Flow Sensor

2010 12th Electronics Packaging Technology Conference

Release and Mounting of Partially Flexible Devices Inside and Around Tubes for Smart Invasive Medical Applications

B. Mimoun1, V. Henneken2, R. Dekker1,2

[1] Delft Institute of Microsystems and Nanoelectronics (DIMES) Delft University of Technology, Feldmannweg 17, 2628 CT Delft, The Netherlands, [2] Philips Research, High Tech Campus 4, 5656AE Eindhoven, The Netherlands

E-mail: [email protected], Tel: +31 (0)15 27 83700, Fax: +31 (0)15 27 87369

Abstract

There is a strong trend to add microsensors and actuators at the tip of minimally invasive medical instruments such as catheters and guidewires. Simultaneous measurement of both dynamic blood flow and pressure in small coronary arteries would be of great value for cardiologists in the assessment of certain types of Coronary Artery Diseases (CAD). This paper reports on the successful dicing, handling and mounting of partially flexible demonstrators inside and around tubes of diameter ranging from 600 µm to 850 µm. The presented approach gives new prospects for the fabrication and the packaging of smart miniature sensors that have to be mounted on or in catheters or guidewires.

Introduction In recent years, micromachined sensors have seen

application in many fields. The improvements in Micro-Electro-Mechanical-Systems (MEMS) technology have opened doors to the miniaturization and integration of more sensing and/or electronics functionalities onto smaller chips. One field that will benefit from this technology is that of minimally invasive diagnosis and therapy such as endoscopy and catheterization. The application discussed in this paper focuses on Coronary Artery Diseases (CAD). Typical problems to be treated are deposits of coagulated blood, fatty tissues or calcium on the vessel walls, leading to partial or total blocking of the blood flow (stenosis). The affected area is reached by making a small incision in a main artery or vessel, usually in the groin. Through this opening, a thin flexible wire (a guidewire or a catheter) is inserted and pushed along the blood vessels to the target destination. The diameter of such wires can vary from a few millimeters to a few hundreds of micrometers. There is a strong trend to add microsensors and actuators at the tip of such minimally invasive medical instruments. Also, since those medical instruments are usually cylindrical and hollow, the use of conventionally shaped (square or rectangle) silicon chips can be problematic. Using flexible sensors in order to comply with the rounded shape of the tubes is a possible solution.

Haga and Esashi published in 2004 a review of catheter-based devices including pressure sensors, flow sensors and imagers, alone or combined [1], while only few compliant devices have been reported in the literature [2], [3], [4]. The diameter of the reported flexible catheter-based applications varies from a couple of centimeters down to 700 micrometers.

A 300 µm diameter guidewire containing a pressure sensor at its tip is already commercially available [5]. Adding instantaneous flow rate sensing capabilities to this guidewire would be of great value. This would yield important

information on the dynamics of the micro-vascular system being fed by the coronary artery system. The simultaneous measurement of both dynamic blood flow and pressure in small coronary arteries has indeed been proven to be very useful for cardiologists in the assessment of certain types of Coronary Artery Diseases (CAD) [6]. The proposed flow sensor is based on a calorimetric flow measurement principle: it consists of a small heating element, and two temperature sensing parts that measure flow-induced temperature differences over the sensor. To achieve this, the fabrication of a flexible thermal flow sensor which remains functional while bent around the 300 µm diameter guidewire is necessary. Moreover, due to the very limited amount of space inside the guidewire for the connection of cables, signal processing and Analog-to-Digital (A/D) conversion electronic functions have to be implemented at the tip of the guidewire. Fig. 1 is a schematic representation of such a flexible sensor connected to a thin silicon chip and mounted inside the 300 µm diameter guidewire.

Fig. 1. Flexible sensor bent around a 300 µm diameter

guidewire and connected to the 50 µm thin Si sensor chip.

Previous work and fabricated demonstrator In a previous publication we presented the so-called Flex-

to-Rigid (F2R) technology which is ideally suited for the fabrication of miniature sensors and circuits that have to be mounted on or in catheters or guidewires [7]. In the F2R technology, silicon chips of arbitrary thickness and shape connected by ultra-flexible polyimide-based interconnects

Guidewire

50 µm thin ASIC bonded to the Si sensor

chip

Rigid 50 µm thin Si pressure sensor chip

Side flexible flow sensor

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978-1-4244-8561-1/10/$26.00 ©2010 IEEE


were fabricated. Fig. 2 depicts a demonstrator fabricated using this technology. The unique system of polyimide tabs allowed the structure to remain fixed to the wafer frame and to be easily released just before the last step of the process (i.e. the mounting step). However, the 50 µm thin silicon chips still needed to be separated from the wafer and an Application-Specific Integrated Circuit (ASIC) still needed to be added to the fabricated device.

The goal of the present paper is to report on the final part of the process, namely the addition of the ASIC, the dicing, the handling and the mounting of the demonstrator described above, inside and around tubes of diameters ranging from 600 µm to 850 µm. Although the final combined flow / pressure sensor should fit inside (and around) a 300 µm guidewire, *2 scaled devices were used for the work presented in this paper. For these mechanical tests, dummy sensor and ASICs that did not contain any sensing or electronic functionality were used. Their fabrication process is reported in [7].

Fig. 2. Device attached to the silicon wafer frame.

Size related issues Decreasing the dimensions of a device usually brings up

size related issues such as a reduced mechanical integrity of the device and an increase complexity of its handling.

In our case, not only the fabricated devices were small but they also were very thin. The dimensions of the device were indeed determined by the diameter of the medical instrument it had to fit in (or around). As shown in Fig. 1, the diameter of the guidewire was of 300 µm. If the width of the Si chip was about 170 µm, its maximum thickness should be around 200 µm. Because of other mechanical components present inside the guidewire, the total thickness of the Si chips had to be smaller than 150 µm. In addition, due to the very small diameter of the guidewire, only 3 electrical wires could fit inside the tube. Therefore, electronics had to be implemented directly at the tip of the guidewire, implying that an ASIC containing the desired electronic functions had to be added to the sensor device. In the final version of the product, the ASIC is flip-chipped. It then means that the total thickness of the stack Si chip / ASIC had to be smaller that 150 μm. Supposing that the flip-chipping process (e.g. bondpads on each chip, solder, underfilling) uses a space of about 50 µm, the

thicknesses of both the ASIC and the fabricated Si chip also had to be in the order of 50 µm.

Although the demonstrator presented in this paper was mounted inside tubes of diameters 2 to 3 times bigger than 300 µm, the thickness of each Si chip was kept to 50 µm as a “proof of concept”.

In addition, handling a device can sometimes (if not often) cause damage to the device itself, a fortiori when the parts are very small and thin. Adding a removable “handling stripe” enables the user to handle, mount and finally fold the device inside and around the tube without any risk of damage.

Gluing of the dummy ASICs on the sensor chips Dummy ASICs were fabricated on the same wafer as the

devices using the F2R technology (cf. Fig. 3.) They could easily be released from the wafer by cutting the polyimide tabs. The space available on the sensor chip for the dummy ASICs to be flip-chipped was 600 µm * 500 µm. As discussed in the previous section, their thickness should be of 50 µm. However, handling such small chips can be problematic. In order to facilitate their handling, the dimensions of the fabricated ASICs were 600 µm * 2 mm. The thickness of the “unneeded” part of the ASICs was 400 µm (the full thickness of the Si wafer) while only the part of the dummy ASICs which was kept on the sensor chip in the end was 50 µm thin.

Fig. 3. Dummy ASIC attached to the wafer frame. Once released from the wafer, the ASICs were attached to

the devices. In order to simulate the flip-chipping and underfilling steps, the dummy ASICs were glued directly on top of the sensor chip (cf. Fig. 4). A low viscosity epoxy encapsulant was used as glue (STYCAST 1266 part A and part B [8]).

Fig. 4. Dummy ASIC glued to device, before dicing.

Dummy ASIC glued to the device

400 µm thick

50 µm thin

400 µm thick wafer

Flexible polyimide

foil

Handling stripe

Released polyimide tabs

Flexible interconnect

50 µm thin Si sensor chip

Polyimide tabs

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Dicing of the devices The F2R technology enables the fabrication of devices that

can be attached to the wafer by means of easily removable polyimide tabs only. However, in our case ASICs needed to be flip-chipped to the device after its fabrication. A flip-chipping process usually requires the “base” chip to withstand a certain amount of pressure. The polyimide tabs were much too fragile to support the pressure reached during flip-chipping. By leaving one side of the sensor chip attached to the supporting frame, the base chip should withstand the force applied during the flip-chipping of the ASIC. The release of the Si platform / dummy ASIC stack was done by means of dicing.

The dicing step appeared to be critical. Indeed, the 50 µm thin Si chips and their glued dummy ASICs were very fragile due to their very small thickness. Breaking of the Si chips was observed during the dicing step, independently of the dicing saw speed (cf. Fig. 5. (a), (b)).

(a)

(b)

Fig. 5. SEM pictures of a device after dicing. The 50 µm thin

chip is broken.

In order to solve this problem, the devices were embedded in different kind of materials such as resist and paraffin wax (cf. Fig. 6 and Fig. 7. (a), (b), (c)).

Fig. 6. Backside view of the wafer before dicing.

(a) (b) (c)

Fig. 7. Backside of the devices before dicing. (a) No embedding. (b) Embedded in resist. (c) Embedded in

paraffin wax. Both paraffin wax and resist appeared to be acceptable

solutions as the Si chips were found to be intact after the dicing step. However, due to stress arising during resist curing, some of the polyimide tabs were found to be broken. The resist was removed in acetone while the paraffin wax was dissolved in “White-spirit”. Resist was selected for embedding the devices before dicing because it was much easier to remove it than the paraffin wax.

Release of the device by dicing while keeping the full wafer in one piece was possible due to the thickness difference between the 150 µm Si chip / dummy ASIC stack and the 400 µm thick Si wafer. The dicing saw was set-up to dice down to a thickness of 200 µm only, allowing the Si chips / ASICs stack to be released while keeping the full wafer intact (cf. Fig. 8). Cutting the polyimide tabs completely released the device (cf. Fig. 9).

400 µm thick wafer frame

Flexible PI foil

Handling stripe

Released polyimide tabs

Flexible interconnect

50 µm thin Si chip

Polyimide tabs Resist

Paraffin No embedding

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Fig. 8. Wafer after dicing. The wafer is kept in one piece.

Fig. 9. Released device.

Mounting of the devices inside and around tubes The devices were mounted in tubes of diameter ranging

from 600 µm to 850 µm provided with 50 µm slits at their tip (cf. Fig. 10). The tubes were clamped horizontally in a rotating tool (cf. Fig. 11).

Fig. 10. 800 µm diameter tube with a 50 µm wide slit at its tip.

Fig. 11. 800 µm diameter tube clamped on a rotating tool.

Handling of the devices was done using the “handling stripe”, consisting of a piece of 50 µm thick silicon covered with a layer of polyimide of 10 µm. This stripe was attached to the flexible polyimide foil with polyimide tabs (cf. Fig. 9). Those tabs were easily cut, releasing the “handling stripe” once the device was mounted inside the tube.

Once the tube was clamped and the device released from the wafer, the device was inserted into the tube, leaving the polyimide foil sticking out through the slit (cf. Fig. 12). Glue was applied to the backside of the foil and a dedicated “fork” tool was used to bend the 10 µm thin polyimide foil around the tube (cf. Fig. 13).

Fig. 12. Insertion of the device inside the tube.

Dicing line

Released device Clamping tool

Tube

Handling stripe

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Fig. 13. Bending of the polyimide foil around the tube using the “fork” folding tool.

This folding tool was custom made in order to fit around

tubes of different sizes. The polyimide foil was wrapped around the tube simply by rotating the “fork”. Once the foil was fully wrapped around the tube, the “fork” stopped on the “handling stripe” (cf. Fig. 14). The stripe could be separated from the polyimide foil either by cutting the polyimide tabs keeping it attached to the polyimide foil or by rotating the “fork” a little bit more.

Fig. 14. Device inserted in the tube and polyimide foil folded around it. Rotating the “fork” a little bit more allows the

release of the “handling stripe”. Keeping the foil folded around the tube for 15 minutes

allowed the glue to cure. Fig. 15 (a) and (b) depict the final situation with the sensor chip / ASIC stack inserted inside the tube, and the polyimide sensor foil wrapped around it after removal of the “fork”.

(a)

(b)

Fig. 14. Device mounted in and around the 800 µm tube.

Conclusions and future work The Flex-to-Rigid” (F2R) technology was especially

developed for the fabrication of very small ASIC / sensor assemblies for catheter and guidewire applications. In this work, we studied the mechanical integration and assembly of a pressure / flow sensor combination on a 800 µm tube. By using otherwise non functional ASICs and sensor chips, the successful assembly of such a system in a 800 µm diameter wire was demonstrated. After release from the wafer, the device was inserted into the tube. A dedicated “fork” folding tool allowed the wrapping and gluing of the 10 µm thin polyimide foil around the tube.

This separation and mounting process combined with the post-processing F2R technology open new doors for the fabrication and the packaging of smart miniature sensors that have to be mounted on or in catheters or other medical implantable devices such as drug delivery devices or e-pills.

As future work, filling of the tube with a biocompatible material such as PDMS or parylene might be considered. Although further packaging of the device is not required since polyimide is a Food and Drug Administration (FDA) approved

“fork” folding tool

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material, blood is a corrosive and electricity-conductive fluid. Also, heat shrinking tubes could be used as an alternative for the fixation of the polyimide foil around the tube. In addition, even if dicing has proven to be a successful technique for the separation of the devices from the wafer, lasering might be a faster, cheaper and cleaner solution. This technique might then be considered as a next step towards the fabrication of a fully functional miniature dynamic blood flow and pressure smart medical guidewire.

Acknowledgments This work was supported by the Dutch Technology

Foundation (STW), under project STW 10046. The authors would like to thank Wim van der Vlist for his help with dicing and Rik van den Hoeven for the fabrication of the tubes and the “fork” and his help with the mounting of the devices.

References 1. Haga, Y., and M. Esashi, “Biomedical microsystems for

minimally invasive diagnosis and treatment,” Proceedings of the IEEE, 2004, pp. 98-114.

2. Park, K.T., and M. Esashi, “A multilink active catheter with polyimide-based integrated CMOS interface circuits,” Journal of Microelectromechanical Systems, Vol. 8, No. 4 (1999), pp. 349-357.

3. Li, C., J. Han, and C.H. Ahn, “Flexible biosensors on spirally rolled micro tube for cardiovascular in vivo monitoring,” Biosensors and Bioelectronics, vol. 22, No. 9-10 (2007), pp. 1988-1993.

4. Shikida, M., et al., “A catheter-type flow sensor for measurement of aspirated- and inspired-air characteristics in the bronchial region,” Journal of Micromechanics and Microengineering, Vol. 19, No. 10 (2009), pp. 105027.

5. www.sjmprofessional.com/Products/Intl/Intravascular-Sensors/PressureWire-Certus.aspx

6. Escaned, J., et al., “Assessment of Microcirculatory Remodeling With Intracoronary Flow Velocity and Pressure Measurements: Validation With Endomyocardial Sampling in Cardiac Allografts,” Circulation, Vol. 120, No. 16 (2009), pp. 1561-1568.

7. Mimoun, B., et al., “Flex-to-Rigid (F2R): A Novel Ultra-Flexible Technology for Smart Invasive Medical Instruments,” Mater. Res. Soc. Symp. Proc, Warrendale, PA, 2010, In press.

8. www.cmr-direct.com/images/products/02-33-003/1266specs.pdf

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Flow Sensor

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