3D Interconnect Technology for Out-of-Plane Biomedical Probe Arrays. A Modular Approach with Slim-Base Solutions (3D Interconnect technologie voor orthogonale biomedische microprobe arrays. Een modulaire aanpak)
3D Interconnect Technology for Out-of-Plane Biomedical Probe Arrays. A Modular Approach with Slim-Base Solutions
Aarts, Arno; S0180659
This study reports the microfabrication of a novel interconnect technology for out-of-plane biomedical probe arrays. This type of interconnect technology can be used to interconnect probe arrays perpendicular onto a backbone resulting in slim-base three-dimensional probe arrays. The microfabrication of such interconnect has been realized using bio-tolerable materials and CMOS compatible prost-processing techniques. This type of interconnect can also be used in many other non-biomedical fields which require a modular, out-of-plane integration concept. CMOS structures can be connected perpendicular with each other, without the need to significantly increase the re-distributed fan-in / fan-out area.Currently, silicon microfabricated probe arrays are often used to record and stimulate neurons at the cellular level. They can contain embedded electronics such as filters, amplifiers and multiplexers, which are integrated into the base of the probe array. Integrated electronics consume a significant amount of surface area and increases the height of the probe array. Implantation of such probe array into the cortical tissue requires extra space above the implanted area, which is most often not available. Consequently, the probe array could be caught between the skull and the cortical tissue, resulting in tissue damage or probe failure. Besides, the brain is constantly moving due to physical movement or macro movements caused by blood pulsation. The ideal probe array for long term and chronic recordings moves along with the cortical movements rather than remain attached or clamped against the skull. To fulfill the floating requirement the base of the probe array should be reduced in height and become as thin as possible. This study describes a technique to connect the shafts perpendicularly to the tall base of the probe array. This approach realizes a slim-base three-dimensional probe array while retaining the ability to integrate complex functions such as filtering, amplification, and multiplexing of the recorded neural signals. Realizing such slim-base out-of-plane biomedical probe array requires a new type of modular high density interconnect, which electrically interconnects the shaft with the tall base. An assembly technique has been developed to align and assemble the probe arrays perpendicular into the base of the probe array. Once the shafts are assembled perpendicularly into the tall base (better known as platform), a non-separable, mechanical stable electrical connection is established. The platform or tall base contains several cavities, which act as socket for the perpendicularly assembled shafts and provide mechanical stability. The electrical interconnects consist of gold clips overhanging the edge of a cavity. During the perpendicularly assembly of the probe array the metal clips will be bent and squeezed between the insulated wall of the cavity and the contact pad located on the probe array.Two microfabrication techniques have been developed to realize the interconnect for slim-base three-dimensional probe arrays. Bothmethods have their own specific technique to deposit the electrical interconnects over the edge of the cavity. The first technique uses a sacrificial filling method, while the second technique uses a thin film transfer bonding method. Several test structures have been developed to measure the contact resistance as well as the perpendicular fit. An interconnect density with a pitch of 35μm and spacing of 15μm has been achieved with contact resistances of about 3Ohm. The slim-base three-dimensional probe array has been implanted in a Long-Evans rat under ketamine/xylazine anesthesia. Successful recordings were measured showing local field potentials and multiunit activity. A post-process tip sharpening technique has been developed to sharpen the chisel shape tip of the probe (prior to assembly), enabling to implant the three-dimensional probe array in bigger size primates without the need to open the dura and thus minimizing the change on in subdural infections.