WO1996012935A1

WO1996012935A1 - Expansion gauge for measuring the expansion of single-crystalline semiconductor material

Info

Publication number: WO1996012935A1
Application number: PCT/DE1995/001446
Authority: WO
Inventors: Jakob Schelten; Reiner Jumpertz; Olaf Ohlsson; Frank Saurenbach
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 1994-10-19
Filing date: 1995-10-14
Publication date: 1996-05-02
Also published as: DE4437306C2; DE4437306A1

Abstract

The invention relates to an expansion gauge for measuring the expansion of single-crystalline semiconductor material. A highly sensitive signal generator, i.e. with a high signal component and low interference component, is to be applied to the single-crystalline semiconductor material to indicate its expansion. The problem is solved by the use of a MOSFET as the signal generator. The source-drain current is kept constant by means of a constant current source (as a means therefor) and the variations in the source-drain voltage are measured. The change in the source-drain voltage is a measure of the expansion.

Description

Strain gauge for measuring the strain of single-crystalline semiconductor material

The invention relates to a strain gauge for measuring the strain of single-crystal semiconductor material, which is used as a semiconductor strain gauge. A MOSFET is attached to the semiconductor material as a signal generator for displaying the strain. The electrical signals of the MOSFET serve as a measure of the elongation of the semiconductor material (1). The invention further relates to a method for strain measurement.

Such a strain gauge is known from DE-OS 2 231 977. The changing source-drain current in the MOSFET channel is measured with constant source-drain voltage. The measured variable is proportional to the strain and can thus be used as a measure of strain (J. Phys. D: Appl. Phys., Vol. 12, page 1973, 1979).

The invention is used in particular to measure the elongation of a lever arm made of semiconductor material and clamped on one side, which is to be used as a scanning probe for a scanning probe microscope. For scanning probe microscopes lever arms clamped on one side are known as scanning probes made of single-crystalline semiconductor material. Fine probe tips are attached to the lever arms clamped on one side on the scanning probe microscope at the free, non-clamped end of the lever arm, scan the material surfaces and thereby detect their topography up to atomic resolution. Here, physical interactions between the material surface and the probe tip, which is moved relative to the material surface, are used. The interaction can be long-range and lead to van der Waals forces, capillary forces, magnetic or electromagnetic forces. It can also be short-range and result in physical or chemical absorption forces, in elastic or plastic deformation forces. Every interaction at the probe tip leads to a deflection of the lever arm with respect to its lever arm end clamped on one side on the scanning probe microscope and to an elastic one

Bending of the lever arm. The amount of deflection is proportional to the force acting on the probe tip and is dependent on location when the probe tip moves over the material surface, so that the locally specific topography structure of the material surface can be deduced from the lever arm deflection. The amounts of the deflection are generally evaluated digitally and converted into data images that make it possible lent to display the topography clearly, for example on screens.

To measure the deflection of the lever arm, it is also known, in addition to electron tunneling, optical interferometry, light beam deflection or the measurement of changes in capacitance, to install a signal transmitter on the lever arm itself which supplies electrical signals proportional to the deflection (M. Tortonese et al, "Atomic resolution with an ato ic microscope using piezoresistive detection", Appl. Phys. Lett. 62 (1993) pp. 834 ff). In this last-mentioned signal transmitter, an electrical conductor track is doped on a lever arm made of single-crystalline silicon as semiconductor material, which changes its resistance if, when the lever arm is deflected, material strains are caused in the semiconductor material due to tensile and compressive stresses. For many applications, in particular with deflections of the probe tip in the order of 0.1 nm, signal transmitters with a small drift and fewer interference signals are required.

The object of the invention is to provide a signal transmitter with high sensitivity, ie high signal component with a small interference component, on single-crystal semiconductor material for indicating the elongation of the semiconductor material. The object of the invention is also to provide a method for strain measurement with these advantages. This object is achieved in a strain gauge of the type mentioned at the outset by the features specified in patent claim 1. Thereafter, a MOSFET is used on the semiconductor material as the signal transmitter, the electrical signals of which are measured as a measure of the elongation of the semiconductor material. It is e.g. B. by means of a constant current source (as a means for generating a constant source-drain current) the source-drain current is kept constant and the changing source-drain voltage measured. The change in the source-drain voltage is a measure of the extension or deflection of the lever arm in the atomic force microscope.

The strain measurement with constant source-drain current has the advantage over the strain measurement with constant source-drain voltage (= change in the source-drain current) that the signal is amplified. The amplification factor is dependent on the ratio R _j / R _D (R _i : differential resistance δU _D / δl _D ; R _D : nominal resistance U _D / I _D ) and can be adjusted if the resistors R and R _{D are} selected accordingly using the voltage at the gate easily lead to a hundredfold amplification. Since external interference between the sensor and evaluation electronics such. B. occur by thermal voltages, microphones, influenza or induction, not be amplified with, this type of amplification has great advantages over the known prior art. The Signal component is significantly increased compared to the external disturbance component.

In order to eliminate thermal drift, it is expedient to integrate at least one MOSFET pair on the semiconductor material which has two MOSFETs oriented at an angle of 90 ° to one another. One of the MOSFETs of the MOSFET pair is preferably arranged parallel to the main component of the mechanical expansion of the semiconductor material. For example, when the

Semiconductor material as a lever arm of an atomic force microscope due to a force interaction on the probe tip of the lever arm of one of the MOSFETs arranged parallel to the main tensile stress generated in the process, the other MOSFET is oriented perpendicular to it.

Single-crystal silicon is preferably suitable as the semiconductor material, but also single-crystal germanium or a III / V semiconductor. The MOSFETs can be applied to n- or p-doped semiconductor material.

For the manufacture of the lever arms and the integration of the MOSFET on the lever arms, it is advantageous if the MOSFETs are attached to the side of the lever arm on which the probe tip is also located on the lever arm.

Analogous to MOSFETs on lever arms for scanning probe microscopes consisting of semiconductor material, MOSFETs are Generally suitable as sensors for strain measurement with the advantages mentioned above, and everything that has been discussed for lever arms also applies to semiconductor strain gauges.

The object is further achieved by measuring the elongation of single-crystalline semiconductor material by means of a MOSFET (8 to 13) attached to the semiconductor material. Its source-drain voltage serves as a measure of the elongation of the semiconductor material (1). The source-drain current is kept constant during this time.

The device is used to carry out the method.

The invention and further refinements of the invention are explained in more detail below on the basis of exemplary embodiments. The drawing shows schematically in detail:

Figure 1 MOSFETs with p-channel on n-doped

Semiconductor material; FIG. 2 shows two MOSFETs arranged at an angle of 90 ° to one another on semiconductor material (shown enlarged)

Section II of Fig. 3); Figure 3 probe holder for a

Atomic force microscope with lever arm clamped on one side Semiconductor material and applied MOSFETs according to FIG. 2.

FIG. 1 shows a schematic representation on n-doped semiconductor material 1 of an applied MOSFET with MOSFET channel 2, in the exemplary embodiment a p-channel. In contrast to the exemplary embodiment, if the semiconductor material is p-doped, a MOSFET with an n-channel is to be used. The effects of MOSFETs with p-channel or n-channel are analogous with regard to their use as signal transmitters for strains in the semiconductor material. In the following, therefore, all exemplary embodiments are based on MOSFETs according to FIG. 1, that is to say p-channel MOSFETs on n-doped semiconductor material.

The MOSFET (metal oxide semiconductor field effect transistor) is constructed as usual: a metal layer 3 is applied to the semiconductor material 1 and is interrupted in the gate area. A gate oxide layer 5 is located under a gate electrode 4, and highly p-doped semiconductor layers 6, 7 are provided on both sides of the MOSTET channel 2, the semiconductor layer 6 being arranged at the source and the semiconductor layer 7 at the drain. The MOSFET channel has one

Gate length 1 and a gate width w. The gate voltage U is adjustable. 12935 PC17

MOSFETs behave piezoresistive, G. Dorda, J. Appl. Physics 4-2, 2053 (1994), and from a publication by C. Canadi et al. , J. Phys. D_ _: Appl. Phys. 12, 1973 (1979), it follows that with an expansion ε (δl / l) of the semiconductor material over the gate length 1 with a constant source-drain voltage Up, a change in the source-drain current IJJ (δlrj / Irj) is measurable. The source-drain current Irj shows strong, anisotropic, piezoresistive effects. The amount of the current change ÖIΓJ / ID can be treated analogously to a change in resistance of an electrical resistance when stretching,

: i) δI _D / ι _D = kj ^• ε,

where the value k_ as a dimensionless gain factor for the elongation ε - comparable in magnitude to the dimensionless quantity k _R with an electrical resistance change - is in the range between ± 100. The dependencies of k _j on the crystal direction in the semiconductor material during charge transport and on the direction of expansion relative to the crystal direction and on the charge carrier type (p-, n-doped) in the MOSFET channel correspond to the dependencies of k ^ on the same parameters.

Compared to the strain measurements with electrical resistors, it is advantageous that when the gate voltage Ug changes, the density of the charge carriers in the MOSFET channel can be influenced. With regard to its measuring sensitivity and its freedom from interference with respect to thermal drift, the MOSFET can thus be set via U _Q to an optimum operating point for the required measurement.

In order to achieve a high sensitivity for an expansion ε of the semiconductor material for a MOSFET used, it is expedient to coordinate the arrangement of the MOSFET with the given crystal directions of the semiconductor material and the main direction of expansion. If the semiconductor level of the single-crystal semiconductor material in the boundary layer between semiconductor material 1 and metal layer 3 is, for example, a (001) crystal surface (for example in the case of single-crystal silicon), the MOSFET channel is advantageously placed in a (110) direction and the main direction of expansion in the semiconductor material parallel or perpendicular to the MOSFET channel in order to obtain the largest possible value for k. For the MOSFET shown in FIG. 1 with semiconductor material made of monocrystalline silicon and the orientation of the MOSFET channel in the (110) direction, the k values for expansions parallel to the MOSFET channel are ε ":

- Extensions perpendicular to the MOSFET channel - right ^: k _l7l = -85, the gate voltage Ug being chosen to be much larger than the threshold voltage. With other doping or other crystal surfaces of the semiconductor material, there are other excellent directions for the alignment of the MOSFET channel.

Compared to the strain measurement with a MOSFET by determining the change in the source-drain current I - _Q (δIrj / I _D ), there is a signal amplification if, with a constant Irj, the change in the source-drain voltage U _j (δUrj / U] }) is measured as a function of the strain ε,

(2) δU _D / U _D = kr _j ^• ε.

Because with R _ as a differential resistor (R_ = δUrj / δlrj) and Rp as a nominal resistor (R _D = U _D / I _D ) of the MOSFET channel, (1) results in the gain factor ky

(3) ky = kj. ^• Ri / R _D ,

If, for example, as with a lever arm of a scanning probe microscope, deflection of the lever arm is to be measured, the values to be determined can be falsified by thermal drift, the disturbance variable k-p of which at the working point is caused by

(4) k _τ = (δI _D / I _D ) / δT can be described. A current change due to thermal drift cannot be distinguished from a current change δl _D / I _D due to an expansion ε of the semiconductor material. Accordingly, is constant

Current feed a voltage change δUr3 / U _D due to expansion indistinguishable from a δUp / U _D = K _T - ⁽ R ₁ / R _D ) δT.

The temperature dependence of the source-drain current is generally somewhat less than the temperature dependence of semiconductor material resistors.

If a MOSFET pair with MOSFETs 8, 9 is integrated on the semiconductor material in the manner shown in FIG. 2 such that the MOSFET channel of the MOSFET 8 is oriented perpendicular to the MOSFET channel of the MOSFET 9, and one of the MOSFET Channels parallel to the tensile stress and the main direction of elongation of the semiconductor material, the other arranged perpendicular to it, so the thermal drift can be largely eliminated. For the difference δlj 8/9 between the two source-drain currents I _{D 8} , I _D g in the MOSFET channels, namely with uniform expansion ε and the same temperature difference ΔT = T - T _Q in the semiconductor material

(5a) δl _Dβ / I ° ₈ = [k _Iβ ^■ ε + k _TB (T - T ₀ )] (5b) δl _Dq / I ° ₉ = _kτ ^• ε + k _τ (T - T ₀ ) l If one thus ensures identical construction for both MOSFETs 8, 9 and ensures that k _I8 = -k _I9 , the result is:

(6) δl _Dg / l0 ₉ - δl _Dβ / l0 ₈ = _ε (| k _I9 | + | k _Iβ |)

In the case of a constant current feed with the same current through both MOSFETs, this leads to analogy to equation (3)

(7) δUn / U ° ₉ - δUn / U ° ₈ = ε {| k „I + lk _Iβ I) (Ri / R _D )

The condition of structurally identical MOSFETs is met if they are attached in such a way that their MOSFET channels are arranged in crystal directions of the semiconductor materials that are equivalent to the strain to be measured (for example in the case of single-crystal silicon in the (110) and (110) directions). The aforementioned K _j condition can be achieved due to the anisotropic behavior of k when the two MOSFETs 8, 9 are oriented at a 90 ° angle to one another.

In order to eliminate additive disturbance variables, a correlation method can also be used under the above-mentioned conditions, as is described in the unpublished patent application P ... Thereafter, for measuring strains due to compressive and tensile stresses acting on the semiconductor material, two MOSFETs are first used when the semiconductor material is not loaded the measured value deviations of both MOSFETs are determined when ambient parameters change, and a calibration function is determined, which then serves to correct the displayed measured values during the measurement carried out. In this way, changing additive disturbances from environmental influences are eliminated.

If only one MOSFET integrated on the semiconductor material is available as a signal generator, the above-described correlation method for eliminating disturbance variables can also be used in that the change in the source-drain current Δlrj / Irj for determining the calibration function in the case of unloaded semiconductor material in the case of a change exclusively of disturbance variables at two different gate voltages U, U JJ is evaluated. The inversion in the MOSFET channel and thus the value of the respectively available gain factor kj depends on the set gate voltage. The two gate voltages U _Q J, U _Q JJ are thus to be selected such that the third condition for the gain factors kj / j and k _j / _j required for the correlation method results, namely:

_{I τ} = - k ^ ü.

In this way, a calibration function can also be determined, which is then used to correct the measured value. Another advantage of using MOSFETs as strain gauges on semiconductor material is the possibility of superimposing the measurement signal Irj or UJJ of a MOSFET with a base frequency using a correspondingly frequency-modulated gate voltage U _Q and the modulated measurement signal in a known manner by means of To strengthen lock-in technology. The thermal drift is then eliminated in the amplifier chain used here.

An important area of application of MOSFETs on semiconductor material for strain measurement is scanning probe microscopy. The topography of surfaces can be determined with scanning probe microscopes, and steps and terraces on the surface can be measured up to atomic resolution. The interaction mechanisms between the scanning probe and the sample surface are also examined.

In atomic force microscopes, lever arms made from semiconductor material are used, the deflection of which is to be measured as a result of the action of surface forces during the scanning of the surface. The lever arms clamped on one side on the scanning force microscope are deflected by surface forces acting on their free lever arm end. The strains resulting from tensile and compressive stresses in the semiconductor material when bending the lever arm are measured. sen. The elongation values are a measure of the deflection of the lever arm.

FIG. 4 schematically shows a holder 14 which is fixedly attached to an atomic force microscope and has a lever arm 15 made of single-crystal silicon which is clamped on one side. A section II of the lever arm 15 is shown enlarged in Figure 2. On the lever arm 15, two MOSFETs 8, 9 are attached, one of which has a MOSFET with its MOSFET channel in the main direction of expansion when the lever arm is deflected, and the other MOSFET is used at an angle of 90 ° thereto.

The lever arm 15 has the usual dimensions for raster probes of this type: lever arm length L = 400 μm,

Width B = 40 μm, thickness S = 4 μm. The two MOSFETs 8, 9 and their conductor tracks 16 on the lever arm are correspondingly small, see FIG. 2. The MOSFET channel lengths 1 and channel widths w are 4 μm, the width of the conductor tracks 5 μm. Such lever arms with integrated MOSFETs are produced in the usual way on single-crystalline semiconductor materials with mask technology. Four masks are required:

1. Generation of the p- or n-doped semiconductor layer in the semiconductor material; 2. Application of the metal layer 3 with conductor tracks 16; thereafter producing the gate oxide layer 5 and finally 4. Manufacture of the gate electrode 4 with conductor strand 7.

All manufacturing steps, such as oxidation, diffusion doping, lithography and metallization, are conventional.

In the exemplary embodiment according to FIG. 3, for reasons of production, the MOSFETs are applied to the same lever arm side on which there is also a probe tip 17 for scanning the sample surface of the atomic force microscope. The position of the probe tip 17, which is only very small relative to the dimensions of the lever arm, is only indicated schematically on the lever arm 15 in FIG.

MOSFETs on semiconductor material can not only be used for strain measurement on lever arms of scanning probe microscopes. In general, they can be attached to semiconductor strain gauges (semiconductor strain gauges) to determine the strain of the semiconductor material or the material on which the semiconductor strain gauge is placed. The manner in which MOSFETs are to be attached to semiconductor strain gauges corresponds to the previously described exemplary embodiments. One or more MOSFETs can be integrated on the semiconductor strain gauge, the MOSFET channels being arranged in selected crystallographic directions on the n- or p-doped semiconductor materials. With single-crystal silicon the MOSFET channels are preferably aligned in the direction of the (001) or (110) planes.

In addition to single-crystal silicon as a semiconductor material, germanium or III / V semiconductors of any orientation are also suitable.

Claims

Patent claims

1. Strain gauge for measuring the strain einkri¬ stallinen, used as a semiconductor strain gauge semiconductor material with attached to the semiconductor material MOSFET (8 to 13) for displaying the strain, the electrical signals serve as a measure of the strain of the semiconductor material (1) characterized in that means for Generation of a constant source-drain current are provided.

2.Method for measuring the strain of single-crystalline semiconductor material by means of a MOSFET (8 to 13) attached to the semiconductor material, the electrical signals of which serve as a measure for the strain of the semiconductor material (1), characterized in that the source-drain voltage with constant source-drain Current is measured.