0026-2714/$ - see front matter � 2006 Published by Elsevier Ltd.doi:10.1016/j.microrel.2006.07.019

Microelectronics Reliability 46 (2006) 1554–1557

www.elsevier.com/locate/microrel

Evaluation of Scanning Capacitance Microscopy Sample Preparation by Focused Ion Beam

N. Rodriguez1,2, J. Adrian1, C. Grosjean1, G. Haller1, C. Girardeaux2, A. Portavoce2

1 STMicroelectronics, RCCAL, Rousset France

2 L2MP UMR CNRS 6137 Université Paul Cézanne Aix Marseille, France Email: nicolas.rodriguez@st.com

Abstract Due to the continuous reduction of the critical dimensions of semiconductor devices, it becomes very important to know the two dimensional (2D) doping profile for electrical performance of devices. Scanning Capacitance Microscopy (SCM) is a powerful technique for qualitative analysis of 2D doping species distribution, measuring small capacitance variations with high spatial resolution. For 2D carrier profiling, the region of interest must be accessible to the profiling instrument. SCM samples require cross-sectioning to expose the inner sample at a visible surface. In some analysis, the failure is localized at a very accurate address up to hundreds of nanometers. With the traditional polishing method of sample preparation it is very difficult to reach the exact location. For this reason we are investigating a new way to prepare SCM sample with Focused Ion Beam (FIB) and plasma etch in order to accurately choose the scanning zone. This paper presents a method to obtain SCM scans after a sample preparation by FIB and the influence of the FIB and the Plasma etcher on cross-sectioned SCM samples. 1. Introduction

Information about implant distribution at a failing site can provide root-cause and/or help to isolate the source of a failure to specific production step. Scanning Capacitance Microscopy (SCM) is an effective method to get clear evidence of device doping failure with spatial resolution of about 10-20 nm and a dynamic range of 1015 to 1020 atoms/cm3 [1]. SCM measures capacitance variations between the metalized probe tip and a semiconductor sample while scanning in contact mode. Since the variations are directly related to dopants, or carrier concentration, the SCM generates a 2D image with contrast corresponding to near-surface variations in carrier density (fig. 1). In order to acquire a SCM image of an integrated circuit (IC), the IC samples must be conveniently prepared. The samples were prepared to reveal the site being investigated. For

top-view sample preparation, a chemical solution was used. For cross-sectional samples, polishing was performed with diamond film from 3µm to 0.3 µm. Usually SCM samples are cross-sectional samples. The most important criteria for a cross-section surface are: low roughness, cleanliness and no surface damage. When some specific failure addresses have to be reached, it is very difficult to control the attack speed and to stop at the exact required location. Optical control in cross section is limited on a target under 1 µm. Today the Focused Ion Beam technique is the more precise technique to perform cross sectioning of specific structures. However this technique based on highly energetic focalized ions (Gallium mostly used) is known to induce damages in the samples. The two main damages induced by FIB, are the amorphisation of the silicon surface and gallium implantation. Some recent SCM studies on FIB prepared samples [2]

N. Rodriguez et al. / Microelectronics Reliability 46 (2006) 1554–1557 1555

have shown that the amorphous silicon layer thickness is not a primary factor to explain the lack of SCM data and the authors suggested that it could be mostly due to charging effects. FIB induced damages on silicon have also been studied by SCM, in the direction parallel and perpendicular to the incident beam [3]. In this case the authors reported a SCM visible damage to an extent region of hundred nanometers. FIB is also used to mark and approach the location of interest of the semiconductor device and a final polishing step is intended to remove most of the FIB induced surface damaged layer and to isolate the site of interest [4].

To eliminate FIB induced artifacts, we decided to use Reactive Ion Etching (RIE). We will present below the experimental procedure and the first results of SCM measurements. Some experiments aimed to distinguish the influence of FIB and/or plasma etching will also be presented.

Fig. 1. Basic SCM detection system

2. Experimental procedure In this study, all the experiences are realized on

a Digital Instruments Dimension 3100 AFM equipped with a Nanoscope III controller, a SCM module and using conductive, metal coated OSCM-Pt probes. Topography and differential capacitance images were obtained simultaneously. The sample preparation is realized on a FIB FEI 800, with standard parameter (30KeV and 3000pA), and on a UNAXIS Nextral 860 Plasma Etcher. The plasma etching used is based on oxygen and fluorine gas chemistry, for 100 seconds.

Two experiments were carried out during our

study. The first one being to prove that it is possible to obtain SCM images after a FIB preparation, the second being to understand the influence of FIB and

plasma on the sample chemistry and structure.

2.1. First experiments plan: SCM image after FIB and Plasma preparation.

For the first experiment plan, each sample was

cleaved with a SELA MC 600 micro cleavage tool near the target. Then four different sample preparations were tested.

For the first one we used only plasma etching (sample 1). For the second we used only FIB polishing (sample 2), on the third we coupled FIB and a plasma etching step (sample 3) and the last one was obtained by classic mechanical polishing method (sample 4), using diamond film and felt film with colloidal silica.

Then, the SCM scan is performed after a waiting time of around 12 hours, time needed for a native oxide growth on the surface of each sample.

2.2. Second experiments plan: Understanding damage and influence of FIB and Plasma

To understand the influence of the FIB and

Plasma on the sample preparation, we prepared two special samples and characterized them by Transmission Electron Microscopy (TEM).

The first type of samples (sample 5) was prepared by FIB only: we made a box (40µm x 6µm x 8µm using a 30KeV 3000pA Ga+ beam), to characterize the FIB influence (thickness of the amorphous layer).

The second type of sample (sample 6) was prepared by FIB as previously described followed by a plasma etching of 120s, to characterize the plasma etch.

3. Results and discussion

3.1. First results and discussion On sample 1 preliminary results not detailed in

this paper, have shown that a SCM signal can be obtained after a plasma etching used to clean the surface.

On sample 2 we confirmed that it was not possible to perform SCM directly after FIB milling. A wide range of AC bias sample was investigated without any success in signal obtaining.

However on sample 3 prepared by FIB followed by plasma etching, we have shown it was possible to

1556 N. Rodriguez et al. / Microelectronics Reliability 46 (2006) 1554–1557

image implants as seen in fig 2.

Fig. 2. Scan SCM of P type implants on samples prepared

by FIB (topographic data on the left and SCM data on the right)

In this scan, P doped drain and source implants

are imaged at the expected place with the expected profile.

We successfully tested the reproducibility of this method by preparing other samples and observed the expected implant type (P type in fig. 2 and N type in fig. 3) and the expected implant profiles.

Fig. 3. Scan SCM of N type implant on sample

prepared by FIB (topographic data on the left and SCM data on the right)

In this scan, N doped drain and source implants

are seen at the expected place with the expected profile.

Nevertheless, when comparing the depth of implants obtained on samples prepared by FIB (sample 3) to that obtained on sample prepared by classic polishing method (sample 4) we noticed that the implant profile was thinner on sample prepared by FIB and that the junction depth was of about 20 nm shorter. This discrepancy corresponds to the SCM resolution limit, and may be explained by statistical reproducibility (two samples and two different scans).

To conclude, our first experiments revealed that

a FIB followed by plasma etching sample preparation is a promising method for SCM sample preparation. The implant revealed on the so-prepared samples was at the same place with the same profile than the one obtained by scanning a traditional mechanically polished sample.

3.2. Second results and discussion

On sample 5, the TEM image revealed an

amorphous layer of around 20 nm induced by the focused ion beam (fig. 4).

On sample 6 we have shown that our structure after the plasma etch step had been etched on a thickness of 50 nm (measured between the Co which is not etched by our plasma recipe and the Silicon) and we showed an unknown amorphous layer of about 4.3 nm as presented on TEM picture in fig. 5.

Fig. 4. TEM analysis of the surface after FIB

Fig. 5. TEM analysis of the structure after FIB and

plasma With the previous TEM information we can

explain the results of the first experiments plan. The thickness of the amorphous layer after FIB (20nm) seems to be too thick to be crossed by the current applied by the SCM probe on the sample and/or there is too much Ga ions. Therefore it is impossible to do SCM after FIB and it confirms our previous result. However when we have made plasma etcher after FIB we removed 50 nm of material. The plasma removed the entire amorphous layer and a sample

Si

Co

Si

Si

Co

Amorphous layer

Si

Amorphous layer (Si)

Oxide

Amorphous layer

Poly

Poly

Poly

Oxide

Amorphous layer

Platinum deposition

Platinum deposition

N. Rodriguez et al. / Microelectronics Reliability 46 (2006) 1554–1557 1557

part of the silica. The presence of an amorphous layer, of about 4.3nm, probably comes from a native oxide induced by plasma. Thanks to this thickness, the current can go through the amorphous layer and then we can obtain SCM data.

4. Conclusion and Perspectives SCM is a valuable tool for characterization of

2D doping profiles in failure analysis. To achieve best results, the sample preparation is a very important step, prior to measurements and thus significant time has to be spent at this step. As integrated circuits have become more and more complicated with the device feature shrinking into the deep sub-micron range, there is an increasing need to have accurate localization of physical defects. In this paper we have demonstrated the ability to get useful information about a 2D dopant profile analysis on samples prepared by FIB and plasma etcher with SCM. Therefore we can localize exactly our target by FIB and then obtain SCM data at a specific location. It is possible to make SCM measurements after FIB because we have shown that plasma etch after FIB removes amorphous layer and creates an oxide layer of about 4.3 nm thick.

In the future we will optimize this process to better reveal implants. For example it can be achieved by improving the FIB box, reducing the FIB voltage and by adjusting process of the plasma step (characterization and optimization of this process step). We have possibility to develop a new methodology for defect localization for SCM cross sectioned prepared by FIB (BITMAP SCM for example). We are trying to perform Scanning Spreading Resistance Microscopy (SSRM) on samples prepared by FIB to show implants with a better resolution method. Acknowledgements The authors would like to acknowledge the Rousset Central Characterization & Analysis Laboratory (RCCAL) for its support. The authors would especially like to thank Pierre BOUSSUGES for his ideas, and helpful competences and Ludovic BEAUVISAGE for TEM analysis. This work is part of the thesis of one of the authors (N. R.) who was supported by the CIMPACA project and granted by European funding.

References [1] Digital Instruments Veeco Metrology, Application

Modules: Dimension and MultiMode Manual Rev D, 2003.

[2] Fillmore D. K. and Wang S., Investigation of Low-Energy Focused Ion Beam Milling for Scanning Capacitance Microscopy Sample Preparation, ISTFA 2004.

[3] Brezna W. and Co, Focused ion beam induced damage in silicon studied by scanning capacitance microscopy, Semicond. Sci. Technol. 18, 2003, pp 195-198.

[4] Jarausch K., Richards J., Denney L., Guichard A. and Russell P., Site specific 2-D implant profiling using FIB assisted SCM, ISTFA 2002.