ELSEVIER Sensorsand Acluators A 54 (1996) 606-611 An active, microfabricated, scalp electrode array for EEG recording Babak Alizadeh-Taheri a,., Rosemary L. Smith b, Robert T. Knight e • SRI International, Computer Sciem'e Laboratory, 333 Rat+en.vwood Ave, Menlo Park. CA 94025. USA Department of Electrical and Computer Engineering. University ~ f California, DoyLe, CA 95616. USA Department ¢~f Nearology alld Centcr fi~r NeurtJsciencex, Univer.vitypf California, Davis, CA 95616. USA Abstract We describe the microfabrication,packaging, and testing of an active,dry, scalp electroencephalogram (EEG) electrode. The electrode consists of a silicon sensor subs(tale and a custom circuit substrate (2 #m CMOS technology ). A via-holelechnologyhas been developed using reactiveion etchingwilh SFJO2 gas mixture 10 make electricalcontacts between the sensor and circuit subs(rates. These substratesand batteries (power source) are then assembled in a custom package for testingon bench and human subjects. Ke~wordv: Electroencephalograms ; Electrodearrays ; Microfabrication L Introduction Electrodes that employ impedance transformation at the sensing site via active electronic devices or circuits are referred to as active electrodes. Published research on active biopotential recording electrodes began in the late 1960s and temporarily ended in the early 1970s, until the recent reports by Padmadinata [ 1] in 1990 and Taheri etal. in 1993 [2]. Ko and HTaecek [3] and Richardson [41 demonstrated that both dry and insulated active electrodes can be used to pick up electrocardiogram (ECG) signals with good signal char- acteristics compared to those of wet electrodes. But dry elec- trodes were found to have the following disadvantages: ( 1) bulky size due to additional electronics and power sources; (2) noise due to the limitations of mieroeleetronics available in the 1970s; (3) motion artifacts due to poor skin-to-elec- trode contact; (4) higher cost; and (5) corrosion of the elec- trode material due to contact with skin [4,51. The literature on active electrodes has focused mainly on ECG recording, with one report on electromyography ( EMG ) recording. We found no reports of electroencephalogram (EEG) studies with active electrodes. In addition, no previous work on active electrodes appears to have been done for low-level signal recording (below 100 p.V), a region critical for EEG recording. The dry-electrode array presented here requires no electrolyte and no skin prep- *Correspondingaulhor Phone: ÷ I (415)859 2844. Fax: +1 (415) 859 6165.E-mail; Tahefi@CSL sri.com. 0924-42471961515.00~*1996 Elsevier Sciqmce S.A.Allrights reserved PII $0924 4247 ( 96 ~,O 1264-2 aration. It has fast setup and cleanup times, and has redundant sensing sites that are significantly smaller than those of con- ventional wet electrodes. These features make it highly attrac- tive for long-term EEG recording, where problems with traditional wet electrodes are encountered because of drying of electrolyte paste, and for high-resolution EEG recording, where shorting of neighboring electrodes through the electro- lyte paste precludes close placement of electrodes. The dry electrode is fully compatible with commercial EEG monitor- ing systart~s. The electrode array contains four capacitive sensors with local circuits. 11 is designed to detect EEG signals in the 15-200/zV range in the frequency range 0.5 Hz to 5 kHz. 2. Electrode system The active electrode consists of an interconnecting circuit subs(rate and sensor substrate bonded together with a silver paste. The electrical contacts between the substrates are made by ultrasonic wire bonding. A side view of the bonded sub- strates is shown in Fig. I. The sensor substrate contains a planar array of four thin- film electrodes on one side and bonding pads on the other. The electrodes are electrically isolated from one another and the substrate by a 1.5/.tin film of silicon dioxide. Each elec- trode is attached to a bonding pad on the opposite side of the sensor subs(rate by a connecting thin film of aluminum run- ning through oxide-coated via holes. The surface of each