Recent Advancement in Charge and Photo-Assisted Non-Contact Electrical Characterization of SiC, GaN, and AlGaN/GaN HEMT
Authors: Findlay Andrew David , Wilson Marshall , Savtchouk Alexandre , D'Amico John , Lagowski Jacek , Hillard Robert 
Published: February 2020

Development of semiconductor devices and control of the manufacturing process require cost effective metrology with rapid feedback to pilot or manufacturing lines. In this respect, silicon IC’s have been benefiting from inventions (by IBM, Fishkill, NY and by SDI, Tampa, FL) that created unique corona-Kelvin non-contact electrical metrology [1]. Corresponding commercial tools designed for Si wafers [2] reduced the need for fabrication of electrical test devices and electric contacts, reducing the manufacturing cost and shortening the data feedback time from weeks to less than one hour. Developments, reported in this work are directed toward new possibilities for corona-Kelvin application for wide-bandgap semiconductors, including SiC, GaN, and GaN/AlGaN heterostructures.

In charge-assisted corona-Kelvin metrology the low kinetic energy (thermalized) corona ions are placed on the surface. They produce an electrical field in dielectric and/or in semiconductor surface space charge region. The change of the surface voltage, DV, induced by charge dose DQ, is measured in a non-contact manner with a vibrating Kelvin probe. Non-contact differential capacitance is obtained as C=DQ/DV. Knowing the net surface voltage, V and the net charge density Q = SDQ the set of (C,V,Q) characteristics is determined. This unique corona-Kelvin data set enables determination of dielectric, interface, and semiconductor parameters [3].

Applications in silicon IC concentrated on characterization of wafers with dielectrics. Extension of the metrology to wide-bandgap semiconductors includes very important measurement of wafers with bare surfaces. Critical is charge-assisted profiling of dopant concentration and use of corona-kelvin CV as a non-contact alternative to mercury probe CV, MCV [4]. Large charging range and superior precision of corona charging for such measurement achieved only recently with the constant surface potential method [4].

In wide-bandgap semiconductors, the bare epi-surfaces in depletion were found to have excellent corona charge retention. In addition, the long- time constant of deep bulk traps and interface traps practically eliminates trap contribution to deep depletion capacitance. This increases the precision of dopant measurement and dopant-depth profiling.

Repeatability of measurements is very important for metrology acceptance in semiconductor manufacturing. In the charge-assisted measurement repeatability requires neutralization of the deposited charge. According to recent developments complete removal of corona deposited charge can be achieved using a photo-assisted process, namely, illumination with UV light with photon energy larger than the semiconductor energy gap. Photogenerated excess carriers neutralize corona ions and neutralized ions detach from the surface. This process enables to return to initial pre-charging condition as verified by the value of surface voltage.

The results of repeated dopant density measurement on epitaxial SiC and GaN demonstrated 1s in 10 repeats of 0.06% for doping in 1014 cm-3 range and about 0.1% for dopant from 1018cm-3 to 1019cm-3. In the range from 1014cm-3 to 2 x 1019cm-3 very good correlation was obtained for SiC and GaN between corona-Kelvin noncontact CV and standard mercury probe MCV results. These results demonstrate present performance levels and confirms that corona-Kelvin represents an industry ready alternative to Hg-CV [5].

Recent results obtained on heteroepitaxial GaN/AlGaN/GaN with two dimensional electron gas, 2DEG, demonstrate the unique advantage of capacitance-charge characteristic C-Q available in corona-Kelvin method. This characteristic enables direct determination of the fully depleted 2DEG condition, and corresponding value of “charge to deplete” the 2DEG, and the pinch-off voltage value based on Q-V data. For GaN/AlGaN/GaN heterostructures these measurements confirmed the 2DEG location at the bottom AlGaN/GaN interface. The electron density depth profile of 2DEG determined from noncontact CV agreed very well with MCV results. The techniques gave the same pinch-off voltage values and the same total capacitance corresponding to AlGaN and GaN layers above the 2DEG.

Wafer level interfacial instability testing is the newest corona-Kelvin application for oxidized SiC and for GaN/AlGaN HEMT with dielectric capping. The fundamental advantage of charge-assisted technique is twofold: 1. larger dose corona charging provides a bias-stress inducing instability and 2. low dose corona charge-Kelvin measurement gives (C,V,Q) characteristics used for evaluation of instability magnitude. In HEMT heterostructures the instability is manifested by the shift in pinch-off point, that in accelerated photo-assisted testing is enhanced using illumination. In oxidized SiC the instability is manifested by CV shift corresponding to threshold voltage shift caused by charging of near-interfacial oxide traps.

The practical advantage of wafer level instability testing is rapid data feedback and elimination of test device fabrication cost, similar to corona-Kelvin advantages established in silicon IC development and manufacturing. An additional well proven advantage of the corona-Kelvin technique is whole wafer mapping capability for all measured parameters.

Figure 1