application notes

19MicroscopyandAnalysis | SPM supplement March/April 2016

AM AM

INTRODUCTION Two scanning probe microscopy (SPM) techniques—scanning spread resistance microscopy (SSRM) and scanning capacitance microscopy (SCM)—have been used extensively for carrier distribution evaluation of semiconductor devices. In recent years, scanning nonlinear dielectric microscopy (SNDM) has been developed and improved significantly, achieving up to 1013–14/cm3-level low-concentrated observations1.

Traditionally, SSRM and conductive atomic force microscopy (C-AFM) demonstrate that when a positive voltage bias is applied to the sample stage during conductivity mapping as well as in the current–voltage (I–V) curve acquisition process, anode oxidation increases the resistivity in air resulting in artifacts2. This study was conducted to investigate how specific measurement environments including air, dry nitrogen (N

2), and vacuum influence the SNDM and ∂C/∂V curve

measurements of semiconductor samples.

ADVANTAGE of VACUUM SNDM We have newly developed a high-sensitivity “HS-SNDM II” system for the AFM5300E (Fig.1). Figure 2 shows the SNDM and ∂C/∂V – V , C-V curve measurement results of p-n structures fabricated through photolithography and ion implantation. • The n-type dark spot shown in the lower right

in Fig.2(d) was clearly observed under vacuum conditions (10-4 Pa). However, it was not clear in air Fig.2(a) .

• In air, the ∂C/∂V – V , C-V curves demonstrate wide variance at each p-type / n-type region with p-like signal curves occurring in the n-type region.

• In vacuum, irregularity in the ∂C/∂V – V, C-V curve measurements was greatly reduced. P-type and n-type regions demonstrated two clear curve groupings.

• Vacuum vs. air results show significant impact of adsorption water.

• The experiment in dry nitrogen provided median result between vacuum and air3.

Si MOS FET and SiC POWER MOS FET • Figure 3 (a) shows SNDM measurements of a

cross-sectional Si MOS transistor. The 3D image is a SNDM overlay on an AFM image. The impurity diffusion layer was clearly observed.

• Figure 3(b) shows the vacuum SNDM result of SiC power MOS FET. The solid line is the cross-section profile of the dotted line. It is composed of two n -type regions (SiC epi n- / n+ substrate).

CONCLUSION We conducted SNDM and ∂C/∂V-V, C-V curves measurements in air and vacuum. Air environment resulted in low resolution / inaccurate data due to adsorbed water interference. In contrast, SNDM images obtained under vacuum had high-resolution and ∂C/∂V-V, C-V curves measurements had much greater repeatability.ACKNOWLEDGEMENTWe would like to express our gratitude to Professor Y. Cho at Tohoku University for guiding the develop-ment of SNDM.

HITACHI HIGH-TECH SCIENCEVacuum SNDM enhances characterization of carrier distribution in semiconductor materials

REFERENCES 1 Y. Cho et al., Scanning nonlinear

dielectric microscope, Rev. Sci. Instrum., 67, 2297-2303N, 1996.

2 T. Yamaoka et al., Nano-scale physical property observations

by SPM: Electromagnetic measurements in vacuum and SPM/SEM observations, The 34th Annual NANO Testing Symposium, 3, p.13-18, 2014.

3 Jing-jiang Yu et al., Environmental

control scanning nonlinear dielectric microscopy measurements of p-n structures, epi-Si Wafers, and SiC crystal defects", ISTFA 2015: Conference Proceedings, 341-348, 2015.

FIGURE 1 Schematic diagram of HS-SNDM II and Hitachi High-Tech Science Environmental Control SPM Unit AFM5300E

FIGURE 2 SNDM and ∂C/∂V – V , C-V curves measurements of p-n pattern in air / vacuum

FIGURE 3, below, SNDM applications: (a) Si MOS FET, (b) SiC power MOS FET

(a) (b)

Photodetector

Ring electrode

Sample electrode

Lock-in amplifier

FM demodulator

Semi-microwave oscillating circuit

Laser diode

Conductive probe

REF

VDC

C L f

VAC

SIG

∂C /∂V

[a.u

.]

∂C /∂V [a.u.]

+

0

-

SiC epi (n-)

SiC (n+) substrate

p

n

p-well

n+

0

+ - 2 µm

SPM014 AM App Notes.indd 19 09/03/2016 11:33