WO1998013663A1

WO1998013663A1 - Atomic force microscope probe

Info

Publication number: WO1998013663A1
Application number: PCT/EP1997/004840
Authority: WO
Inventors: Ewald Guenther; Rainer Leuschner; Olaf Ohlsson
Original assignee: Siemens Aktiengesellschaft; Nanosensors Dr. Olaf Wolter Gmbh
Priority date: 1996-09-23
Filing date: 1997-09-05
Publication date: 1998-04-02
Also published as: DE19638977A1; TW360894B

Abstract

In an atomic force microscope probe with an electroconducting spring-mounted stylus and an electroconducting probe point, the spring-mounted stylus is provided with a shielding electrode and an electrically insulating layer is arranged between the shielding electrode and the spring-mounted stylus. Electrostatic forces produced during operation may thus be eliminated.

Description

description

Force microscopy probe

The invention relates to a force microscope probe with an electrically conductive spring beam and an electrically conductive probe tip.

In semiconductor technology and in microelectronics, the structural dimensions are getting smaller and smaller. Structures with a width of less than 400 nm are produced today in memory production; optical lithography is used in conjunction with mask technology. Due to diffraction effects, a limit can be expected in optical lithography at approx. 150 nm. For novel applications, such as one-electron transistors or molecular electronic components, structures with even smaller dimensions are required. In the case of very high integration densities, this also applies to conventional electronics.

X-ray lithography offers the possibility of producing such small structures, with which - due to the shorter wavelength - dimensions below 100 nm can be imaged. However, this results in problems with the required masks and with the positioning. This is not the case with electron and ion beam lithography. Since these are direct-writing methods, they do not require masks. In electron and ion beam lithography, structures down to 10 nm can be produced with high-energy particles. However, complex vacuum systems and beam guidance systems are required for this. In addition, there are problems due to electron scattering, which in some cases lead to radiation damage in the substrate. A new type of structuring is offered by scanning near-field techniques, in particular scanning tunneling microscopy (STM = Scanning Tunneling Microscopy) and scanning force microscopy (SFM = Scanning Force Microscopy). With all scanning near-field techniques, a fine, pointed probe is moved at a constant distance over the sample surface and the topography is scanned in this way. Interactions between the sample surface and the probe tip serve to regulate the distance. In addition to pure imaging, the sample surface can also be modified with an appropriate near-field probe using mechanical, electronic, chemical and optical effects. No focusing is required, because in the immediate vicinity of the probe only the shape and size of the probe tip are important for the dimensions of the structures produced.

In atomic force microscopy, a probe that has a tip with a radius of curvature of 10 to 100 nm is passed over the sample surface. This tip is located at the end of a rectangular or triangular cantilever that is a few hundred micrometers long and approx. 0.5 to 2 μm thick. Repulsive or attractive forces between specimen and tip bend the cantilevers accordingly. The bending of the cantilever is measured using optical or other methods and allows the shape of the sample surface to be recorded on a nanometer scale.

For the structuring of surfaces, the probe tip is used as a source for highly localized, low-energy electrons in order to expose resist materials - similar to electron beam lithography. Beam guidance and focusing systems are not required due to the near field. In contrast to electron beam lithography, work can be carried out under ambient conditions, so that complex vacuum systems are not required. Besides, there are none Radiation damage is to be expected because the electron energy is too low for this.

For exposure, the electron-sensitive lacquer is applied to a conductive substrate and an electrical voltage is applied between the substrate and a conductive force microscope probe. Electrons then flow between the probe tip and the substrate and cause chemical reactions. This creates a latent image, which is transformed into real structures in a subsequent development step.

The thin insulating resist layer (dielectric) is located between the conductive force microscope probe and the conductive substrate. This arrangement thus represents an electrical capacitor which is charged by the application of an electrical voltage, in the present case the exposure voltage. The charge leads to an electrostatic attraction between the force microscope probe and the substrate, which significantly increases the contact force of the tip on the lacquer. Without applied voltage, the contact force is determined by the much smaller forces that result from the elastic bending of the cantilever. The surface hardness of the lacquer is high enough to prevent the tip from penetrating due to the elastic forces. However, the much higher electrostatic forces cause the tip to penetrate the entire paint layer.

No mechanical damage to resist materials caused by electrostatic forces or necessary elimination measures has been reported to date. There are two main reasons for this:

- Up to now, exposure experiments using the atomic force microscope as a resist material have generally methyl ethacrylate (PMMA) used. Recently, however, so-called CARL coatings (CARL = Chemical Amplification of Resist Lines) have been used for the following reasons. In contrast to CARL varnishes, PMMA has a significantly higher surface hardness, but serious disadvantages of PMMA compared to CARL varnishes are the significantly lower sensitivity and the lack of the possibility of chemical reinforcement after development. - So far, only relatively thin PMMA layers (20 to 25 nm) have been used. However, the exposure voltages required for such layer thicknesses are relatively small at around 20 V. Since the electrostatic forces are quadratically dependent on the voltage applied, no mechanical damage to hard paints can be expected for such low voltages.

In other applications of force microscopy, for example in the measurement of potential distributions on surfaces, in the electrical characterization of thin dielectric layers (approx. 2 to 50 nm) by local tunnel current measurements and in local capacitance measurements, the application of an electrical voltage to the conductive is SFM probe required. Here, the electrostatic forces also have a negative effect on the sensitivity and the resolution.

The object of the invention is to design a force microscope probe of the type mentioned at the outset, which has a spring bar and a probe tip, in such a way that the electrostatic forces which occur during operation are largely eliminated. In particular, this is intended to prevent the resist layer from being mechanically damaged during the exposure by the movement of the probe tip when structuring resist layers by means of scanning force microscopy. This is achieved according to the invention in that the spring bar is provided with a shielding electrode and that an electrically insulating layer is arranged between the shielding electrode and the spring bar.

A force microscopy sensor of the usual type consists of a cantilever with an integrated tip at the end. For this type of scanning probe lithography, both elements, the spring bar and the tip, are usually conductive and thus generate electrostatic forces. The force contributions from the cantilever and tip are to be estimated below.

The length and width of the cantilever are relatively large compared to the working distance between the cantilever and the conductive sample surface, so that this arrangement can be regarded as a plate capacitor to a good approximation. The following applies to the electrostatic attraction force F _e ι _st at between two capacitor plates with the area A, which are parallel to one another at a distance d and to which a voltage U is applied.

F _e ι _ε ta _C = 1/2 ε ₀ ε A (u / d) ² .

Here εo is the constant of influence and ε is the dielectric constant of the surrounding material, which here is essentially air; ε = 1 is therefore a very good approximation. For a rectangular cantilever with a width of 50 µm and a length of 450 µm, the area is A = 0.0225 mm ² . The distance d is equal to the sum of the height of the tip (approx. Approx.

12 µm) and the layer thickness of the resist material (approx. 50 nm). Typically, the cantilever is slightly inclined (= 15 °) towards the tip, so that the average distance is slightly larger than Is 12 μm and thus the forces generated are slightly less than the force resulting from the specified values

Fβiacat = 0, 7-U ² [nN / V ² ].

For the electrostatic attraction caused by the tip, a simple estimate due to the cone geometry is not possible. The decomposition of the cone into small concentric circles, which can be interpreted as plate capacitors with a constant distance from the sample surface, allows a rough estimate of the forces. The integration over the entire cone with radius R and half the opening angle α then results in:

Fβiεtat = π εo ε U ² tan ² (α) - [ln (l + R / s-tan (α)) -R / s-tan (α) + R].

S is the distance of the cone tip from the sample surface, i.e. the resist thickness of = 50 nm. With the tips used, the opening angle is approx. 35 ° and thus the radius R approx. 4 μm. This results in the electrostatic attraction caused by the tip

Feistat = 0.004-U ² [nN / V ² ].

This rough estimate shows that the attractive forces caused by the tip are significantly (= factor 100) lower than those of the cantilever. The power contribution of the tip is not eliminated by the measures according to the invention. The effectiveness of these measures is only guaranteed if the force contribution of the spring beam is reduced by at least two orders of magnitude. However, this is achieved by the shielding electrode. The choice of material for the shielding electrode is relatively uncritical. Practically all metals are considered that are adhesive and can be deposited. These are, for example, chrome, aluminum, titanium, nickel and gold.

The principle of the shielding electrode integrated in the force microscope probe can be implemented in various ways. It is essential that the working electrode, which brings about the current supply to the tip, is electrically separated from the shielding electrode. Either the silicon of the cantilever and tip itself or a metal layer can be used as the working electrode, especially if the cantilever and tip are made of silicon nitride.

Metals that do not form an insulating oxide layer are used for the working electrode. These are in particular precious metals such as gold, platinum and palladium. It is also advantageous to use an amorphous carbon-containing layer (great hardness and sufficient electrical conductivity), as is proposed in German published patent application DE 195 26 775 AI as a coating for atomic force microscopy probes and scanning tunneling microscopy tips.

The electrically insulating layer, which advantageously consists of silicon dioxide or of materials such as polyimide, polybenzoxazole, benzocyclobutene and organic spin-on glasses, advantageously has a layer thickness of> 50 nm, preferably> 500 nm given sufficient dielectric strength. If a layer of one of the materials mentioned is used for electrical insulation, there is a metal layer between this layer and the spring bar as the working electrode. Preferred embodiments of the force microscope probe according to the invention are described in more detail below.

If silicon (Si) serves as the working electrode, the insulation layer is produced by oxidation of the silicon. A thin metal layer, for example made of chrome, is then deposited on this oxide layer as a shielding electrode. In a subsequent step, the Si tip is freed of metal and oxide.

If the working electrode is made of metal, then this metallic working electrode is first applied to the probe. The cantilever and tip can either be coated over the entire surface or a narrow stripline is structured. The stripline has the advantage that stray fields are also shielded by the significantly larger shielding electrode. The metallic working electrode is then provided with an insulation layer, for example made of polyimide or polybenzoxazole. The metallic shielding electrode is then applied and the probe tip is then exposed.

In both cases, the shielding electrode - with the appropriate geometry - completely eliminates the force contribution of the cantilever. Only the much smaller portion of the tip remains.

The above-described compensation or elimination method for the electrostatic forces can be used - in addition to lithography - in other force microscopy applications. This applies to all applications in which a voltage is applied to the SFM probe, but no electrostatic forces may occur.

Claims

claims

1. Force microscope probe with an electrically conductive cantilever and an electrically conductive probe tip, so that the cantilever is provided with a shielding electrode, and that an electrically insulating layer is arranged between the shielding electrode and the cantilever.

2. Force microscope probe according to claim 1, that the shielding electrode consists of an adhesive, separable metal.

3. force microscope probe according to claim 1 or 2, d a - d u r c h g e k e n n z e i c h n e t that the electrically insulating layer has a thickness> 50 nm.

4. force microscopy probe according to one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t that the electrically insulating layer consists of silicon dioxide.

5. force microscope according to one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t that the electrically insulating layer consists of polyimide or polybenzoxazole, and that there is a metal layer between the polyimide or polybenz oxazole layer and the cantilever as a working electrode.

6. force microscope probe according to claim 5, d a d u r c h g e k e n n z e i c h n e t that the working electrode is a stripline.