Development of probes for cochlear implants.

Nishant S. Lawand and Paddy J . French Electronic Instrumentation Laboratory, Delft University of

Technology, 2628 CD, Delft The Netherlands.

E-mail: n.s.lawand, p.j.french@tudelft.nl

J. J. Briaire and J. H. M. Frijns ENT Department, Leiden University Medical Centre

P. O. Box 9600, 2300 RC Leiden The Netherlands.

E-mail: J.J.Briaire, J.H.M.Frijns@lumc.nl

Abstract— In this paper the development of the probes with respect to the fabrication aspect is shown. The stiff probe is formed by defining the probe thickness and releasing at the end by deep reactive ion etching. These stiff probes were fabricated in order to study the problems involved in the fabrication procedure and the behaviour of the probes when they are inserted perpendicular to the auditory nerve. The probes were fabricated with lengths ranging from 3.5 mm up to 13.5 mm and thickness between 50 to 60 microns.

I. INTRODUCTION The traditional wire electrodes of the cochlear implants

(CI’s) have gone through a development phase showing a promising future to replace themselves with a silicon micromachined electrode array for the use of the cochlear prosthesis device for the deaf patients. The silicon microprobe probe technology is the first applied technology for devices in human use. K. D Wise and his colleagues fabricated the first biological microprobe arrays using lithographic techniques with successful performance in the brain for recording from small populations of neurons [1]. Since then this technology is been used for fabricating microprobes to date and also to fabricated flexible ribbon cable substrates which houses and supports the stimulation sites in it [2]. This technology have certain advantages over the traditional wire electrodes by contributing towards smaller array dimensions, flexibility and safe insertion into the scala tympani (ST) of cochlea, more stimulation sites and easy batch production with reduced cost.

CI is an auditory prosthesis which is implanted under the skin bypassing the non-functional inner ear and directly stimulating the auditory nerve with a specific sequence of electric currents in order to experience speech and other sounds for the profoundly deaf people. It actually overlooks the damaged or the missing hair cells within the cochlea which normally would do the decoding of the sound. These hair cells perform the function of converting the fluid vibrations created by the external sound waves into an electrical neural signal which is in turn perceived as sound by the auditory cortex. The electrodes which are connected to the receiver magnet of the CI are inserted through the round window of the cochlea as shown in Fig. 1 to stimulate the auditory neurons which otherwise would be stimulated by hair cells. The frequency of

the sound is determined along the spiral cochlea in such a manner that the low-frequency sound is more towards the apical contacts in the cochlea, and high-frequency sound to more basal ones. This is imitated in the cochlear devices by the distribution of the stimulation current to respective electrode stimulation sites along the array which corresponds to appropriate frequencies. So for better frequency resolution of the device, greater number of stimulating sites are required in the electrode array. Also the proximal distance between the electrode array and the auditory neurons is an important factor for increased frequency resolution and decreased stimulation current required by the device.

Figure 1. Sketch of a human ear with an Cochlear Implant.

The benefit of single site stimulation over multiple sites is to gain the freedom of perceiving multiple frequencies over a broader range to match the so-called tonotopy in the cochlea. Also multiple sites gives the audiologist an extra room for a better choice of stimulation pattern, allowing fine-tuning of the device for individual patients. Silicon semiconductor technology provides the fabrication of advanced high-density CI electrode arrays with greater number of stimulation sites,

This research is funded by the Dutch Technical Foundation (STW) underproject code: 10056.

978-1-4244-9289-3/11/$26.00 ©2011 IEEE

integration of electronics, reduced size, multiplexing and specific site selection as per frequency. This results in less area requirements for placing the device with low power for the electronics and the other components. Sensor integration by this technology gives the device more functionality in monitoring the precise shape and the position of the electrode array inside the cochlea. This valued tool will help the surgeons to insert the device in the cochlea safely and with minimal insertion trauma to the patient.

Figure 2. Schematic sketch (top view) for the basic structure of the implant and SEM photograph of the stiff probe before the Aluminium wet etch.

II. PROBE DESIGN AND FABRICATION We present the design and the fabrication of alternative

stiff probes for auditory nerve stimulation. Potentially this array will be modified further both in its design and fabrication procedure to fabricate flexible devices which will allow ST insertion.

The stiff probe was fabricated to investigate the photolithographic limits and the problems encountered in the fabrication procedure. Fig. 2 shows a schematic sketch of the stiff probe along with the SEM photograph of the probe without stimulation sites and before the release of the probe. The probe is 13.5 mm long, with the insertable front-end portion tapering from a width of 235 µm to 25 µm. Eighteen platinum-iridium stimulation sites of 250 µm in diameter are positioned along the probe with centre to centre distance of 500 µm. The fabrication procedure for the stiff probe without the stimulation sites is as illustrated in Fig. 3. The process begins with a single crystalline p-type (100) wafer with the specifications: thickness: - 525 ± 15 µm, diameter: - 100.0 ± 0.2 mm and resistivity: - 2 to 5 Ωcm. Photoresist of 3 µm thick is patterned with the photolithographic techniques.

Deep reactive ion etching (DRIE) process on the front side of the wafer defines the thickness of the probe. This is done by timed etch stop in silicon on the front side to achieve a thickness of 50 µm approximately. The wafer undergoes the standard cleaning procedure which includes stripping of photoresist in oxygen plasma, residual removal in alternate

fuming nitric acid bath and 65% nitric acid bath with intermediate cleaning in quick dump rinsing (QDR) bath. A 6 µm thick dielectric silicon dioxide (SiO2) layer is deposited by plasma enhanced chemical vapor deposition (PECVD) technique in a plasma reactor on the back side of the wafer. The deposition temperature is approximate 400 °C. This PECVD oxide layer will act as a masking layer during the DRIE process while etching from the back side. The process is followed with metallization step on the front side of the wafer. Aluminum (Al) metal is sputter deposited on the wafer by 100% Al target at a room temperature (25 °C) with an argon flow of 100 sccm to achieve a thickness of 4 µm. Sputter deposited layer is uniform with better step coverage in the trenches of 50 µm on the front side. Further in the back side processing of the wafer, lithography defines the windows which will be used for patterning of the 6 µm PECVD oxide. Window etching of this oxide is done in a plasma etcher to land on silicon surface. This etching process takes place in the presence of chloro-fluorocarbon gases C2F6 and CHF3 which are useful in the oxide etch process as well as surface passivation. The etching is carried for approximate 18 minutes with etching parameters: RF power:- 200W, Helium gas pressure:- 12 Torr. Then the standard cleaning follows to remove traces of photoresist.

Figure 3. Fabrication steps for stiff probe without stimulation sites.

In the next step, thru etching of the silicon from the back side is performed by the DRIE process to land on the aluminum which acts as an etch stop layer during the etching process. This is the crucial step in the complete fabrication

process in which the machine parameters like gas flow and the temperature has to be maintained and tuned accordingly to obtain the required anisotropy of the trenches. At the end the probes are released by aluminum wet etch. The wet etching bath contains a mixture of phosphoric acid (H3PO4), nitric acid (HNO3), acetic acid (CH3COOH) and deionised water. The probes are in position due to the supports at the base end of the device as seen in Fig. 2. These supports are of full wafer thickness (525 µm) thick with a width of 50 µm each at the base end on the either side of the device. These supports are purposely given a tapered shape at the point of contact with the base end which permits the easy breakage from the wafer. After the wet etching process the devices are released individually by dicing from the supports. Before the dicing procedure the whole wafer is coated with parylene to increases stiffness as well as the biocompatibility since parylene is used as a biocompatible material in most of the medical implants [5]. Parylene is deposited by vapor deposition method from the vapour phase by a process which in some respects resembles vacuum metallizing. This process consists of three distinct steps as outlined in Fig. 4 at the end. The first step is vaporization of the solid dimer at approximately 175 °C. The second step is the quantitative cleavage (pyrolysis) of the dimer at the two methylene-methylene bonds at about 650 °C to yield the stable monomeric compounds. Finally, the monomer enters the room temperature deposition chamber where it simultaneously adsorbs and polymerizes on the substrate wafer. No liquid phase has ever been isolated and the substrate temperature never raises more than a few degrees above the ambient. Other necessary components in this system are the mechanical vacuum pump and associated protective traps. By this deposition process a thin-film, pinhole free parylene conformal coating is applied to the devices. Parylene provides excellent moisture, chemical and dielectric barrier protection [5]. Prior to fabrication simulation results of the stiff probe in the cochlear model gives the stimulation pattern of stimulation sites on the electrode array [3].

III. RESULTS AND DISCUSSIONS. The silicon stiff probe has been fabricated to check the

mechanical stability and investigate the issues encountered in the fabrication process. In order to study difficulties and the limits of the fabrication process the devices of various lengths (3.5 mm to 13.5 mm) and thickness ranging between 50 µm to 60 µm have been fabricated. During the fabrication process handling of the wafer especially after the DRIE process is important since after this process the fragile devices are only supported by the two supports at the base end of the device. Also for the DRIE process the masking layer (in our case PECVD oxide layer of 6 µm thick) must be sufficient thick to withstand the complete etching process. This is to ensure that the silicon surface underneath the PECVD oxide is not being exposed or etched by the etching gasses during the DRIE process. The parylene layers deposited over the probes not only add up to the mechanical characteristics and strength of the probe but also the biocompatible properties of the insertable electrode array.

The external signal processing circuitry which is connected via an inductive link to the current generation circuitry which is in turn connected to the back end of the

implant in the middle ear is been researched here in TU Delft. Various speech processing strategies are being studied to develop an optimum strategy. A design of an analog complex gammatone filter is developed in order to extract both envelope and phase information of the incoming speech signals as well as to emulate the basilar membrane spectral selectivity [4].

IV. FUTURE WORK AND CONCLUSIONS The work presented here shows the initial fabrication

results of the stiff probe which laid the foundation for the development of the future new generation cochlear implant electrode arrays. The next step is to perform mechanical tests of these probes and also to do insertion tests by inserting into the human cadaver cochlea, to verify the ability to enter without any breakage. The next devices to be fabricated will include several significant modifications with respect to the fabrication process. The devices will be defined in heavily doped substrates which will also act as an etch stop layers during the release of the devices in the wet etching process. Stress compensated dielectric layer stack consisting of silicon dioxide, silicon nitride and silicon dioxide will be incorporated in the process in order to insulate the conducting layers from the doped structures underneath. Another similar dielectric stack will be on top of the metal interconnects for shielding those lines from the saline environment of the ST inside the cochlea. The geometry, the material and the deposition technique for the stimulation sites will also be looked upon during the fabrication process. The sizes will be decided on the current delivery values for each electrode sites. Research will continue further towards fabricating flexible electrode arrays for insertion inside the ST of the cochlea. Also work on

different polymers and its fabrication compatibility has to be explored for biocompatibility and long term functioning capabilities of the device which will be placed inside the fluid filled chamber of ST in the cochlea.

ACKNOWLEDGMENT The authors would like to acknowledge the financial

support for this work by STW, the Dutch Technology foundation and The Delft Institute of Microsystems and Nanoelectronics (DIMES), Delft University of Technology, The Netherlands for their processing support. The authors would like to thank Advanced Bionics for their support in this project.

REFERENCES [1] K. D. Wise, J. B. Angell and A. Starr, “An integrated-Circuit approach

to Extracellular Microelectrodes,” IEEE Trans. Bio-Medical Engg., Vol. BME-17, No. 3, July 1970.

[2] J. F. Hetke, J. L. Lund, K. Najafi, K. D. Wise, and D. J. Anderson, “Silicon Ribbon Cables for Chronically Implanatable Microelectrode Arrays,” IEEE Trans. Biomed Engg., vol. 41, April 1994, pp. 314-321.

[3] N. S. Lawand, P. J. French, J. J. Briaire, J. H. M. Frijns, “Silicon probes for cochlear auditory nerve stimulation and measurement,” Advanced Materials research vol. 254, pp 82-85, 2011.

[4] Wannaya Ngamkham, Chutham Sawigun, Senad Hiseni, Wouter A. Serdijn, “Analog complex gammatone filter for cochlear implant channels,” ISCAS, pp. 969-972, 2010.

[5] Web information: http://www.scscooks.com/parylene/properties.cfm

Figure 4. Parylene vapor deposition procedure [5].