Sensor structures generated with combination of SU8 and PCBMEMS

Stefan Gassmann*, Antonio Luque**, F. Perdigones**, J. M. Quero**, Lienhard Pagel***

*Jade University of Applied Sciences, Department of Engineering, Friedrich-Paffrath-Str. 101 D 26389 Wilhelmshaven

*Stefan.Gassmann@jade-hs.de **University of Seville, Dept. Electronic Engineering

Av. Descubrimientos s/n E41092 Seville, Spain

***University of Rostock, Institute of Electronic Appliances and Circuits A.-Einstein-Str.2

D 18059 Rostock, Germany

Abstract - The usage of standard PCBs (Printed Circuit Boards) for the creation of microfluidic systems was already reported. Here the combination of a standard microtechnology using photolithography and photo sensitive layers (SU8) and the PCB MEMS technology is presented. Using PCBs as the substrate the integration of electronics is simple and the substrate is available at low cost. Putting the sophisticated SU8 based micro technology using photolithography on top, new systems can be created. This combination leads to interesting low cost solutions for micro systems. Both technologies deliver their best, in the PCB technology integrated electrodes and electronics can be created while the SU8 technology adds high resolution high aspect-ratio micro systems which are not possible in a PCB implementation. Using this technology combination several sensor structures can be created. In this paper as successful examples an electrochemical sensor cell array and a flow sensor will be presented.

Keywords: PCB, fluidics, SU8, PCB MEMS

I. INTRODUCTION

Printed circuit boards (PCBs) have in all electronic devices the holding and wiring task for the electronic elements. Using special technologies they can take over other functionalities and can become functional elements itself. Examples like pumps [1], valves [2] and sensors [3] are already reported. The so called PCB-MEMS have reached a sophisticated level where a lot of functionalities can be realized. The basic of the building technologies is that PCB techniques are used to build the MEMS structures. The basic building materials of the PCB, Copper and FR4 are structured. Using this approach there are some drawbacks. The smallest possible features are limited by the structuring of the copper lines. This is in the moment around 50µm. These smallest features made in copper, so smaller features are always conductive. When nonconductive features and higher resolutions are needed a new approach has to be used.

Here a combination of the PCB MEMS and a sophisticated standard MEMS technology will be presented. In this case a MEMS technology using SU8, a photosensitive epoxy, will

be used to create microstructures on top of a PCB MEMS. So both technologies will be combined. On the PCB electrodes, electrical connections, electronic components and sensors will be realized and the PCB is in the same time the substrate for the SU8. In the SU8 layer microstructures like channels with feature sizes down to 1 µm can be created.

The advantage of this approach is the combination of the low cost PCB technology for the creation of electronic functions with the sophisticated micro technology based on SU8 for the creation of the micro structures.

In the following the technology steps, problems and two examples, a flow sensor and an electrochemical cell array will be described.

II. TECHNOLOGY

The technology starts with a normal PCB process. This could be a single, double or multilayer PCB. The description here starts with a single layer PCB for simplification.

The raw material for the PCB production is an isolating carrier (normally FR4) with a copper coating (see Fig. 1 Step 1). The FR4 layer and the copper are available in different thicknesses. The creation of the structure in the copper layer will be done in a subtractive process. The copper layer is covered with a photo sensitive masking layer; this layer is structured with a photolithography process after the etching of the copper is performed. After the etching the mask is stripped. Such a PCB contains now the structure of copper lines on top of the basis material (see Fig. 1 Step 2).

Such a PCB is now used as a substrate for the SU8 deposition. In the first step the SU8 is coated on the substrate (see Fig. 1 Step 3). This can be done by spin coating as usual in micro technology. Since the edge bead can be important when using rectangular substrates, care has to be taken. After the coating a soft bake must applied to evaporate the solvent, after this a mask is put on the SU8 film and must be aligned to the structures on the PCB (see Fig. 1 Step 4). After the alignment the exposure takes place (see Fig. 1 Step 5). Than a post exposure bake has to follow in which the cross linking of the exposed material is performed. In the development step

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the not-cross linked parts of the SU8 are dissolved (see Fig. 1 Step 6). After washing the SU8 structures are ready to use. An additional top cover can be placed on top of the structures (see Fig. 1 Step 7).

The SU8 process is an additive process. The deposition can be done in several layers. Up to 500 µm thick layers can be realized in one step using normal UV exposure. Higher thicknesses are possible. The creation of top covers or several layers of SU8 micro structures can be created for example with the help of the BETTS process [4].

Fig.: 1: technology steps of the combination of SU8 on top of PCBs

Although this combination seems to be straight forward a

few things have to be taken into account. The used PCBs are normally in a rectangular shape. Coating a rectangular shaped substrate by spin coating can be challenging since the edge bead on rectangular substrates can be important. The excess material from the corner flows back during the soft bake and

the layer thickness will vary. Normal edge bead removing techniques where a solvent is applied near the edge of the substrate are not suitable. Using a recessed chuck gives the best results for spin coating. As an alternative coating procedure dip coating or the lamination of SU8 dry film can be used.

Adhesion of SU8 can be a problem on large copper areas. Here an appropriate cleaning with 40°C warm Butanone for 5 min in ultrasound is a good approach to overcome adhesion problems. When creating thicker layers than 100µm it is a good practice to spin on first a thinner layer of about 10 µm as a seed layer to achieve better adhesion.

In the following two examples that use the technology are

explained. First a flow sensor with a moving wheel and second an electrochemical cell using bond wires are depict.

III. FLOW SENSOR

A flow sensor based on a movable structure can be fabricated using a variation of the techniques described above. In order to have a structure that can move under the action of a fluid a sacrificial layer must be used to release it. Fortunately, copper is available on every PCB substrate, and the etchant used to remove copper in the standard PCB process is very selective with respect to SU8.

A. Fabrication process The process starts by patterning the copper layer on a PCB

substrate, removing all copper except for an area with a torus shape. After that, the substrate is coated with an SU8 layer, 100um-thick. Then the layer is patterned using pholithography and a tooth wheel is created over the copper torus. Also, a circular axis for the wheel is patterned in the center of the torus, where no copper remained. The copper is then etched in HCl and the wheel is released. Care must be taken in this step, as the released wheel can be lost in the etchant bath. Finally the device is covered with a transparent solid cover.

Fig.: 2 Released wheel made of SU8 by etching a copper sacrificial layer. The input and output channels for flow measurement can be seen on bottom and right edges

4 mask alignment on SU8

3 SU8 coated on top of PCB

2 finished PCB (FR4 with structured copper lines)

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5 exposure of SU8

6 developed SU8 structure

7 top cover on the SU8

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The fabricated device can be seen in Fig. 2. The fabricated wheel has an inner diameter of 4mm, an outer diameter of 7mm, and eight blades.

B. Characterization The fluid entering the device at the input channel and

leaving it at the output port makes the wheel rotate at an angular speed which is proportional to the flow rate. An optical detector at a fixed point near the perimeter of the wheel can be used to count the number of times that a blade passes through that point in its rotation.

It is clear from the fabrication process that there is a gap between the wheel and the substrate and top cover. The two gaps together have a length equal to the initial thickness of the sacrificial copper layer (35um in our case). Although there is fluid flowing through these gaps which does not contribute a momentum for the rotation, this is not important as long as the rotation speed is proportional to the total flow rate.

Fig.3: Final device with the optical sensor mounted on top

Fig. 3 shows the final device with an OPTEK OPB703 reflectant object sensor. SU8 is quite transparent, but the change in reflectivity with respect to water is enough to be detected by the sensor. Fig. 4 shows the captured signal from the sensor and the relationship between the measured frecuency of the pulses and the flow rate.

Fig. 4: Measured pulses from the optical detector (top), and flow-date/frequency characteristic curve (bottom)

In the future, the optical components might be added directly to the PCB substrate, improving the integration of the system. This would need some modifications to the fabrication process, because the copper etch step would etch all copper present in the board, including the pads and electrical connections for the optical components. In order to solve this problem, a protective adhesive must be applied over the parts of the board that will hold the electrical circuit. This adhesive would be manually removed after copper etching and before putting the electronic components in place.

IV. ELECTROCHEMICAL SENSOR CELL ARRAY

In electrochemical measurement a defined sensor cell is needed. This cell needs three electrodes: working, counter and reference electrode. Thin gold wires (25µm diameter) are usable when the working electrode needs to be heated. [6]. Since the manual handling of these gold wires is very challenging an approach is proposed where bond wires are used. Bond wires can be easily attached on the PCB using a bonding machine. Around of these bond wires a channel for the liquid that will be investigated can be created with SU8.

The here presented fluidic chip does not only contains the electrochemical cells but also the preparation of the sample. This is here the polymerase chain reaction (PCR) for the amplification of DNA since with the help of this fluidic chip an electrochemical DNA detection should be carried out. The here presented chip is based on a design presented by Pagel [7].

A. Technology for the electrochemical cell array The PCB with all needed structures for heating, fluid

movement, fluid sensing and bond pads was created. This PCB is the substrate for the fluidic chip. Details about the design of this chip can be found in [6]. On top of this PCB the electrochemical cells and the fluidic channel has to be created. For this a first layer of SU8 with a thickness of about 30µm was created. In this layer all bond pads and all contact areas are left open. This layer works like a solder mask. In the next step the bond wires are attached to the bond pads creating a high profile loop whre the middle section of the wire is about 100 µm over the ground. With the help of the half automatic bonder HB16 from tpt the bond loop was programmed so that the geometry is repeatable. In the next step the channel should be created around the existing bond loops. In this step the bond pads needs to be covered totally because of the very high sensitivity of the electrochemical measurement against contaminant metals that can be found in the area of the bond pads. The channel should be created only in the middle of the bond wire. For this purpose a 230µm thick SU8 layer was deposited by spin coating 2 layers of 115µm subsequently. The exposure was made in one step with a diffuse light exposure unit. This was needed to avoid shadows and unexposed regions under the bond wires.

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B. Realization and characterization The realization of this chip is shown in Fig. 5. On the left-

hand side an overview of the chip is seen. The channel is visible as a meander. The heaters that are realized on the backside can be seen as dark areas. The electrodes for the liquid sensing are also visible inside the channels (arrangement of three gold electrodes). In the lower bottom part the area with the electrochemical cells takes place. This part is magnified in the right hand picture. Here the copper wires (covered with gold for bonding) can be seen. In the middle the channel with the bond wires is visible. A total covering of the bond pads could be achieved by the above described approach.

Fig. 5: DNA detection chip created with SU8 on top of a PCB. Left picture: overview, right picture: magnified view of the bond wires used as electrochemical sensor

C. Measurements With the produced prototypes several measurements and

test where carried out. First the usability of the sensor cell was checked by cyclic voltammetry. For all wires voltage sweeps from -0.3 to 1.5V in a 0.05 M H2SO4 solution using the reference and counter electrodes build also by bond wires where recorded. All created electrochemical cells worked without any error. In the Fig. 6 the cyclic voltammogram of sensor 9 is depict. The slope down and up at the ends marks the ends of the scanning area. The down and the up peak in the curve are the oxidation and reduction peaks of gold, these peaks are very clear. This is a sign that the electrodes work perfect and there is no other contaminant metal.

Fig. 6: Cyclic voltammogram of electrode 9, gold oxidation and reduction peaks are visible without any contaminant.

The next test was the heating calibration of the electrodes. This shows if the heat transfer from the sensor to the liquid works. For this test a chronopotentiometry measurement was carried out. Fig. 7 shows the result of electrode 9. A heating current was applied to the electrode and the potential was recorded. The potential drop shows the reaction of the liquid to the temperature. The potential drops correlate to the heating pulses and the potential reaches the baseline after the heating pulse quick. This is the proof of the heating capability of the created sensor structure.

Fig. 7: chronopotentiometry of electrode 9. Heating pulses of 200 mV, 400 mV, 500mV, 600mV, 700mV and 800mV was applied.

After the successful heating calibration tests of the DNA sensing capability where carried out successful. This shows that the bond wire electrochemical cells with SU8 channels works for the target application.

V. CONCLUSION

The here presented technology combines PCB MEMS technology and the SU8 technology for the creation of micro fluidic systems. The PCB technology can be used for the creation of electrodes and electrical connections to sensors or other electronic elements. The PCB is in the same time the substrate for the following SU8 process. The SU8 is used to generate channel structures or movable parts on top of the PCB. The SU8 technology as a sophisticated MEMS technology is able to create structure sizes down to 1 µm in a photolithographic process. The combination of both technologies leads to low cost solutions for MEMS devices. In this paper two examples for the creation of sensors are depict: one flow sensor based on a paddle wheel and the second is an array of electrochemical sensors. Both sensors are characterized as working.

The technology combination does not only work for sensors also actuators or passive MEMS structures can be created. The here presented approach opens new possibilities to the PCB MEMS technology. The PCB MEMS technology can overcome the resolution limitation and is able to build high resolution nonconductive channels.

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VI. REFERENCES

[1] Wego A., Pagel L., “A self-filling micropump based on PCB technology” Sens. Actuators, vol. A88, pp. 220-226, 2001

[2] Hämmerle M., “Pressure Sensor in Printed Circuit Board Technology“, IMAPS Device Packaging, 5th International Conference and Exhibition on Device Packaging, 8.-12. March , Scottsdale, Arizona USA.

[3] Anastasios Petropoulos, Dimitris N. Pagonis, Grigoris Kaltsas, Flexible PCB-MEMS Flow Sensor, Procedia Engineering, Volume 47, 2012, Pages 236-239

[4] Aracil, C., Perdigones, F., Moreno, J. M., & Quero, J. M. (2010). BETTS: bonding, exposing and transferring technique in SU-8 for microsystems fabrication. Journal of Micromechanics and Microengineering, 20(3)

[5] Microchem Corp. Datasheet: SU-8 2000 Permanent Epoxy Negative

Photoresist PROCESSING GUIDELINES FOR SU-8 2025, SU-8 2035, SU-8 2050 and SU-8 2075, available at: http://microchem.com/pdf/SU-82000DataSheet2025thru2075Ver4.pdf, last visited: 2013-02-15

[6] Gründler P, Flechsig G-. Principles and analytical applications of heated electrodes. Microchimica Acta. 2006;154(3-4):175-89

[7] Lienhard Pagel, Stefan Gassmann: “Integrated Analysis System with Polymerase Chain Reaction and DNA Analysis in Printed Circuit Board Technology” IMAPS 8th International Conference on Device Packaging, Scottsdale (Arizona) USA, 6.-8. March 2012

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