective packaging of laser modules using LTCC substrates

The modeling, realization and characterization of photonic module based on the use of Low Temperature Co-fired Ceramics (LTCC) technology is reported. The 3D modeling of the system provides possibility to optimize structures, materials and components in order to achieve optimal performance for the final product and still maintain reasonably low fabrication costs. The cost-effectiveness in the product can be further optimized using an iterative optimization process, in which the effect of module manufacturing tolerances and assembly process tolerances is simulated by a VisVSA Monte-Carlo simulation. The tolerance distributions produced by a VisVSA simulation are used as input parameters together with optical component tolerances in an ASAP Monte-Carlo simulation, in which the final module optical performance distribution in simulated production is obtained. The module cost, performance and optical performance limited yield is possible to define with this iterative process. As an example of this kind of packaging modeling, we present a demonstrator module having a high-power multimode laser diode with a 1μm x 100μm emitting area coupled to a 62.5/125μm graded-index (NA=0.275) multimode fiber. The tolerance modeling results are verified by experimental characterization of the packaged prototypes. Measured coupling efficiencies were in good agreement with simulated ones, when the fiber NA was 0.2 or larger. The measured coupling efficiency, however, was 38% lower than simulated, when the fiber NA was 0.12. This was probably due to the laser mode structure difference between simulation model and reality. Coupling efficiency of 0.46 was obtained in a passively aligned demonstrator module, when the nominal value was 0.48.

Printed hybrid systems

This paper presents research activities carried out at VTT Technical Research Centre of Finland in the field of hybrid integration of optics, electronics and mechanics. Main focus area in our research is the manufacturing of electronic modules and product structures with printed electronics, film-over-molding and polymer sheet lamination technologies and the goal is in the next generation of smart systems utilizing monolithic polymer packages. The combination of manufacturing technologies such as roll-to-roll -printing, injection molding and traditional component assembly is called Printed Hybrid Systems (PHS). Several demonstrator structures have been made, which show the potential of polymer packaging technology. One demonstrator example is a laminated structure with embedded LED chips. Element thickness is only 0.3mm and the flexible stack of foils can be bent in two directions after assembly process and was shaped curved using heat and pressure. The combination of printed flexible circuit boards and injection molding has also been demonstrated with several functional modules. The demonstrators illustrate the potential of origami electronics, which can be cut and folded to 3D shapes. It shows that several manufacturing process steps can be eliminated by Printed Hybrid Systems technology. The main benefits of this combination are small size, ruggedness and conformality. The devices are ideally suited for medical applications as the sensitive electronic components are well protected inside the plastic and the structures can be cleaned easily due to the fact that they have no joints or seams that can accumulate dirt or bacteria.

Fiber pigtailed multimode laser module based on passive device alignment on an LTCC substrate

A concept that utilizes structured planar substrates based on low-temperature cofired ceramics (LTCC) as a precision platform for a miniature passive alignment multimode laser module is demonstrated. The three-dimensional shape of the laminated and fired ceramic substrate provides the necessary alignment structures including holes, grooves, and cavities for the laser-to-fiber coupling. The achieved passive alignment accuracy allows high coupling efficiency realizations of multimode fiber pigtailed laser modules. Thick-film printing and via punching can be incorporated in order to integrate electronic assemblies directly on the optomechanical platform. The platform is scalable, and it allows embedding of subsystems, such as silicon optical bench (SiOB), but it also provides the interface for larger optical systems. Temperature management of high-power laser diodes is achieved by realizing heat dissipation structures and a cooling channel into the LTCC substrate. The measured maximum laser metallization temperature was 70degC when a thermal power of 0.5 W was applied at the laser active area using a liquid cooling of 50 mL/min. The measured maximum temperature of the laser surface was about three times higher without liquid cooling. Optical coupling efficiency of the multimode laser systems was simulated using optical systems simulation software. The nominal coupling efficiency between 100times1 mum stripe laser and 62.5/125-mum graded index fiber (NA=0.275) was 0.37. The simulated coupling efficiency and alignment tolerances were verified by prototype realization and characterization. The measured alignment tolerance values between laser and fiber in AT prototype series were Deltax=7.7 mum, Deltay=7.6 mum, and Deltaz=10.8 mum (SD values). The corresponding values in A2 prototype series were Deltax=3.1 mum, Deltay=9.1 mum, and Deltaz=10.2 mum. The measured average coupling efficiency was 0.28 in AT series and 0.31 in A2 series. The coupling efficiencies of all operational prototypes varied from 0.05 to 0.43

Prototyping of miniature plastic imaging lens

The prototyping process of miniaturized plastic imaging lens is described. The sequence is divided into five phases: specification, optics design, optomechanical design, manufacturing and characterization. During specification, the optical and mechanical requirements of the lens are defined. In the optical design phase, the lens is optimized, and a tolerance analysis is carried out. Simulation tools, especially, an image quality simulator, can be used to visualize and verify the performance of the design. Mechanical design is performed considering the geometrical specifications and optical tolerances of the system. In addition, stray light analysis is carried out to verify the optical performance of the optomechanics. Plastic optics are particularly vulnerable to stray light due to the integrated mountings, which provide additional paths for unwanted light. If the prototype is used for preliminary performance evaluation of a future product, the differences between prototype and mass manufacturing methods need to be considered carefully. After the lenses are manufactured they are characterized, and the experimental results are compared with the original specifications and estimations obtained from the previous design verification simulations. New error analysis simulations can be performed in order to pinpoint faults in manufactured modules. If the performance of the prototype is not sufficient, a new prototyping iteration circle is needed. The whole process is described and analyzed using a miniature, plastic imaging lens as an example, but it can also be applied to other optical prototyping tasks.

Fiberoptic in-vessel viewing system for the International Thermonuclear Experimental Reactor

A viewing system was designed and a prototype realized for the in-vessel inspection of the International Thermonuclear Experimental Reactor. The viewing is based on the line scanning principle, and the system consists of ten identical units installed on top of the reactor at 36° intervals. Each device contains a laser, beam steering mirrors, and viewing probe with insertion mechanics. The probe has an outside diameter of 150 mm and a length of 14 m. The illumination design applies frequency-doubled Nd: yttrium–aluminum–garnet lasers whose beams are guided through hermetically sealed windows into the vacuum vessel. The diffuser optics creates a vertically oriented light stripe onto the vessel surface that is viewed by the imaging optics, consisting of 16 modules altogether covering horizontal and vertical field-of-views of 2° and 162°. The optical images are transferred to charge coupled device cameras via coherent fiber arrays. The multifocus design uses stacked fiber rows whose ends are assembled into di...

2.5.5 Microimmersion lens LEDs for portable photoacoustic methane sensors

Microimmersion lens mid-infrared light emitting diodes (mid-IR LEDs) operating in the 3.4 m wavelength range, which covers the absorption bands of many important hydrocarbons (CH4, C3H8…), have been studied with respect to their application in cantilever enhanced photoacoustic trace gas detection technique