Scanning probe microscope

Details Scanning probe microscope Scanning probe microscope

1998-07-15

2001-06-05

Anderson, Bruce C. (Department: 2881)

Radiant energy

Inspection of solids or liquids by charged particles

C250S307000

Reexamination Certificate

active

06242736

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a scanning probe microscope represented by a scanning interatomic force microscope (AFM: Atomic Force Microscope), and more particularly to a scanning probe microscope adapted to convert a surface geometry of a sample into color information depending on its surface frequency in order to effect color display.
The scanning probe microscope such as an AFM uses a cantilever provided with a probe needle at a tip of a cantilever beam in order to detect a microscopic texture or structure of a sample surface by utilizing an interaction between the sample surface and the probe.
The scanning of the probe needle utilizing such a cantilever over a sample surface causes an attractive force or a repulsive force between the sample surface and the probe needle on the basis of interatomic force. Consequently, if this interatomic force is detected as a cantilever strain amount and a sample stage is slightly moved in a Z direction so as to make this strain amount constant, that is, so as to make a gap between the sample surface and the probe needle constant, a slight-movement signal thereof or a detected strain amount itself will represent a geometry of the sample surface.
FIG. 13
is a block diagram showing one example of a signal processing system of a conventional scanning probe microscope. A sample
52
is rested on a three-dimensional sample stage
55
, and above the sample
52
there is oppositely arranged a probe needle
54
fitted at a free end of a cantilever
53
. The strain amount in the cantilever
53
is detected by measuring, using a position detector
73
, an incident position of a laser beam
72
output by a laser generator
71
.
The position detector
73
is constituted by a four-segment light detecting electrode, and aligned in position such that a spot of a laser beam
72
comes to a center of the four-divided electrode when the strain amount of the cantilever
53
is 0. Accordingly, if a strain occurs on the cantilever
53
, the spot of a laser beam
72
moves over the four-segment electrode, thereby producing a difference in the voltage output by the four-segment electrode. This difference in voltage is amplified by a differential amplifier
74
and input as a strain signal S
1
representative of a gap between the sample surface and the probe needle
54
to a non-inverted terminal (+) of a comparator
75
. The comparator
75
has an inverted input terminal (−) to which a target value signal as to the strain amount in the cantilever
53
is input from a target value setting section
79
.
An error signal S
2
output by from the comparator
75
is input to a proportional integration (PI) control section
76
. From the PI control section, a resultant signal of the error signal S
2
and its integration value is input, as an observed image signal S
3
and also as an actuator slight-movement signal for controlling the gap between the sample surface and the probe needle
54
to a predetermined value, to an amplifier
81
and an actuator driving amplifier
70
.
A scan signal generating section
78
supplies a slight-movement signal for slightly moving the sample
52
in XY directions to the actuator driving amplifier
70
. The position detector
73
, the differential amplifier
74
, the comparator
75
, the PI control section
76
and the actuator drive amplifier
70
constitute a feedback circuit.
The observed image signal S
3
is appropriately amplified by the amplifier
81
and thereafter supplied to an A/D converter
82
where it is converted into image data and stored in an image memory
83
. An image memory control section
84
outputs an address signal and a lead signal to the image memory
83
in synchronism with a clock signal output by a synchronous signal generator
85
. The image data output by the image memory
83
in response to the address signal and the lead signal is supplied to a RAM-DAC
86
. The RAM-DAC
86
converts the image data into an analog signal in response to horizontal and vertical synchronizing signals, and the converted image data is output to a monitor unit
87
.
Where a roughness of a sample surface is to be monochromatically displayed with accuracy, a gradation representation of approximately 16 bits is ideally required. The monitor unit
87
, however, is low in gradation representability. In addition, the increase in gradation requires an increase in the resolving power of the A/D converter
82
or the memory capacity of the image memory
83
, thereby resulting in expensive apparatus cost. To avoid this, the above-stated prior art apparatus is typically designed to represent each pixel concentration with 8 bits (64 gradations), so that there has been a problem that the sample surface roughness cannot be accurately represented.
In order to solve such problem, there has been proposed a method in which image data is put into a computer and the data is subjected to image processing so as to convert it into a three-dimensional representation. However, since the image data processing requires a high-speed processor and a large-capacity image memory, there has been the problem that the apparatus becomes expensive as well.
Further, in the above-stated prior art apparatus, if the space frequency as to the sample surface roughness is high, and the probe needle
1
is comparatively quick in scan speed, the probe needle
1
cannot follow the roughness as the case may be. Where the feedback circuit is insufficient in gain, the comparator
75
outputs an error signal S
2
depending upon a difference between the strain signal S
1
and the target value. The PI control section
76
outputs an observed image signal S
3
as an actuator slight-movement signal in order to effect feedback control for approximating the error signal to zero. However, the error signal S
2
cannot be reduced completely to zero. Consequently, the observed image signal S
3
always becomes insufficient in signal component corresponding to the error signal S
2
, thereby resulting in bluntness at its edge portion.
In order to solve such problem, there has been a proposed structure, as in the prior art apparatus shown in
FIG. 14
, that is provided with a switching section
77
for selectively outputting either one of the strain signal S
1
or an observed image signal S
3
to the amplifying section
81
depending upon a switching signal separately input.
In the above-stated structure, however, the switching section
77
has to be switched to the strain signal S
1
side when an edge portion is to be recognized with preference, while the switching section
77
switched to the observed image signal S
3
side when a roughness state is to be recognized with preference. Due to this, there has been a problem in that an edge portion and a roughened portion are impossible to be recognized with accuracy at the same time.
It is an object of the present invention to provide a scanning probe microscope which is capable of converting a sample surface geometry into color information depending on its surface frequency to provide color display, thereby making it possible to accurately recognize the sample surface geometry.
SUMMARY OF THE INVENTION
In order to achieve the above-stated object, according to the present invention there is provided a scanning probe microscope adapted to scan a probe needle in proximity to a surface of a sample in XY-axis directions over the sample surface while moving at least one of the probe needle and the sample in the Z-axis direction, characterized by comprising: a signal generating means for outputting a surface geometry signal representative of a surface geometry of the sample on the basis of a change in gap spacing between the sample surface and the probe needle; a band-pass signal generating means for generating plural band-pass signals by extracting predetermined frequency bands different one another; an image memory for memorizing the respective band-pass signals by putting correspondence to positions on the sample surface; and a color image outputting means for outputting a color image by deeming that

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