WO2009118164A2

WO2009118164A2 - Nanoscale charge carrier mapping

Info

Publication number: WO2009118164A2
Application number: PCT/EP2009/002173
Authority: WO
Inventors: Oleg Lourie
Original assignee: Nanofactory Instruments Ab
Priority date: 2008-03-25
Filing date: 2009-03-25
Publication date: 2009-10-01
Also published as: SE0800670A0; WO2009118164A3

Abstract

The present invention relates to a measurement device (2) for characterization of physical properties of a nano-sized structure of any semiconductor device using a scanning probe in an electron microscope. The invention further relates to a measurement system (1) comprising such a measurement device (2), a method of testing and verifying nano-sized structures of any semiconductor device and a quality analysis method.

Description

NANOSCALE CHARGE CARRIER MAPPING

TECHNICAL FIELD

The present invention relates to a measurement device for characterization of physical properties of a nano-sized structure of any semiconductor device. The invention further relates to a measurement system comprising such a measurement device, a method of testing and verifying nano-sized structures of any semiconductor device and a quality analysis method.

BACKGROUND OF THE INVENTION

The size of small structured electronic devices are becoming increasingly smaller and today devices with structures of size orders in the range of a few tenths of nanometres are being developed. This sets up new demands in testing and measurement devices for these small sized structures in order to understand the operation of devices and functions built using such small structures and in order to verify operation of large scale manufacturing of these types of devices.

There are a number of measurement devices available today that can operate at these scales, such as electron microscopes (operating in transmission, TEM, or scanning, SEM, modes) and scanning probe microscopy devices (SPM). Two often used SPM instrumentations are the atomic force microscope (AFM) and the scanning tunnelling microscope (STM). Each of these instrumentations has their benefits but also their drawbacks which will become apparent.

The TEM and SEM are very good at looking at both the micro- and macro-scale of an object and thus being able to locate structures, however, none of these two are able to test and verify electrical properties of the sample.

The AFM is good at determining structural properties but not the electrical properties, whereas the STM is primarily good at determining electrical properties of a sample surface but since it measures a tunnelling current between a tip probe and the sample surface it will probe the local electrical conductance of the sample at the nanoscale. The AFM and the STM are not good at working on the macroscale, i.e. it is difficult to repeatedly find a certain location on the sample surface.

SIMS (secondary ion mass spectrometry) provides a solution for determining the chemical structure and depth profile of the sample; however, this method is not suitable for small structures since it provides only a few micrometers of resolution at best.

There is thus a need for an instrument that can provide both a macro- and nano-scale positioning while at the same time being able to determine the electrical and/or chemical properties of the sample.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. This is achieved by providing a measurement device as claimed by claim 1.

In a first aspect of the present invention there is provided a measurement device for characterization of physical properties of a nano-sized structure of any semiconductor device. The measurement device comprises a probe, a probe holder for holding the probe, a positioning device, and a control unit. The positioning device is arranged to position the probe holder and sample in relation to each other. The control unit is arranged to send control signals to the positioning device for positioning of the probe in relation to the sample, and to acquire at least one measurement signal relating to the charge carrier density of the nano-sized structure. Simultaneous imaging is possible by means of using an electron microscope and having located the sample in situ of the electron microscope. The positioning device is able to move the probe relative to the sample, such that a charge carrier density map of an area of the sample can be achieved.

The invention provides for measuring charge carrier maps in situ of an electron microscope using a scanning probe microscopy technique simultaneously with acquiring images with the electron microscope. The images of the electron microscope are helpful for locating the structure of interest and for verifying the position of the probe during characterization. In one embodiment, the measurement signal is a plurality of measurements of electrical current flowing between the probe and sample at given voltages. The current may be used to determine electrical and/or chemical properties.

The positioning device may be a piezo-electric device. It may be adapted to either move the probe or the sample. The same positioning device may be adapted to move the probe or the sample in both micro-scale and macro-scale displacements.

The measurement device may further comprise at least one force sensing device associated with the probe and/or sample, the force sensing device being arranged to measure the force exerted when the probe makes contact with the surface of the sample.

In a second aspect of the present invention there is provided a measurement system for determining a charge carrier density map comprising a measurement device as described above and an analysis processing unit.

In a third aspect of the present invention there is provided a method of testing and verifying nano-sized structures of any semiconductor device, comprising the steps of: (a) positioning an electrically conducting probe in relation to a sample; (b) determining the position of the probe relative the sample using an electron microscope;

(c) monitoring the position of the probe in relation to the sample while positioning the probe at a measurement position of the sample;

(d) acquiring measurement signals related to the charge carrier density from the probe;

(e) scanning an area of the sample and acquiring a charge carrier density map.

The method provides for measuring these charge carrier maps in situ of an electron microscope.

The method may also repeat steps (b)-(e) securing that the same area of the sample is measured by using the electron microscope to keep track of where on the sample measurement is done. In a fourth aspect of the present invention there is provided a quality analysis method for quality monitoring of semiconductor structure devices, using the method of testing and verifying described above for measurement of samples randomly chosen from a production series or batch of semiconductor devices, integrated circuits or nems/mems devices. This quality analysis method would for example be useful for quality monitoring in a manufacturing plant.

In a fifth aspect of the present invention there is provided a use of the above-mentioned measurement device for quality monitoring of semiconductor structure devices, the semiconductor structure devices being samples randomly chosen from a production series or batch of semiconductor devices, integrated circuits or nems/mems devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be explained in greater detail by means of non- limiting examples and with reference to the appended drawings in which:

Fig. 1 illustrates schematically a measurement system according to the present invention;

Fig. 2 illustrates schematically a measurement device according to the present invention;

Fig. 3 illustrates schematically the probe/sample area in a close up of Fig. 2;

Fig. 4 illustrates schematically in a block diagram a method according to the present invention; and

Fig. 5 illustrates schematically an I-V curve according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In Fig. 1 reference numeral 1 generally indicate a measurement system comprising a measurement device 2, control/measurement electronics 3, and optionally a control and analysis processing device 4, which as an option may be connected to a network 6. The measurement device 2 may be built as a microscope sample holder for an electron microscope 5, for instance for a transmission electron microscope (TEM) or for a scanning electron microscope (SEM). The illustrated electron microscope 5 of Fig. 1 shows the configuration of a standard TEM. The microscope 5 essentially comprises an electron gun 7, able to produce an electron beam 9 passing through a collimator 8. The electron beam passes through various components, such as a lens system 10 and the measurement device 2, and is ultimately detected by a detector 13, for example by being projected on a screen. The function of the TEM instrument is well known, and will not be closer described herein. Moreover, it shall be noted that the structure of the electron microscope per se is not important for the present invention, but the invention may be used with various kinds of electron microscopes, and is not limited to the TEM disclosed in Fig. 1.

The measurement device 2 may include a sample holding structure 11 and a probe holding structure 12. One of the sample 11 and the probe holding structure 12 is located in mechanical cooperation with a nano-positioning device such as a piezo-electric device. The piezo-electric device changes its mechanical dimensions when a voltage is applied to it. The control/measurement electronics 3 is arranged to control signals applied to the piezo and/or other positioning devices. The control/measurement electronics 3 is further arranged to receive signals from measurements performed at the sample/probe area. It should be noted that pre-conditioning electronics may be present close to the sample/ probe area on the measurement device 2 depending on type of measurement to be done, since in some cases the measurements are subject to electrical and/or mechanical disturbances.

In some configurations the piezo-electric device is complemented with some other electromechanical device for operating the sample or the probe on a macroscopic scale (up to a few millimetres of range). The total operating range will thus be from a few Angstroms (or even smaller) and up to a few millimetres.

In one embodiment of the present invention the piezo-electric device is arranged to operate with a movement solution that operates within the entire displacement range for the relative position between the probe and the sample. This is shown in relation to Fig. 2. The components of the measurement solution are in mechanical connection to a frame 22 of an electron microscope sample holder. A positioning device (e.g. controlled by a piezoelectric tube 24) is at one end in fixed mechanical connection to the frame (directly of indirectly). A probe holder receiving structure 23 (e.g. a structure with part of the outer circumferential surface being spherical) is attached to the piezo tube 24, and in turn a probe holder 12 is clamped to the probe holder receiving structure 23 using clamping structures pressing on the receiving structure 23. A probe 20 may be attached to the probe holder 12. The operation of the positioning device may be as follows: by applying a voltage to one or several electrodes on the piezo tube it will be elongated, retracted, or bent in some suitable direction: this type of displacement operate in the sub Angstrom range. By rapid changes of the applied voltage it is possible to displace the probe holder 12 relative the probe holder receiving structure 23 using the mechanical inertia of the probe holder 12. This type of displacement may provide movement in the range of several millimetres using repeated changes of the voltage (each change may for example give a displacement of the order a few micrometers depending on applied voltage change). Furthermore, a sample 28 is attached to the sample holder 11 which in turn is fixed by fixation means 21 to the frame 22. Both the sample 28 and the probe 20 may be electrically connected (indirectly via sample holder 11 and probe holder 12) with electrical connections 26 and 27 respectively. The electrical connections may be connected to an optional pre-conditioning unit 25 before being connected to the control and measurement electronics 3. Further, electrical connections may be present to the piezo tube for applying voltages to a suitable electrode of the piezo tube, for grounding purposes, or for other physical measurements available in the setup.

The control/measurement electronics 3 may optionally be connected to a processing device 4, e.g. a computer. This may be provided with software for interfacing with the electrical control/measurement electronics 3 and with the user of the system. The processing device 4 may be arranged to analyze measurement data and to control the overall operation of the system while the control and measurement electronics 3 may be arranged to control operation of the measurement setup on a local scale, i.e. handling feedback loops, taking of measurements, noise cancellation, pre conditioning of signals, and so on, as understood by the skilled person.

A suitable probe 20 is positioned close to the sample 28 and the electrical interaction between the probe and the sample surface is measured using at least one of the available techniques. In one embodiment the probe 20 is pressed against the sample 28, preferably with a known force and/or known distance into the sample. This ensures a good and reproducible contact configuration enabling reproducible measurements to be done of the electrical and/or chemical properties of the sample at the probe position on the sample. However, in order to measure small structures the probe needs to have a tip that has small enough dimensions and a high aspect ratio. Such tips can be produced for example by means of electrochemical etching, ion sputtering or by FIB processing (Focused Ion Beam). They can also be functionalized in different ways, by chemical or physical methods, for example by growing nanotubes at the end of the tip. Furthermore, the sample tip may also be functionalized with respect to other characteristics such as chemical or magnetic interactions.

In some cases, the sample also requires some preparation. For instance, if a TEM is to be used, the sample need to be prepared by reducing it to suitable dimensions for fitting in the sample holder and reducing the thickness so as to allow electrons to pass through it (i.e. make it electron transparent) in order to view the structures of the sample. Also, in the case of using a SEM, the sample might need cleaning and/or dimensioning (i.e. reduced in dimensions so as to fit in the sample holder).

In one embodiment a TEM is used for locating a structure which is of interest to characterize. When the structure has been located the probe is positioned close to or in contact with the structure while still imaging with the TEM in order to verify the location of the probe during the characterization. This ensures the relevance of the data (at least with respect to the geometrical location). The TEM may also be used to determine the distance the probe enters into the sample which may be used for reproducing similar depths between different samples or different structures on the same sample: thus again ensuring the reproducibility of the technique.

By using a force sensing device associated to the probe or the sample, it is possible to determine the force exerted when the probe makes contact with the surface. The force sensing device may be directly or indirectly connected to the probe or the sample. This has the advantage of allowing for a reproducible measurement between measurements (i.e. the same measurement repeated several times, for different structures on the same sample, or different structures on different samples) and/or determining the mechanical characteristics of the interaction area of the sample, for instance, by comparing the force exerted and the depth of the probe in to the sample, the mechanical hardness may be determined. In Fig. 3 is shown a setup with a probe 30 positioned in relation to a sample 31. In or on the sample is located a number of structures 32, 32', and 32" which are of interest to characterize. Furthermore, a force sensing device 33 is located as to measure the force exerted on the sample 31 by the probe 30. However, it should be understood by the skilled person that the force sensor may be located associated to the sample 31 instead of the probe 30 in some configurations. When dealing with nano-sized structures on or in semiconductor or nems/mems devices, it is of interest to determine the depth information of the nano-sized structures, for instance doping profiles of n- or p-doped structures. The terms nems and mems stands for nano electro-mechanical systems and micro electro- mechanical systems.

When the probe 30 is close to or in contact with the surface, the system may perform different types of measurements depending on the type of probe. In one embodiment an electrical conducting tip is used and the voltage is scanned over a range and the resulting current through the probe/sample system is detected, generating an I-V curve, a current- voltage curve, presenting the current as a function of the voltage. This I-V curve may be used to determine for instance the electrical and/or chemical characteristics of the structure. With appropriate signal analysis the depth profile of the sample may be obtained. The probe may be scanned over the surface producing a 2-dimensional map of the structure of interest. By stopping at a number of points during the scan and taking I-V curves for each point, a 2-dimensional map with depth information or charge carrier information may be obtained. This so called CC-map (charge carrier map) may be used to understand the electrical (or chemical) performance or behaviour of the structure in question. The structure may be any type of semiconductor structure, conducting structure or insulating structure as used in small scale electrical devices.

Fig. 5 shows a typical I-V curve obtained from a probe/surface measurement according to the present invention.

Scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) are methods suitable for two-dimensional profiling of localized resistance/conductance on a semiconductor surface of na no/micro-scale structures. These methods may provide depth profiles from bulk structures due to that the bulk structures may provide a surface signature affecting the surface sensitive measurement. SCM measures capacitance variations between a metallised probe and a sample, e.g. a semiconductor sample while scanning in e.g. contact mode. Since the variations are directly related to charge carrier concentration, the SCM may generate a 2D-image with contrast corresponding to near-surface variations in charge carrier density. 5

Fig. 4 illustrate schematically a measurement process according to the present invention with the steps of,

401. Positioning the probe in relation to the sample.

402. Determining the position of the probe in relation to the sample using the electron 10 microscope.

403. Positioning the probe in relation to a structure of the sample.

404. Continuously monitoring the probe/sample area during measurements.

405. Measuring electrical or chemical characteristics in relation to the structure of the sample.

15 406. Optionally repeating measurements for several structures. 407. Removing the probe from the sample.

The present invention is advantageously used in sample characterization of production grade samples of for instance any semiconductor structure devices (semiconductor may

20 be Si based, GaAs based or any other semi conducting device), i.e. devices manufactured in a batch or series production in professional fabrication facilities or in laboratories. The measurement solution according to the present invention may be used for instance for quality analysis of random inspection samples of production series of semiconductor devices, integrated circuits, or nems/mems devices.

25

It should be noted that the word "comprising" does not exclude the presence of other elements or steps than those listed and the words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, and that several "means", "devices",

30 and "units" may be represented by the same item of hardware.

The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent 5 claims should be apparent for the person skilled in the art.

Claims

1. A measurement device (2) for characterization of physical properties of a nano- sized structure of any semiconductor device, said measurement device (2) comprising a probe (20), a probe holder (12) for holding the probe (20), a positioning device (24), and a control unit (3), wherein the positioning device (24) is arranged to position the probe holder (12) and a sample (28) in relation to each other, and the control unit (3) is arranged to send control signals to the positioning device

(24) for positioning of the probe (20) in relation to the sample (28), and to acquire at least one measurement signal relating to the charge carrier density of the nano- sized structure, while simultaneous imaging is possible by means of using an electron microscope

(5) and having located the sample (28) in situ of the electron microscope (5), and said positioning device (24) is able to move the probe (20) relative to the sample (28), such that a charge carrier density map of an area of the sample (28) can be achieved.

2. The measurement device according to claim 1 , wherein the measurement signal is a plurality of measurements of electrical current flowing between the probe (20) and sample (28) at given voltages.

3. The measurement device according to claim 1 , wherein the positioning device (24) is a piezo-electric device.

4. The measurement device according to claim 1 , wherein the positioning device (24) is adapted to either move the probe (20) or the sample (28).

5. The measurement device according to claim 1 , wherein the same positioning device (24) is adapted to move the probe (20) or sample (28) in both micro-scale and macro-scale displacements.

6. The measurement device according to claim 1 , wherein the device further comprises at least one force sensing device (33) associated with the probe (30) and/or sample (31 ), said force sensing device (33) being arranged to measure the force exerted when the probe (30) makes contact with the surface of the sample (31 ).

7. A measurement system (1 ) for determining a charge carrier density map comprising a measurement device (2) according to claim 1 and an analysis processing unit (4).

8. A method of testing and verifying nano-sized structures of any semiconductor device, comprising the steps of: (a) positioning an electrically conducting probe (20) in relation to a sample

(28);

(b) determining the position of the probe (20) relative the sample (28) using an electron microscope (5);

(c) monitoring the position of the probe (20) in relation to the sample (28) while positioning the probe (20) at a measurement position of the sample (28);

(d) acquiring measurement signals related to the charge carrier density from the probe (20);

(e) scanning an area of the sample (28) and acquiring a charge carrier density map.

9. The method according to claim 8, further comprising the step of repeating steps (b)-(e) securing that the same area of the sample (28) is measured by using the electron microscope (5) to keep track of where on the sample measurement is done.

10. A quality analysis method for quality monitoring of semiconductor structure devices, using the method according to claim 8 for measurement of samples randomly chosen from a production series or batch of semiconductor devices, integrated circuits or nems/mems devices.

11. The use of the measurement device (2) according to claim 1 for quality monitoring of semiconductor structure devices, said semiconductor structure devices being samples randomly chosen from a production series or batch of semiconductor devices, integrated circuits or nems/mems devices.