DE19531802A1 - Scanning near field optical microscope - Google Patents

Abstract

The microscope has a light source for scanning the examined object, and is operated in the reflection mode, with the incorporation of an optical near field interference feedback. A special arrangement of optical fibres is used for the collecting the reflected light, with a magneto-optical data memory being used for storing the optical data. Preferably the optical fibres (1) enclosed by a metal layer (2) are supplied with light from a laser light source, and are moved across the scanned surface (3) with the detection of the intensity of the reflected light by a detector (6).

Description

The resolution of optical instruments is generally due to the Wavelength λ of the light used is limited. This means that with conventional optical microscopes that work with visible light (400 nm < λ <800 nm), a maximum resolution of about 1 µm is achieved. The Storage density of magneto-optical data storage devices (Scientific American 8/95, Page 31) using a laser in combination with a lens system consequently also limited to this spatial resolution.

Recent advances in scanning probe microscopy (SPM) Research area by scanning tunneling microscopy (RTM) (see Physical Review Letters 50, 120 (1983) G. Binnig, Ch. Gerber, E. Weibel, H. Rohrer) have led to the fact that the resolution in microscopy with visible light was extremely increased.

RTM can make atoms "visible", so it has resolutions in the sub Å range. For a variety of applications such as B. Examination of biological materials, Semiconductor structures, in particular optoelectronic components, are suitable but particularly visible light.

Scanning Near Field Optical Microscopy (SNOM) works roughly according to the following principle:
(see Science 257, 189 (1992); General description of SNOM; E. Betzig and JK Trautman)
(see Science 264, 264 pp. 1740-1745 June 1994; Near-Field Spectroscopy of a Luminescent System; HF Hess, E. Betzig, TD Harris...)
(see Journal Applied Physics 73 (3) 1 February 1993: Resolution in collection mode scanning optical microscopy; EL Buckland, PJ Moyer and MA Paesler)

(Fig. 1a)
An extremely small-sized light source (sample) ( 1 ) is moved with the aid of a piezo scanning system ( 3 ) over any surface (sample) ( 2 ).

The representation of the light source can be realized by different methods, what but is irrelevant to the elementary operating principle of the SNOM.

For each point p (x, y), the intensity of the reflected and / or transmitted light is measured by means of a light collection system ( 4 ) and detector ( 5 ), so an intensity profile I (x, y) is obtained in the reflection and / or transmission mode . Furthermore, by "connecting" a spectrometer parallel to the detector, it is possible to create a spectroscopy profile I (x, y, λ) of a surface.

Different methods of displaying the light source:

(a)
Semiconductor lasers can already be produced in dimensions of 100 nm. Such semiconductor lasers are sometimes used directly as a light source for scanning.

(b) (Fig. 1b)
It is possible to manufacture glass fiber, the end of which tapers to a circular area with a diameter below approx. 50 nm ( 1 ). If you couple coherent light from a laser into this glass fiber, which was previously coated with an opaque metal layer ( 2 ), the laser light at the end of the fiber is "compressed" to a cross-sectional area with a diameter of less than 50 nm whose dimension is far below the wavelength of the light used.

The main problem in general in the Scanning Probe Microscopy SPM as well as in the Scanning Near Field Optical Microscopy is to find a feedback (feedback mechanism) that guarantees a constant distance between sample and sample. Calculations and experiments show:
The sample (ie the light source) must be brought to the sample up to a distance of approx. 10-100 nm in order to obtain a favorable optical near field, which ultimately results in a good resolution.

To achieve this, feedback is currently being used for Atomic Force Microscopy (AFM) and RTM were developed and not based on special properties of the optical near field.

Brief explanation of these feedbacks:
- Atomic Force Feedback:
In this case, the glass fiber tip from (b) is excited to oscillate at a certain frequency. If this oscillating glass fiber tip is brought close to atoms, this oscillation frequency changes. Exactly this, as yet unexplained effect is used to construct feedback.
- RTM feedback:
(see: Applied Physics Letters: 62 (12), March 22, 1993: "Simultaneous Scanning tunneling and Optical Near-Field imaging with a micropipette" Authors: Klony Lieberman, Aaron Lewis)
These techniques mark the current state of the art.

Disadvantages of this feedback are mainly what in the low scan speed a low "refresh rate" of the microscope using these methods work results. Furthermore, some of them are very limited and applicable far more complex and costly.

Novel feedback for which patent protection is requested:
Description of the mechanical arrangement ( Fig. 2):
Tube scanners ( 3 ) normally have a "scan area" in the x, y, z direction of a few µm.

With the help of an x, y, z positioning platform ( 1 ), the tube scanner ( 3 ) with the sample ( 4 ) is therefore "roughly" moved up to the glass fiber tip ( 6 ) up to a distance z less than 1 µm. The sample is attached to the positioning platform using a holder ( 2 ), as is the sample on the tube scanner. The collection glass fiber ( 7 ) are arranged symmetrically around the glass fiber tip ( 6 ), the angle α between ( 6 ) and ( 7 ) is 45 ° in each case.

( 6 ) and ( 7 ) are precisely positioned with the help of a special holder ( 5 ) so that the imaginary axes of the collection glass fibers ( 7 ) intersect the tip of (6). The distance to this focal point can be selected according to the numerical aperture of these glass fibers.

In summary, one end of the collection fiber ( 7 ) and the tip of the glass fiber ( 6 ) are clamped by the holder ( 5 ), the other ends can be connected to other optical devices as described below.

Schematic structure ( Fig. 3)

A computer with digital / analog and analog / digital converter controls using various analog and digital output signals ( Fig. 3., (x1, y1, z1), ( 2 ), ( 3 )), which are amplified accordingly with a high-voltage amplifier , on the one hand the positioning platform (amplifier output signals ( Fig. 3, (x2, y2, z2)) and on the other hand the position of the tube scanner (amplifier output signals ( Fig. 3, (x3, y3, z3))) Laser fiber optic coupler over the other end of the input fiber ( Fig. 3, ( 4 )) to its tip ( Fig. 2, ( 6 )). The other ends of the collection fiber ( Fig. 2, ( 7 )) ( Fig. 3 , ( 5 )) are connected to photodiodes and spectrometers and possibly to other detectors ( Fig. 3, ( 11 )) as required.

For control and data acquisition, the computer receives signals from the spectrometer ( 8 ) and the photodiode ( 7 ) via an A / D converter. Both detectors are connected to the SNOM in parallel via the collection fiber ( 5 ). The spectrometer requires suitable light filters, the output signal of the photodiode has to be amplified accordingly for the A / D card of the computer.

How it works: ( Fig. 3)

The elementary functioning of the feedback is based on the dependence of the intensity of the reflected light I on the distance sample-sample z. This function I (z) depends on the type of sample and the wavelength of the laser light used, but topologically you get a typical curve shape I (z) as shown in ( Fig. 6). The x-axis of the graph corresponds to the distance z from the sample, the y-axis to the intensity I.

This course can be roughly divided into two areas:

• - area (a): optical near-field interference,
• - Area (b): optical far field interference, z »λ.

When roughly positioning the tube scanner, the computer simultaneously controls the positioning platform ( Fig. 2 ( 1 )) or the scanning system ( Fig. 2 ( 3 )) and reads the output signal of the photodiode, which is proportional to I (z). By comparison with the reference curve ( Fig. 6) it is thus possible to "find" the optical near field and to achieve a sample-sample distance z <λ <1 µm. A reference intensity I _{R is then} defined in the optical near field and the scanning process is started. The computer gives the signals to scan a certain area (x, y).

The near-field dependence of I (z) (see reference curve ( Fig. 6)) is used to construct the feedback for which patent protection is requested.

Basically there is the possibility to control the feedback with the help of analog ( Fig. 3, option ( 6 ), ( 10 ), ( 9 )) or digital electronics ( Fig. 3, signal ( 7 )):
A digital solution to the problem is:
The following algorithm in pseudo code for the feedback is run for each point (x, y) of the scan area:
Definitions:

• - I = I (x, y, z): intensity signal read by the computer from the photodiode ( Fig. 3, signal ( 7 )).
• - V: signal from the computer to the high-voltage amplifier for controlling the z-expansion of the tube scanner. The z-expansion is proportional to V. ( Fig. 3, signal (z 1), (z2))
• - dI: tolerance range for intensity I defined by the user
• - zero crossing = 1: previously read intensity signal was less than the reference signal I _R and the instantaneous intensity is greater. Or the other way around.
• - State = 0: feedback is active
• - Condition = 1: feedback is ready
• - Abs (x): absolute value of x

FUNCTION feedback (V _start , I _R );
BEGIN
State: = 0;
V: = V _start ;
Read I;
WHILE (state = 0) DO
BEGIN
IF ((zero crossing = 1) OR (Abs (I - I _R ) <dl)) THEN state: = 1;
ELSE BEGIN
IF (I <I _R ) THEN lower V;
IF (I <I _R ) THEN increase V;
Read I;
END;
END;
Feedback: = V;
END;
The two instructions:
IF (I <I _R ) THEN lower V;
IF (I <I _R ) THEN increase V;

make use of the functional dependence of the near-field interference I (z) ( Fig. 4, area (a)), ie if the intensity I is too high, the sample-sample distance must be reduced or the z-extension and the voltage V the scanning tube can be enlarged. And vice versa.

For each point (x, y) of the scan area, the algorithm outputs the value of the voltage V that is stored. A profile of the constant reflection intensity of the surface V _R (x, y) is thus obtained.

If you connect to another collection glass fiber parallel to the photodiode Spectrometer, you also get a spectrum for each point (x, y) of the Scan area I (x, y, λ).

Particular advantages of the invention are speed. In principle, the feedback described in (2.2.1) and (2.2.2) is limited. There is no fundamental limit for the mechanism described in (2.4). The scan frequency can therefore be chosen higher; consequently, you get much higher frame rates. Furthermore, this opens up completely new application options (see 2.7). The multiple arrangement of collection glass fibers ( Fig. 2 ( 6 ), ( 7 )) is of great advantage for this type of feedback. On the one hand, it allows the reflected intensity to be measured from several different angles α, which is important depending on the surface structure of the sample. On the other hand, this type of collection of light is far more cost-effective than that with extremely complex lens systems.

applicability

This invention allows optical microscopes to be constructed whose Resolving power is in the range of approx. 10-50 nm, in contrast to 1 µm conventional optical microscopes.

The range of applications is therefore very broad. Because of the high spatial and Such microscopes are particularly suitable for investigations in spectral resolution biology and semiconductor research, especially for optoelectronic Components.

A special embodiment of the invention is ( Fig. 4, Fig. 5):
(Magneto-optical data storage: see Scientific American, August 1995; p.31)

If you use a magneto-optically active surface as a sample, it is open Due to the high speed of the feedback possible to read and read data save with a much higher compression density than with conventional magneto-optical storage systems.

Mechanical structure ( Fig. 4)

A magneto-optically active disk ( 1 ) is set in rotation by a holder ( 2 ). A 1-dimensional positioning system ( 3 ) moves the head ( 4 ) exactly parallel to the x-axis over a distance from the center of the disc ( 1 ) to the edge of the same. The head ( 4 ) itself has a 2-dimensional fine positioning system in the direction of the x and y axis, with which the distance of the glass fiber tip ( 5 ) to the surface of the disk along the y axis can be changed with a precision in the nm range . At the same time, the translation error of the 1-dimensional positioning system ( 3 ) parallel to the x-axis is also corrected into the um range. Technically, this can also be achieved with the help of piezoelectrically active crystals.

Schematic structure and mode of operation ( Fig. 5)

The basic idea is to add a second system to the conventional magneto-optical drive that works completely independently of the first. The conventional system works roughly as follows:
The control electronics ( 1 ), on the one hand controls the translation of the 1-dimensional positioning system with the output signal x1 ( Fig. 4, ( 3 )), and on the other hand with the output signals o1-on various parameters such as B. the rotational frequency of the rotary motor on which the holder ( Fig. 4, ( 2 )) is attached; the intensity of the laser 1, etc. . At the same time, ( 1 ) receives various other control signals i1-ik in addition to the intensity signal ( Fig. 5, ( 7 )) from the photodiode. Reading and writing processes are controlled by ( 1 ) using the laser 1 and the output signal ( Fig. 5, ( 7 )) of the photodiode 1 , which receives laser light reflected from the surface of the pane via an optical filter 1 and an optical system.

The added second system, for which patent protection is sought, consists of the control electronics ( 2 ) with an amplifier, a laser 2 with the wavelength λ₂, a glass fiber multiplexer, the head ( Fig. 4, ( 4 )) and a second detector system. The glass fiber multiplexer couples the light of the two lasers 1 and 2 with the respective wavelength λ₁ and λ₂ of the two glass fibers ( 3 ) into a single glass fiber ( Fig. 5, ( 4 )) and ( Fig. 4, ( 5 )).

The photodiodes 1 and 2 receive light reflected from the surface of the disc ( Fig. 4, ( 1 )) via the collection glass fibers ( Fig. 4, (6)), ( Fig. 5, ( 5 )). For the following reasons, each photodiode is given a corresponding optical filter.

The control electronics ( 1 ) control the position of the fine positioning system with the signals (xx1) and (xx2) ( Fig. 4, (4)) parallel to the x-axis. The feedback, which is intended to guarantee a constant distance between the glass fiber tip, is integrated in the control electronics ( 1 ) and can consist of a digital or analog circuit. ( 1 ) compares the output signal of the photodiode ( Fig. 5, ( 6 )) with a defined reference value and uses the output signal (y1) to correct the y position of the fine positioning system ( Fig. 4 ( 4 )). This ensures that the glass fiber tip does not leave the near field interference area. The two systems can be decoupled from each other with a simple trick. Filter 1 ensures that only light of wavelength λ 1 reaches photodiode 1 . Filter 2 only allows light from laser 2 of wavelength λ₂ to the second photodiode. As a result, both systems can work independently of one another.

applicability

Magneto-Optical Discs currently have a storage capacity of approx. 1 GB. One bit requires the area of 1 µm × 1 µm. Glass fiber tips with a diameter of 20 nm can already be manufactured today. In theory, this could be done Storage capacity of optical storage media by a factor of 50² to approx. 2500 GB enlarge.

Claims (2)

1. Optical near-field optical microscope (Scanning Near Field Optical Microscope - SNOM), which uses a light source for scanning and works in reflection mode, characterized in that an optical near-field interference feedback is used. In combination with a special arrangement of glass fibers for the collection of the reflected light. 2. Identical arrangement according to claim (1) in combination with a magneto optical data storage for storing and reading data.