White Paper

Technology for Medical Human Implants
Vision Chip on Thin-film Multilayer
Abstract

Dr. Alexander Kaiser
Karin Ruess
Dr. Bruno Holl
Josef Vanselow
Dr. Ernst Feurer
Reinhardt Microtech GmbH
Sedanstrasse 14
89077 Ulm I Germany
alexander.kaiser@cicor.com
A company of the Cicor Group

Dr. Liu LiuProf. Dr.-Ing. Albrecht Rothermel
University of Ulm
Andreas Möller
Multi Channel Systems GmbH
Reutlingen
Dr. Alex Harscher
Retina Implant AG, Reutlingen

Increasing complexity of today’s electrical circuits’ lead to a growing demand for thin film “interface”
substrates. Such substrates provide an important link between integrated electronics and peripheral
accessories like amplifiers or signal processing units. Possible applications lie in the field of laser sub
mounts with electrical connections for re-routing or even more sophisticated in the field of medical
applications or implants. Whereas conventional thin film multilayer are mostly realized on stiff ceramic
substrates (aluminum oxide, aluminum nitride), the extraordinary challenge with medical applications
often comes from the required flexibility of the circuit. Biocompatibility and practical handling of theassembly play an important role in these applications.
This publication will show general aspects of the fabrication of thin film multilayer, stiff as well as
flexible, and show the main differences which may appear, and be requested, in such systems.
A special focus shall be laid on an application for a retinal implant. In this application a vision chip,developed by the University of Ulm, is combined with a flexible thin film multilayer forming a human
medical implant for regaining, at least partly, the eyesight for blind people. The special issues of theimplantable chip are the low supply voltage and DC free external supply of the circuit.
The vision unit itself consists of a 40 x 40 pixels array with light sensors and electrode drivers, which
are addressed sequentially to improve power consumption and spatial resolution of perception. Thisguarantees for a long-time operation and minimization of chemical reactions on the periphery1

Introduction

Due to growing experience in the fabrication of multilayer substrates, which Reinhardt Microtech
gained over the last years, the step from multilayer circuits on rigid substrate towards flexible circuits
was a rather small one. First experiments showed the principle functioning of the technology and withthe retinal implant application a first product was being developed.
The aim of the retinal implant is to help blind people to regain at least partially their eyesight. Unfortunately at the moment the retinal implants are no cure-alls. With the implant discussed here only humans suffering from Retinitis Pigmentosa can be helped.
Retinitis pigmentosa is a degenerative disease of the outer retina that can result in complete blindness. Part of the inner retina is still intact and electrical stimulation of this neural network has shownto elicit light sensations in blind patients [1]. With the so-called subretinal implants (which we havehere) the intact inner retina is stimulated and the rest of the “natural” path as well as the optical system of the eye are still used.
A first clinical pilot study has been carried out and the detailed analysis of the data with 10 patients so
far resulted in a redesign of the existing CMOS chip that is implanted in the subretinal space.
Fig. 1
Optical photograph of a human eye with the installed
retinal implant.

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Figure 1 shows an optical image of the implanted CMOS chip of the first pilot study. One can clearly
see the chip itself as well as the connection with the attached flexible substrate.

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CMOS Vision Chip

The novel chip, which was designed based on results out of the first study, operates with a symmetrical supply voltage (±2V) and has all DC free terminals for long life wired operation. 40 x 40 pixel
cells including photo diodes, amplifiers, control logic and electrode drivers are addressed sequentiallyto improve power consumption and spatial resolution of perception. Pad count is limited to 6. The
3 x 3.5 mm2 design is fabricated in a 0.35 µm CMOS technology [2].
The block diagram of the silicon chip is shown in figure 2. The chip has six input pads, which have to
be connected to the supply box.
Fig. 2Block diagram of retina chip

Dr. Alexander Kaiser
Karin Ruess
Dr. Bruno Holl
Josef Vanselow
Dr. Ernst Feurer
Reinhardt Microtech GmbH
Sedanstrasse 14
89077 Ulm I Germany
alexander.kaiser@cicor.com
A company of the Cicor Group

Dr. Liu LiuProf. Dr.-Ing. Albrecht Rothermel
University of Ulm
Andreas Möller
Multi Channel Systems GmbH
Reutlingen

Three supply terminals are used for VH, VL and GND potentials. Three other supply terminals areused to provide control signals IGL, IIMP and ISEL. On the chip, a rectifier generates internal DC supply
VDDCONST and VSS from the external AC supply VH and VL. Three control signals are required (brightness reference level IGL, stimulation timing and amplitude limit IIMP, operating mode ISEL) to control thechip functions.
Low input impedance amplifiers are used to convert externally supplied control current to internal
control voltage. The input voltages are clamped to levels below 300mV.
On the chip, each pixel cell includes a photo diode, an amplifier and an output driver. The pixels are
addressed by the digital control logic sequentially to reduce chip power consumption. The photo
diode current is read out by a standard logarithmic photo sensor which has highest dynamic range ofincident light and corresponds to human perception of brightness.

Dr. Alex Harscher
Retina Implant AG, Reutlingen

Fig. 3
Optical image of CMOS vision chip with photo diodearray and input/output circuitry

The inputs of each differential amplifier are connected to local photo diode output and mean luminance signal, its bias current is determined by IIMP. The amplifier DC output signal is converted to abiphasic output stimulation signal by the output driver. The driver output will be connected to TiNstimulation electrodes, which will then be able to stimulate the optic nerve layer. Figure 3 shows an
optical image of the CMOS vision chip, also denoting its main parts. More details on the circuit’s
operational principles can be found in [2].

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Thin-film Multilayer Substrates

As it has been for long years, the thin film technology nowadays is still settled in between the widely
known PCB field and semiconductor circuits and device technology. Providing the ability of fine line
patterning (down to 10 µm lines and spaces) and the use of nearly arbitrary substrate material, theadvantage over PCB is obvious, whereby the high resolution of e.g. CMOS semiconductor circuits
never will be achievable.
Main applications of thin film products lie in the field of RF circuits, like e.g. T/R modules, DC fan-outs
or sub mounts for semiconductor devices. Various kinds of solutions for thermal management by2

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using thin film substrates and comparison with conventional technologies have been developed anddescribed intensively [3, 4, 5].
Especially in the area of sub mounts for optical devices a strong increase has been noted over thelast years. Principal differentiations hereby can be seen in the requirements for thermal management(in such applications normally thick Cu plays an important role) or relatively high pin-count to the‘outer world’. The second option normally will require several layers of metallization to provide thedesired connectivity between circuit parts.

3.1
Dr. Alexander Kaiser
Karin Ruess
Dr. Bruno Holl
Josef Vanselow
Dr. Ernst Feurer

Rigid Multilayer

Due to the increased demand on such circuits, Reinhardt Microtech started to develop a technology
for the production of such multilayer substrates. Figure 4 gives a schematic cross section of a rigid
multilayer circuit in thin film technology.
Rigid multilayer circuits most commonly start with a base metallization put directly on the substrate.
Hereby the substrate may be glass, alumina, and silicon or aluminum nitride for improved thermal
properties. Due to customers demand and required current transportation capabilities, the metal
layers in the stack can be just sputtered metal or also strengthened by electroplating.

Reinhardt Microtech GmbH
Sedanstrasse 14
89077 Ulm I Germany
alexander.kaiser@cicor.com
A company of the Cicor Group

via
metal 3polyimde 2
metal 2polyimide 1
metal 1

open window

Fig. 4
Schematic cross section of rigid thin film multilayer
substrate (3 metal layers, 2 insulator layers, base substrate).

filledvia

substrate (alumina, AlN, …)Andreas Möller
Multi Channel Systems GmbH
Reutlingen

In contrary to what people may know from CMOS semiconductor processing, where normally insulator layers are made of silicon oxide, oxynitrides or similar which is deposited e.g. by CVD 1, thesematerials normally cannot be used in thin film manufacturing. Especially when using electroplated
metal lines, a certain kind of planarization and covering capability is necessary. For this reason often
polyimide insulators are the material of choice, as these can be deposited in sufficient thickness.

Dr. Alex Harscher
Retina Implant AG, Reutlingen

Fig 5
Test substrate with 2 metal layers and 1 insulator layer(left: overview; right: detailed view of metal lines and via,
brown layer: polyimide insulator).

upper metal
via

lower
metal

Dr. Liu LiuProf. Dr.-Ing. Albrecht Rothermel
University of Ulm

Figure 5 shows a test substrate for a two story multilayer chip. Hereby two electroplated Au line layers are separated by a polyimide insulator layer. Electrical contact between the two layers is realizedby fabrication of via interconnects between the different metallization levels.
Different methods for the via drilling in the insulator layer are possible. One can think e.g. about using
laser drilling, which is a well-established technique for producing holes in ceramic substrates. Anothermethod is the use of photosensitive polyimide and the fabrication of holes by photolithography. If thisis not possible, also a dry etching process can be used for patterning of the holes. This method tends
to be suitable for moderate polyimide thicknesses below 10 - 15 µm. Common parallel plate reactors
with gas mixtures mainly containing O2 can be used for etching of the polyimide insulator.

metal 3

metal 2

Fig. 6
High volume multilayer product: detail with 3 metal
layers, 2 insulator layers, via interconnects between
metal layers.

via
interconnects

metal 1
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Figure 6 shows a detail of a rigid multilayer incorporating 3 metal layers and 2 layers of polyimide
insulator. One can easily recognize the surface topology which is resulting when having several layers of electroplated metal, separated by the polyimide, above each other. This issue even becomes
more important during the fabrication of flexible multilayer substrates.

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CVD: Chemical Vapour Deposition

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3.2

Flexible Multilayer for Medical Applications

In comparison to thin film multilayer substrates for fan-outs or sub mounts, where the material selection in principal is totally free, there are some restrictions for medical applications. The choice of themetals one can use is mainly limited to e.g. Ti, Au, Pt, or TiN for electrodes and lines, and also notevery insulator material can be used. This fact also puts some limitations on the usable processes inthe development of medical multilayer substrates.
An additional point is the flexibility of the ready-made circuits. Thus, the carrier which is normallyneeded to support the whole structure during processing is temporary. The manufacturer needs tohave appropriate processes to guarantee this property.

Dr. Alexander Kaiser
Karin Ruess
Dr. Bruno Holl
Josef Vanselow
Dr. Ernst Feurer

Due to the final flexibility, unlike to the rigid multilayer, the flex multilayer processing normally starts
with a polyimide layer of specific thickness which supports the whole structure after release from thecarrier. Mechanical, electrical and handling aspects in the later application have to be taken into
account when choosing polyimide kind, thickness and processing parameters.
Fig. 7
Flexible medical multilayer on 4" rigid glass carrier
substrate, first metal layer deposited.

Reinhardt Microtech GmbH
Sedanstrasse 14
89077 Ulm I Germany
alexander.kaiser@cicor.com
A company of the Cicor Grouppolyimide basemetallization

Dr. Liu LiuProf. Dr.-Ing. Albrecht Rothermel
University of Ulm
Andreas Möller
Multi Channel Systems GmbH
Reutlingen
Dr. Alex Harscher
Retina Implant AG, Reutlingen

In figure 7 one can find a 4”-sized glass substrate carrying the first polyimide and metal layer of theretinal implant multilayer. This substrate is designed to provide re-routing and connectivity of a CMOS
vision IC to the attached periphery while being implanted in the human eye.
The completed flexible substrate then consists of three polyimide layers (one carrier layer and two
insulator/ passivation layers) and two metal layers for control signals and power supply.
The complexity of the flexible multilayer substrate can also be seen in figure 8. A detailed view of acontact pad shows the various layers of the finished substrate. Contact windows hereby are opened
by using a plasma dry etching process.

a)polyimide baseFig. 8
Detailed view of contact
pads:

b)

contactpada) overviewb) detail of metal andpolyimide insulator
stack

Figure 8(b) shows the stack of materials which make up the multilayer: 1st polyimide, 1st metal, 2ndpolyimide, 2nd metal and the polyimide cover layer. Clean surfaces and proper process control are
required to fabricate reliable substrates for medical implants.

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Combination of System Parts

Figure 9 shows the whole retinal implant. It is made up of the three main parts: the CMOS vision chip,
which provides the fundamental function of image sensing and signal processing; the flexible substrate which carries the CMOS chip and goes round the eye; and the RF power supply which enables
DC free operation of the system. Due to this, the chemical interaction of the implant with the periphery(human body) can be minimized.
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flexible substrateconnector cable
CMOS chipFig. 9Retinal Implant with RF power supply, showing the
combination of the different system parts being implanted in the human body (subcutaneous = below the skin).

subcutaneous
RF power supply

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The electrical connection of the chip with its flexible carrier is realized via gold wire bonding onto eachof the six 3-fold contact pads (see figure 10). Utilizing these connections, the electrical supply and thecontrol of the CMOS chip is realized.
Fig. 10CMOS Chip on multilayer with electrical connections
used for supply and control.

Dr. Alexander Kaiser
Karin Ruess
Dr. Bruno Holl
Josef Vanselow
Dr. Ernst Feurer
Reinhardt Microtech GmbH
Sedanstrasse 14
89077 Ulm I Germany
alexander.kaiser@cicor.com
A company of the Cicor Group

Dr. Liu LiuProf. Dr.-Ing. Albrecht Rothermel
University of Ulm
Andreas Möller
Multi Channel Systems GmbH
Reutlingen

Encapsulation of the chip with silicone and connecting it to a subcutaneous hermetically sealed RF
power supply via a flexible cable (see also figure 9) has proven to be feasible.
Attachment of the silicone cable to the flexible multilayer is realized by wire bonding on the flex substrate side and welding of Au wires onto the silicon cable. This guarantees for a strong and reliable
connection between the single parts.

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Results

First tests with the new redesigned multilayer substrates showed good and promising results. Important properties hereby are the electrical resistance of the lines, which has to be kept small to minimize losses. Another critical point, as the power supply is DC free, is the cross coupling capacitance
between the single lines, which also has to be kept on a minimum. This topic has to be addressed byproper design and control of thicknesses of insulators and metal lines.
Fig. 11Output stimulation waveforms of CMOS vision chip
applying different input conditions: (a) current limit of
IIMP: +7µA / -5µA; (b) current limit of IIMP: +28µA / -20µA.

control signals

Dr. Alex Harscher
Retina Implant AG, Reutlingen

output current

output voltage

a)

+2V
-2V

b)

Another point which seems to be very trivial is the ability to do wire bonding on the connection pads.
But due to reliability issues of such implants this has to be considered as an important parameter.
Flexibility also has to be taken into account when characterizing such systems, as this is importantduring the implant surgery as well as in the later use inside the human body.
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Examples of output stimulation waveforms with RC-load models are shown in figure 11. These fundamental tests were performed under ambient light condition to prove the functioning of the CMOS
chip. The discussed design enables charge-balanced biphasic output signals, which shall be able tocouple to the biological interface in the eye and activate the intact cells. Hereby the response of retinal cells to electrical stimulation depends on the charge injected per impulse. The stimulation timing
and amplitude of the chip’s output can be adjusted by the control signal IIMP.

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As the polarities of the stimulation voltage and currents change as can be seen in figure 11, at theend of the stimulation phase the total charge injected can be adjusted to be zero. This is very important to avoid chronic accumulation of electric charge in the nerve layer. In detail, in a first phase anegative charge is injected, followed by a positive charge injection designed to be as large as possible with the given voltage constraints, and finally a third negative charge injection completes thestimulation cycle, with the sum of all three charges being close to zero.
Different conditions were used for testing the principal functioning of the circuits. In figure 11(a), due
to a relatively small stimulation current the output voltage approaches a triangular waveform. Hereby
the stimulation intensity is determined by the stimulation current amplitude.

Dr. Alexander Kaiser
Karin Ruess
Dr. Bruno Holl
Josef Vanselow
Dr. Ernst Feurer
Reinhardt Microtech GmbH
Sedanstrasse 14
89077 Ulm I Germany
alexander.kaiser@cicor.com
A company of the Cicor Group

Dr. Liu LiuProf. Dr.-Ing. Albrecht Rothermel
University of Ulm
Andreas Möller
Multi Channel Systems GmbH
Reutlingen

In figure 11(b), the output voltage reaches the level of the maximum voltage relatively fast. Subsequent increase of IIMP will not lead to increased stimulation intensity, as the output voltage is limited by
the supply voltage amplitude of ±2V.

6Conclusions

The described technology for flexible implantable multilayer substrates provides new possibilities in
the field of medical implants. One first application is demonstrated with a retinal implant, used forhelping people regain a certain portion of visual perception.
For this a new CMOS chip has been developed which is able to operate DC free to minimize theinteractions with the human body. Special attention was laid to design for long lifetime and high stimulation efficiency to improve the vision of the patient as much as possible.
A promising system with new and outstanding technological attempts was described, which hopefully
will be able to help a lot of blind patients in their future life.

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Acknowledgement

The authors would like to thank the staff of Cicor at Reinhardt Microtech GmbH and Retina Implant
AG for support on the technological development and the management for the possibility of publishing this work. Additional thanks are to Volker Burger at EADS Ulm for assisting with the attachment of
the Au wires.

Dr. Alex Harscher
Retina Implant AG, Reutlingen

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References[1]

E. Zrenner: “Restoring neuroretinal function: new potentials”, Doc Ophthalmol, pp. 56 - 59, 2007

[2]

A. Rothermel, V. Wieczorek, L. Liu, A. Stett, M. Gerhardt, A. Harscher, S. Kibbel, “A 1600-pixel
subretinal chip with DC-free terminals and ± 2V supply optimized for long lifetime and highstimulation efficiency”, IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers, pp. 144 - 145,
Feb. 2008[3]

E. Feurer, B. Holl, J. Vanselow, K. Ruess, A. Kaiser, “Advanced Thin Film Substrates in Cu-AlN
Technology”, Proceedings EMPC 2007 European Microelectronics and Packaging Conference,
June 17 – 20, 2007, Oulu, Finland, pp. 522-525[4]

A. Kaiser, J. Kusterer, K. Ruess, B .Holl, J. Vanselow, and E. Feurer, “Improved Thermal Management in High Power Applications by utilizing novel “Filled Via” Thin Film Substrate Technology”, Proceedings IMAPS 2007. The 40th International Symposium on Microelectronics, McEnery Convention Center - San Jose, California, USA, November 11-15, 2007

[5]

A. Kaiser, J. Kusterer, K. Ruess, B .Holl, J. Vanselow, and E. Feurer, “Solid Filled Via: Improved
Interconnects for DC and RF Thin Film Applications”, Proceedings IMAPS/ACerS 4th International Conference and Exhibition on Ceramic Interconnect and Ceramic Microsystems Technologies (CICMT), Holiday Inn City Centre, Munich, Germany, April 21–24, 2008

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