DE102017119551A1 - normal - Google Patents

Abstract

The invention relates to a normal (10) in the form of a microelectromechanical system comprising (a) a reference surface element (12) having (i) a planar reference surface (O) and (ii) a recess (18), (b) a shaft (14), (c) at least one spring (16), (d) wherein the shaft (14) (i) is movable transversely to the reference surface (O), (ii) a flat end face (20 ) and (iii) can be brought to a zero position, in which the end face (20) extends along the reference surface (O), (iv) wherein the at least one spring (16) of a deflection (.DELTA.d) of the shaft (14) counteracts from the zero position, and (e) a deflection detecting device (28) for detecting the deflection (Δd).

Description

The invention relates to a normal. A normal is a metrological comparison object used to calibrate other gauges. The calibration of measuring instruments for particularly small or very large sizes is particularly challenging. For example, the calibration of microscopes, such as optical microscopes or atomic force microscopes, often with high uncertainty afflicted, since very small forces or distances must be measured. This leads to comparatively large measurement uncertainties.

The invention has for its object to improve the calibration of microscopes.

The invention solves the problem by a normal in the form of a micro-electro-mechanical system with (a) a reference surface element having a flat reference surface and a recess, (b) a shank, (c) at least one spring, d) the shank is movable transversely to the reference surface, has a flat end face and can be brought into a zero position, in which the end face extends along the reference surface, wherein the at least one spring deflects the shank out of the zero position. Counteracting position, and (e) a deflection detecting device for detecting the deflection.

An advantage of this standard is that it can be used for the calibration of atomic force microscopes. For this purpose, with a cantilever of the atomic force microscope, a force can be exerted on the end face of the shaft, which thereby shifts transversely to the reference surface against the force of the spring. The displacement from the zero position can be detected with the displacement sensing device and the force determined based thereon. In other words, the normal is a force-transfer standard that can transfer forces down to the pN range. The normal can therefore be referred to in this case as pN-Krafttransferormal. Alternatively, the normal may also be an embodiment of a route. In this case, the end face, which is plane according to a preferred embodiment, is moved by a predetermined desired path and traced this movement with a microscope.

In the context of the present description, the feature that the shank is movable transversely to the reference surface, in particular, means that when a force that is perpendicular to the reference surface acts on the end face, the shank moves in a direction of movement moves, wherein the direction of movement with the reference surface forms an angle of 90 ° ± ε, where ε is as small as possible, for example, at most 5 °.

By the feature that the end face extends along the reference surface, it is understood in particular that the reference surface can be described in good approximation as a plane.

The end face is also approximated by a plane in the mathematical sense, wherein a deviation is also at most 1 micron. The feature that the end face extends along the reference surface is understood in particular to mean that an angle between the two planes is at most 15 °, in particular 3 °. A minimum possible distance between the two planes is for example at most 1 μm, in particular at most 100 nm.

The recess preferably has an opening gap of at least 2 μm and / or at most 500 μm. In this way it is ensured that on the one hand a cantilever can reach sufficiently deep through the recess and on the other hand, the reference surface and the end face offset only a small angle to each other. Under the inner diameter is understood in particular the inner circle diameter, ie the diameter of the imaginary cylinder of maximum diameter, which fits through the recess.

Preferably, the reference surface element and the shaft are electrically connected to each other, that is, they always have the same electrostatic potential. This prevents electrostatic force between the reference surface element and the cantilever of an atomic force microscope to be calibrated.

Preferably, the reference surface element and the shaft are made of the same material. Then the peak-to-surface interactions of a cantilever on both objects are the same, thus avoiding systematic measurement errors. For example, the speaker surface element and the shaft are made of silicon.

The deflection detection device is understood in particular to mean a structure by means of which a deflection, that is to say a change in the position of the shaft relative to an initial position, in particular to the zero position, can be determined. In particular, the Auslenkungserfassungsausrichtung is designed so that by detecting a measured variable in a unique way, the deflection is determined. This measure may be one way, but it is not necessary. In particular, it is possible that the deflection is based on an electrical and / or optical measuring device can be detected.

A standard always comprises an associated calibration certificate in which the relevant variables, in the present case the spring constant / or the dependence of the deflection on a measured variable which can be tapped by the deflection detection device, are indicated.

The feature that the normal is embodied in the form of a microelectromechanical system is understood in particular to mean that the reference surface element, the spring and the shaft, and optionally a drive for the shaft, consist of a substrate, in particular a silicon monocrystal , worked out, and in particular etched out.

According to a preferred embodiment, the normal has a drive, which is connected to the shaft and by means of which the shaft is movable transversely to the reference surface. When the normal, as provided in its preferred embodiment, is a Pikonewton force transfer standard, the stem may also be referred to as a force transducer. Under a Pikonewton force transfer standard is a force transfer standard, with which forces in the range of Pikonewton can be calibrated, in particular with a measurement uncertainty of at most 10 ^-4 .

Particularly preferably, the drive is an electrostatic comb drive and has a first comb electrode and a second comb electrode. Such comb drives are simplified as a micro-electro-mechanical system (MEMS) produced and also allow a capacitive measurement of the deflection.

It is favorable if the normal has an evaluation unit that is designed with the comb electrodes for determining a capacitance of the comb electrodes relative to one another. The comb electrodes are insulated from each other and form a capacitor whose capacitance depends on the distance between the two comb electrodes. By measuring the capacitance of the two comb electrodes relative to each other, therefore, the distance of the two comb electrodes from each other and thus the deflection can be determined.

Preferably, the shaft has a head in which the end face is formed and which has a frusto-conical lateral surface, wherein the frusto-conical lateral surface has a head cone angle. The recess is also preferably frusto-conical and widens away from the reference surface and has a recess taper angle corresponding to the head taper angle. By the feature that the two cone angles correspond to one another, it is understood in particular that although it is possible, but not necessary, that the two cone angles are the same mathematically. Rather, it is possible that both deviate from each other so far that the function of the normal is not impaired. For example, a difference between the two cone angles is a maximum of 5 °.

It is favorable if the deflection detection device comprises an interferometer, in particular a Fabry-Perot interferometer, which has a first mirror, which is connected to the shaft, a second mirror and a light source. Preferably, the first mirror is formed on the shaft as a reflective surface. In other words, a surface of the shaft is made so smooth that it can act as a mirror.

Preferably, the second mirror is formed as a reflective surface of an optical fiber, wherein the optical fiber acts as a light source. In this case, the two mirrors form a Fabry-Perot interferometer, the interference pattern is coupled into the optical fiber and evaluated, for example, by the evaluation unit.

gilt. Dabei ist M eine natürliche Zahl und es gilt $\left|\epsilon \right|<\frac{1}{10}\left(M-\frac{1}{2}\right)\frac{\lambda }{4}.$

Preferably, the light source is configured to emit light of a wavelength, wherein for a zero-force distance between a first compensation plane through the reference surface and a second compensation plane through the face in the absence of an external force on the shaft $G=\left(M-\frac{1}{2}\right)\frac{\lambda }{4}+\epsilon$

applies. Where M is a natural number and it applies $\left|\epsilon \right|<\frac{1}{10}\left(M-\frac{1}{2}\right)\frac{\lambda }{4},$

The light source preferably comprises a laser which emits light of this wavelength during operation. Preferably, it is monochromatic light.

Preferably, the at least one spring has a spring constant with respect to a deflection transversely to the reference surface of at most 1000 Newton per meter. Preferably, the spring constant is greater than 0.01 Newton per meter.

According to a preferred embodiment, the evaluation unit is set up to carry out a method automatically with the steps: (i) detecting a deflection of the shaft, which is caused by a force acting on the shaft, and (ii) calculating the force from the deflection. The deflection is for example based on a spring constant or on the basis of a map in which the relationship between the deflection and the Cantilever force is stored, calculated. The spring constant and / or the map are determined in preliminary tests. For example, a parent normal is used for this.

Alternatively, the evaluation unit is configured to automatically perform a method comprising the steps of: (i) controlling a voltage that is applied between the comb electrodes so that the deflection caused by a force acting on the end face is compensated, (ii) detecting the Compensation voltage required for compensation and (iii) Compute the applied force from the compensation voltage. For this purpose, a data record is preferably stored in the evaluation unit which codes the dependence of the force on the compensation voltage.

According to the invention, there is also a method for calibrating an atomic force microscope comprising the steps of: (i) applying a desired force with a cantilever of the atomic force microscope to an end face of a shaft of a pN transfer standard according to the invention, (ii) measuring an actual force by means of the pN force transfer standard and (iii) calibrating the atomic force microscope using the actual force.

An inventive method is also one in which an optical microscope is calibrated and the steps. (i) determining a first actual position and a second actual position of the end face of a normal according to the invention by means of the normal; (ii) determining the associated first measuring position and the second measuring position of the end face by means of the microscope and calibrating the microscope on the basis of the actual position; includes.

• 1 eine schematische Zeichnung eines erfindungsgemäßen Normals und
• 2 eine Zeichnung eines erfindungsgemäßen Normals.

In the following the invention will be explained in more detail with reference to the accompanying drawings. It shows

• 1 a schematic drawing of a Normals invention and
• 2 a drawing of a standard invention.

1 shows a standard according to the invention, which is formed in the present case as pN Krafttransfernormal and a reference surface element 12 with a reference surface O a shaft 14 and two springs 16.1 . 16.2 , In the reference UI element 12 is a recess 18 introduced, which has an inner diameter w , For example, w = 6 microns, has.

The feathers 16.i (i = 1, ..., N, where N is the number of springs) hold the shaft 14 so that he moves in one direction R led by the springs 16.i is stored. The direction of movement R runs transversely to the reference surface O , In the present case, the angle between the reference surface O and the direction of movement R 90 °.

The shaft 14 has a flat face 20 , which are transverse to the direction of movement R and thus parallel to the reference surface O runs. Under "parallel" is understood in this case that while it is possible, but not necessary, that the reference surface O and face 20 in a strictly mathematical sense parallel to each other. A small deviation of, for example, a maximum of 5 ° is possible.

The shaft 14 has a head 22 , which has a frustoconical surface 24 Has. The surface forms with the direction of movement R , which simultaneously has a longitudinal axis L of the shaft 14 represents a head cone angle α ₂₂ , The recess 18 is also frustoconically bounded and has a conical surface 26 that have a recess cone angle α ₁₈ with the reference surface O forms. The head cone angle α ₂₂ corresponds to the recess cone angle α ₁₈ , In particular, both deviate from each other by at most 5 °.

The normal 10 has a displacement detection device 28 , in the present case a comb drive 30 includes. The electrostatic comb drive 30 has a first comb electrode 32 and a second comb electrode 34 , Which are arranged to be movable relative to each other and electrically isolated from each other, so by applying the comb electrodes 32 . 34 an electrostatic attraction or repulsion force between the two comb electrodes 32 . 34 can be generated. The comb electrodes 32 . 34 are with a schematically drawn evaluation 36 connected by means of a capacity C the two comb electrodes 32 . 34 is measurable to each other. From this capacity can be clearly a distance d between the comb electrodes 32 . 34 be calculated.

Alternatively or additionally, the deflection detection device 28 an interferometer 38 have, which is formed in the present case as a Fabry-Perot interferometer. This has a first mirror 40 passing through a mirrored side surface of the shaft 14 is formed. The interferometer 38 also has a second mirror 42 passing through a mirrored surface of an optical fiber 44 is formed. The optical fiber 44 represents a light source and is with a laser 46 connected. This laser gives light, preferably monochromatic light, of a wavelength λ at which wavelength the vacuum wavelength is meant. Depending on the distance ₃₈ there is constructive or destructive interference of a standing wave between the mirrors 40 . 42 , The corresponding interference pattern is in the optical fiber 44 coupled and from a sensor 48 recorded with the evaluation unit 36 connected is.

If the normal 10 having an interferometer, it is convenient if the comb drive 30 merely as a drive for moving the shaft 14 is used, but not for detecting a position of the shaft 14 because the measurement accuracy of the interferometer 38 usually much larger.

gilt, wobei M eine natürliche Zahl (M = {1, 2, ...}) ist. In der Regel ist M kleiner als 10.The face 20 has from the reference surface O Zero-force distance G from the evaluation unit from the distance d is calculated. For this purpose, a correction value to the distance d added or subtracted. The zero force distance is preferably chosen so that $G=\left(M-\frac{1}{2}\right)\frac{\lambda }{4}+\epsilon$

where M is a natural number (M = {1, 2, ...}). As a rule, M is less than 10.

A spring constant k for all feathers 16.i together, for example, k = 0.05 N / m. Between one on the face 20 applied force F ₂₀ and a change .DELTA.d of the distance d Thus, the relationship F ₂₀ = kΔd.

2 shows a scale cutout of a standard according to the invention 10 , It can be seen that the normal 10 four spring 16.1 . 16.2 . 16.3 . 16.4 All of which are meander-shaped, to a particularly small spring constant k to reach. The shaft 14 has a cell structure 50 on to achieve the lowest possible mass. In the 2 shown components of the standard 10 are etched lithographically from a silicon single crystal. The normal 10 is therefore a micro-electro-mechanical system (MEMS).

For carrying out a method according to the invention becomes the normal 10 introduced into an atomic force microscope. With a cantilever 52 ( 1 ) may be the reference surface first O be scanned. The reference surface O can thus simultaneously serve as a surface normal. To calibrate the force measurement of the atomic force microscope is the cantilever 52 positioned so that its tips on the face 20 pressed and so the force F ₂₀ is exercised.

The deflection constitution direction 28 , in the present case in the form of the Fabry-Perot interferometer 38 constantly measures the distance d , The evaluation unit 36 is designed to be a tension U between the two comb electrodes 32 . 34 applies, the voltage U so chosen is that the resulting electrostatic force F _el the power F ₂₀ equivalent. In other words, the tension becomes U so regulated that the shift .DELTA.d of the shaft 14 is zero.

From the applied voltage U then calculates the evaluation unit 36 the power F ₂₀ , Alternatively, no voltage U or a constant voltage U applied and the deflection .DELTA.d , in particular by means of the interferometer 38 , measured. From the deflection and the spring constant k then becomes the force F ₂₀ calculated.

Alternatively, the normal 10 placed in an optical microscope and on the end face 20 focused. Thereafter, by means of the comb drive 30 a shift .DELTA.d set and by means of the deflection detection device 28 , in particular the interferometer 38 , measured. The corresponding measured value recorded by the optical microscope is compared with that of the normal 10 measured value and calibrated in this way, the optical microscope.

LIST OF REFERENCE NUMBERS

10

normal

12

Reference surface element

14

shaft

16

feather

18

recess

20

face

22

head

24

lateral surface

26

conical surface

28

Auslenkungserfassungsvorrichtung

30

Drive, comb drive

32

first comb electrode

34

second comb electrode

36

evaluation

38

interferometer

40

first mirror

42

second mirror

44

Light source, optical fiber

46

laser

48

sensor

50

cell structure

52

cantilever

C

capacity

d

distance

F ₂₀

force

G

Zero-force distance

i

running Index

k

spring constant

L

longitudinal axis

M

natural number

N

Number of feathers

O

Reference surface

R

movement direction

w

Inner diameter

α ₁₈

Recessing cone angle

α ₂₂

Head Cone angle

λ

wavelength

.DELTA.d

deflection

U

tension

F _el

electrostatic force

Claims (11)

Normal (10) in the form of a micro-electro-mechanical system with (a) a reference surface element (12), the (i) a flat reference surface (O) and (ii) has a recess (18), (b) a shaft (14), (c) at least one spring (16), (d) wherein the shaft (14) (i) is movable transversely to the reference surface (O), (ii) has a flat end face (20) and (iii) can be brought to a zero position, in which the end face (20) extends along the reference surface (O), (iv) wherein the at least one spring (16) counteracts a deflection (Δd) of the shaft (14) from the zero position, and (e) a deflection detecting device (28) for detecting the deflection (Δd). Normal (10) to Claim 1 characterized by a drive (30) connected to the shaft (14) and by means of which the shaft (14) is movable transversely to the reference surface (O). - The drive is an electrostatic comb drive (30) and a first comb electrode (32) and a second comb electrode (34). The normal (10) according to any one of the preceding claims, characterized in that (a) the shaft (14) has a head (22) on which the end face (20) is formed and which has a frusto-conical peripheral surface (24) - wherein the frusto-conical lateral surface (24) has a head taper angle (α ₂₂ ), and that (b) the recess (18) is frusto-conical, - widens away from the reference surface (O) and - has a recess Cone angle (α ₁₈ ), which corresponds to the head cone angle (α ₂₂ ). Normal (10) according to one of the preceding claims, characterized in that the head cone angle (α ₂₂ ) is 30 ° ± 5 °. The normal (10) according to one of the preceding claims, characterized in that the deflection detection device (28) comprises an interferometer (38), in particular a Fabry-Perot interferometer, comprising - a first mirror (40) connected to the shaft (14 ), - a second mirror (42) and - a light source (44). Normal (10) to Claim 5 characterized in that (a) said light source (44) is adapted to emit light of a wavelength (λ) and (b) for a zero force distance (g) between a first level of balance by said reference surface (O) and a second level of balance by the end face (20) in the absence of an external force (F ₂₀ ) on the shaft (14) $G=\left(M-\frac{1}{2}\right)\frac{\lambda }{4}+\epsilon$

where M is a natural number and $\left|\epsilon \right|<\frac{1}{10}\left(M-\frac{1}{2}\right)\frac{\lambda }{4}$

applies. Normal (10) according to one of the preceding claims, characterized in that the at least one spring (16) has a spring constant (k) with respect to a deflection (Δd) transverse to the reference surface (O) of at most 1 millinewtons per micrometer. A normal (10) according to any one of the preceding claims, characterized by an evaluation unit (36) arranged to automatically perform a method comprising the steps of: (i) detecting a deflection (Δd) of the shaft (14) from one to the other Shaft (14) acting force (F ₂₀ ) is conditional, and (ii) calculating the force (F ₂₀ ) from the deflection (.DELTA.d). Normal (10) after one of Claims 1 to 7 characterized by an evaluation unit (36) arranged to automatically perform a method comprising the steps of: (i) controlling a voltage (U) applied between the comb electrodes (32, 34) such that the deflection (Δd), which is caused by a force acting on the end face (20) (F ₂₀ ) is compensated, (ii) detecting the compensation voltage necessary for compensation, and (iii) calculating the applied force (F ₂₀ ) from the compensation voltage. Method for calibrating an atomic force microscope, comprising the steps of: (i) applying a desired force with a cantilever (52) of the atomic force microscope to an end face (20) of a shaft (14) of a standard (10) according to one of Claims 1 to 7 , (ii) measuring an actual force with the normal (10) in the form of a Pikonewton force transfer standard and (iii) calibrating the atomic force microscope using the actual force. Method for calibrating an optical microscope, comprising the steps of: (i) determining a first actual position and a second actual position of the end face (20) of a standard (10) according to one of Claims 1 to 7 , (ii) determining the first measuring position and the second measuring position of the end face (20) by means of the microscope and (iii) calibrating the microscope on the basis of the actual positions.

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