FABRICATION OF THICK COPPER LINES BY FILM TRANSFER METHOD APPLIED TO PDMS STRUCTURAL LAYERS

M. Couty1,2,3, S. Nazeer1,2, T. H. N. Dinh1,2,4, E. Martincic1,2, M. Woytasik1,2,

M. Tatoulian3, E. Dufour-Gergam1,2

1 Univ. Paris-Sud, Laboratoire IEF, UMR-8622, Bât. 220, F-91405 Orsay, France

2 CNRS, F-91405 Orsay, France 3 Laboratoire du Génie des Procédés Plasmas et Traitements de Surface (LGPPTS), Chimie ParisTech,

EA3492, 11 rue Pierre et Marie Curie, F-75005 Paris, France 4 KFM Technology, 11 avenue de Norvège, F-91140 Villebon Sur Yvette, France

Abstract — In this paper, we present a technological process giving access to fully embedded copper tracks into PDMS stacked layers, with adapted processes for single side metallization, double side or heterogeneous flexible layers. Examples of mi-crocoils, double side MRI micro-antennae and heterogeneous capacitive pressure sensor using a hybrid process are presented as demonstration devices. Patterns examples of sizes from 5 µm up to 4 mm are shown. The transfer process yield is higher than 95% in all cases, but the adhesion mechanisms seem to be a function of the patterns geometry together with physical and chemical adhesion mechanisms.

Keywords : PDMS metallization, metallization us-

ing transfer, copper microcoils, capacitive mi-crosensor.

I – Introduction Polydimethylsiloxane (PDMS) is a non-toxic, bio-

compatible and highly flexible material. It is also resistant to most solvents and acids and is optically transparent until Deep UV wavelengths, thus compati-ble with many micro-nanofabrication processes. More-over, it is low cost and easy to handle. For all these reasons, PDMS has become one of the most popular materials in the microsystem field : microfluidic chips, stretchable electronics, implantable devices...

However, PDMS suffers from its low elastic modulus and low surface energy, traduced by a criti-cally low adhesion to its surface. Then, the fabrication of metallic lines on PDMS is not straightforward. Classical metallization processes such as sputtering or evaporation result in cracks in the metallic surface, i.e. a non continuous metallic film [1]. The loss of conductiv-ity since this early step is forbidding low resistance conductors (thick metallic layers) fabrication, for example by electroplating. Metal deposition on small areas do not solve the cracks appearance [2,3], even with plasma treatment or corona discharge, which are usually able to promote metal adhesion to PDMS [4]. The only efficient solution was reported by Lacour et al. [5]. They applied prestretching to the PDMS film before metal deposition by gold evaporation. Releasing the metallized PDMS film results in out of plane 1D corru-gations or waves that eliminate mechanically originated

cracks and allow curving the PDMS without crack appearance.

To avoid the PDMS metallization, a solution pro-posed by some authors consists in fabricating conduc-tive PDMS composed of a mixture of PDMS and conductive metallic particles [6,7], such as carbon nanotubes [8]. Despite their attractive results, these composites do not offer a so high conductivity as a metallic layer. Another way to obtain uncracked metal-lic layers on PDMS films is to deposit the metallic layer (patterned or not) to an other material, cover it with PDMS and release it. The release is done using either a sacrificial (SU-8 [9] or LOR 20B ) or a low adhesion layer [10] (MicroChem, CYTOP® [11], poly acrylic acid (a water soluble polymer) [12]. For transfer proc-esses, adhesion to the carrier substrate and to PDMS could be controlled using Self Assembled Monolayers (SAMs) [13-15]. In those cases, immersion of PDMS into strong chemicals is required. These dangerous chemicals may penetrate into the porous bulk material and that would be especially undesirable for the reliabil-ity and specifically drastic in the case of implantable devices.

In this paper, we propose an alternative method for fabrication of thick copper lines on PDMS using film transfer technology. Different variants have been developed for fabrication of single side, double side and hybrid structures. For each one, an example of device is presented: microcoils, MRI micro-antennae and capaci-tive pressure sensor, respectively.

II - Fabrication process The principle is to fabricate the thick conductive

lines on a donor substrate with a low adhesion, then releasing them to a receiver substrate with a higher adhesion force.

Donor substrate is either silicon or glass wafers (10 cm in diameter), covered with a specific fluorocarboned coating to ensure that the adhesion will be low enough during all fabrication process of the metallic lines, but easy to release at the end of the process. These kinds of films are well-known for their low surface energy and the resulting anti-adhesive properties [16]. This CxFy layer is obtained by plasma polymerization of C4F8 gas and exhibits similar properties to Teflon®, according to its characterization by WCA, XPS and ATR-FTIR published in [17]. The control of the adhesion is

achieved through the deposition parameters and subse-quent treatments i.e. thermal annealing.

We studied three kinds of structures : the first allows us to obtain PDMS directly covered by copper lines, the second one using two controlled-sticking films permit-ting to obtain well aligned copper lines at the both sides of a PDMS thick film and the third, called “hybrid structure” developed to overcome the contacting issues while using PDMS.

A. Single side process

The process flow is summarized in fig. 1. After the deposition of the CxFy layer (step a), the thick copper lines are fabricated using the micromoulding process [18]. Briefly, the donor substrate is first covered with a sputtered Ti/Cu:10/100 nm film as a seed layer (b). Then, a thick AZ4562 photoresist mould is patterned using UV lithography (c), and filled by copper electro-plating (d). The photoresist mould and the seed layer are removed. The fabricated lines are covered with PDMS Syl-gard®184 which becomes a receiver “substrate” (e). The weight ratio is 10:1 and the control of thickness is achieved through spin-coating parameters and curing conditions. The PDMS containing the conductor lines is released from the donor substrate at the interface with CxFy layer (f).

Figure 1: steps of the process for single level metallic patterns embedded in PDMS.

B. Double side process

In the case of multi-layer i.e. double-side structures,

the process consists in carrying out twice the process described previously for single side devices, the patterns on the both sides could be the same or different.

After separate fabrication, the top side and bottom side patterns are aligned using a classical lithography equipment and both sides are bonded with a very thin uncured PDMS layer as glue [19] (fig. 2, step f’). As last step, the release of the aligned tracks embedded into the polymer could be performed simultaneously for the top and the bottom sides (g’).

Figure 2: alternative steps for fabrication of aligned thick copper tracks on PDMS.

Beyond this general overview, thicknesses and re-leases steps have to be adapted to each device. Some examples of single and double side structures are presented in the next section. Note that this process includes only conventional microfabrication steps and thus requires only basic cleanroom equipments. More-over, after removing the release layer, the donor sub-strate may be re-used for new processes.

C. Hybrid process

The metallic patters fully embedded in PDMS may be electrically tested without wire connection for some devices, as in the case of MRI micro-antennae by inductive coupling with a test probe [20]. But others devices require wire connection or industrial connectors and thus present contacting issues while using PDMS because of its very low elastic modulus. A solution is to strengthen some parts of the device to allow the wire bonding on conductor pads, while benefiting of the PDMS properties. For this purpose, some modifications are brought to the process, resulting in a “hybrid” structure. In this variant, only the top donor substrate is coated with the CxFy layer and the bottom tracks are fabricated by micromoulding on an uncoated silicon or glass wafer (fig. 3, steps a”-d”).

Figure 3: steps of the hybrid process.

The track dedicated to electrical contact is protected during PDMS deposition, alignment and bonding (fig. 3, steps e” and f”). Therefore, after the release of the top

side (step g”), we obtain a device with embedded copper tracks in PDMS and a free conductor pad on a rigid substrate for subsequent wire bonding.

III - Results and discussion The processes have been applied for fabrication of

microcoils, capacitive pressure sensor and MRI micro-antennae. The geometry and the dimensions of these devices, including the main dimension (side or diame-ter) D, the width w and the gap s between two patterns, are given in table 1.

Table 1: Dimensions of the transferred patterns.

Device Geometry D (mm) Dimensions Microcoils

(single side)

Circular and square

spirals 1

40 turns, w = 5 µm, s = 5 µm

Squares (capacitors)

3 or 4 s = 1 mm Capacitive pressure sensor

(hybrid) Lines

(tracks) - w = 100 µm

6 6 or 7 turns,

w = 88 or 118 µm, s = 40 or 110 µm

MRI micro-

antennae (double

side)

Circular spirals

4 5 or 7 turns,

w = 66 to 106 µm, s = 36 to 66 µm

A. Microcoils

We investigated the transfer of single side structure

with batch fabrication of microcoils (fig. 4). In this case, the copper lines and the PDMS substrate are 10 µm and 2 mm-thick, respectively. A detail of a transferred microcoil on PDMS is given fig. 5. The successful transfer of small patterns (5 µm) with a transfer yield about of 90% shows the efficiency of the process.

Figure 4: a 10 cm sized PDMS 2 mm-thick wafer with trans-ferred microcoils.

The yield is slightly lower for the pads, thus we sup-pose that mechanical anchorage participates to the good adhesion of patterns into PDMS. It is also possible to increase the PDMS/metal adhesion by plasma treat-ments [4].

Figure 5: a square microcoil (40 turns, 10 µm thick, 5 µm / 5 µm) transferred into PDMS.

B. MRI micro-antennae

We applied the double side process to MRI micro-antennae displayed in fig. 6. The structures consist in aligned copper windings on both sides of a dielectric film i.e. PDMS, designed for resonant frequencies in the 300-600 MHz range. To avoid the increase of series resistance by skin effect, the copper thickness is 10 µm (skin depth is 3.8 µm at 300 MHz). The PDMS thick-ness is 150 ± 2 µm, resulting from 65 µm deposited on each side in step e (fig. 1) and 20 µm for the adhesive layer in step f’ (fig. 2). The transfer yield range from 70 to 90%.

The experimental resonant frequencies are in good agreement with the targeted ones (deviation less than 4%) and the quality factors range from 55 to 70, compa-rable to the ones obtained with similar design on Kapton [21].

Figure 6: MRI micro-antennae, 4 mm in diameter, embedded into a 150 µm-thick PDMS film.

For in vivo applications, both sides are covered with 20 µm of PDMS. The tracks, fully embedded in PDMS, are then submitted to mechanical stress (bending). Their resonant frequencies and quality factors are measured in these conditions. No significant variation is observed until a bending radius equal to the antenna radius. Below this value, in extreme bending conditions, some delamination is observed.

C. Capacitive micro sensor

As a demonstration of the hybrid process, fig. 7 shows a 4x4 cells capacitive sensor array. Each cell row is connected to a side contact on a Molex type footprint on the bottom side, while every cell column is con-nected to a contact on the top side. Although the con-

nectors are designed to be fabricated onto a Kapton™ polyimide film for its use with ZIF Molex connectors, the prototype presented was fabricated on a 10 cm glass substrate.

Figure 7: 4x4 capacitive pressure sensor array of 3x3mm2 elementary cells

The copper tracks thickness is 3 µm, covered with 8 µm of PDMS. Such thin PDMS layers are obtained using long spin coating duration [22]. The adhesive layer is also 8 µm PDMS, resulting in a total thickness of 25 µm. The distance between the elementary capaci-tors is 19 µm.

The designed process, combined with the relatively low thickness of the conductors compared to the other metallic coils presented in this paper, allows a good control of the inter-electrodes distance. 97% of the metallic patterns have been successfully transferred for the top side electrodes. Surprisingly, all electrodes (big patterns) have been correctly transferred, while 50% of 1 connector (out of 8) have not been transferred.

Neglecting side effects, and supposing a relative permittivity of 2.7 for the PDMS layer, each sensor has a capacitance given by:

pF 2110 .d

L.l.rC ≈=εε

A Microchip pic16f707 microcontroller (with built-in capacitance measurement capabilities) based readout circuit has been designed and will be used for measure-ments.

IV - Conclusion Successful fabrication of microcoils, MRI micro-

antennae and micro capacitive force sensors with tracks fully embedded in PDMS has been achieved using film transfer technology. Thick conductive metallic patterns ranging from 3 µm to 10 µm thickness and lateral dimensions ranging from 5 µm up to 4 mm have been successfully manufactured, with a fabrication yield higher than 95%.

Electrical and mechanical characterization of these devices in compressing, bending and stretching condi-tions is currently in progress.

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