Multisite microLED optrode array for neural interfacing

Abstract. We present an electrically addressable optrode array capable of delivering light to 181 sites in the brain, each providing sufficient light to optogenetically excite thousands of neurons in vivo, developed with the aim to allow behavioral studies in large mammals. The device is a glass microneedle array directly integrated with a custom fabricated microLED device, which delivers light to 100 needle tips and 81 interstitial surface sites, giving two-level optogenetic excitation of neurons in vivo. Light delivery and thermal properties are evaluated, with the device capable of peak irradiances >80  mW/mm2 per needle site. The device consists of an array of 181 80  μm×80  μm2 microLEDs, fabricated on a 150-μm-thick GaN-on-sapphire wafer, coupled to a glass needle array on a 150-μm thick backplane. A pinhole layer is patterned on the sapphire side of the microLED array to reduce stray light. Future designs are explored through optical and thermal modeling and benchmarked against the current device.


S1. Utah Optrode Array Fabrication
The fabrication process flow of the BOROFLOAT33 UOA device is detailed fully in Scharf et al., 1 covered briefly in the main text and supplemented by the figure below.

Fig. S1:
Step 1 -Tip formation, pyramidal structures are formed using a Disco DAD3220 dicing saw with a bevel blade on 25 mm BOROFLOAT33 glass piece (2.3 mm thickness).
Step 2 -Pillar formation, deep kerfs are made in between the pyramid tips to create rectangular pillars. In this case the pillars measure 1.5 mm in height.
Step 4 -Annealing step, which further smooths the surface and is required to reduce optical scattering along shank length. Left: before chemical etch, middle: after etch (unannealed), right: annealed.
Step 5 -SEM of the completed device. Step 1: A thin Pd layer is deposited using electron-beam evaporation. This layer is then etched using reactive ion etching (RIE) to form the LED p-contact. Following the Pd etch, the p-GaN is etched using an inductively coupled plasma (ICP) system to create the microLED pixels (square with an 80 µm side).

S2. LED Array Fabrication
Step 2: The n-GaN is now patterned and etched using an ICP etch.
Step 3: The sample is annealed using a rapid thermal anneal (450 o C). A lift off process is then used to pattern a sputter deposited Ti/Au bilayer.
Step 4: A PECVD SiO2 layer is deposited as an insulation layer between n-tracks and p-tracks. Vias are opened in the SiO2 with RIE.
Step 5: The p-tracks are sputter deposited (Ti/Au) and patterned using a lift off process. A subsequent lift off process patterns the metal bondpads (Ti (100 nm): Pt (200 nm): Au (400 nm)) for wire bonding to control electronics.

S3. Pinhole Patterning and Device Integration
Upon completion of the microLED fabrication, the chip was mechanically thinned from the sapphire side to 150 µm and then diced into individual devices. An individual microLED array was then bonded (Norland NOA 61) to each UOA. As both devices are transparent it is possible to accurately align features on both devices. The effect of misalignment was studied using with optical modelling (Fig. S3C). This indicates that we have a tolerance to misalignment of approximately 10 µm before there is significant loss of optical power coupled into the optrodes. In order to study optical cross talk between adjacent needles, a prototype device with the sapphire-side coated in a metal thin film (Ti:Au, 20nm:30nm) was fabricated. This metal layer had 40 µm square side apertures (patterned using a lift off technique) over the microLED illumination sitesallowing light to pass through and couple into the needles while blocking stray light that would out-couple into tissue. Only half the array was covered with this pinhole layer to allow a direct comparison.
Developing an electrical connection scheme to address each microLED separately is challenging. Therefore, a matrix-addressing scheme was adopted. In this approach, all pixels along one column share a common anode (p-contacts) and all pixels along one row share a common cathode (n-contacts) - Fig. S3A. This means that only 38 connections are required, 19 anodes and 19 cathodes, which simplifies the electronic driver scheme dramatically and allows commercial LED current drivers to be employed. This approach reduces the number of connections, at the cost of limiting the available patterns that can be displayed. For example, individual LEDs, horizontal or vertical lines and rectangles/squares are possible. Diagonal illumination patterns and simultaneously displayed horizontal and vertical lines are not possible. Pulse width modulation schemes can be used to realise these restricted patterns. Each of the 38 connections is linked to LED driver circuitry through a 25 micron insulated gold wire (Fig. S3B). This method of interconnection is widely used in the 100-channel Utah Electrode Array system. 2 The wire-bonds have a length of 10 cm and, since they are insulated, can be bundled together. Silicone (MED 4211, NuSil Technology) can be added to secure the bond connections, increasing the strength of the wire bundle, and further encapsulating the interconnect region. The completed device is shown in Fig. 1 B with the wirebonds encapsulated in this manner.

S4. Interstitial Sites
Addressing the interstitial sites is less of a challenge as we do not have to couple light into an optrode with a limited size and numerical aperture. Fig. S4 details the optical performance of these illumination sites. Fig. S4: Measured and Modelled light output from an interstitial site. A) Measured peak irradiance for a given microLED current. B) Modelled volume above an irradiance threshold of 1 mW/mm 2 . C) Modelled effect of the pinhole on the light output from an interstitial microLED. For our prototype device a square interstitial pinhole of side 60 µm was chosen. No significant irradiance (>1 mW/mm 2 ) was detected in needle sites neighbouring the interstitial microLED. D) A cross section of the light emission from an interstitial microLED operating at 50 mA with no pinhole layer. E) A cross section of the light emission from an interstitial microLED operating at 50 mA with a 60 µm pinhole layer. The green contour line highlights the 1 mW/mm 2 irradiance level taken here as a threshold for ChR2 excitation. Table S1: We detail the thermal restrictions on the reported device across a range of typical operating parameters for optogenetic experiments. A certain microLED current delivers a peak irradiance, illuminating a volume of tissue above 1mW/mm 2 . For a given pulse width, the maximum pulse repetition rate is quoted such that the tissue temperature rise is kept below a 1 o C increase.