DE4437306C2 - Strain gauge for measuring the strain of a single-crystal semiconductor material used as a semiconductor strain gauge - Google Patents

Description

The invention relates to a strain gauge Measurement of the strain as a semiconductor strain gauge strips used monocrystalline semiconductors terials according to the preamble of the claim, such as known from US 4 191 057. The invention is used to formation of conventional semiconductor strain gauges, especially for measuring the elongation of a semi-lead made of one-sided clamps arms, which acts as a scanning probe for a scanning probe micros Kop is to be used.

From DE-OS 22 31 977 an arrangement for measuring me chanic strains of a single crystal semiconductor materials using four MOS field-effect transistors knows. The transistors are on the corners of one Diamond whose diagonals are crystallographic axes, the same in terms of electrical properties are valuable. This arrangement is used for an expansion measurement of overcoming disadvantages that arise due to changing concentrations of contaminants in the Adjust semiconductor material.

According to J. Phys. D: Appl. Phys., 12, 1979, pp. 1973-1983 are mechanical strains in a MOSFET by measuring solution of changes in the electronic components electrical currents determined.  

For scanning probe microscopes, e.g. B. known from Appl. Phys. Lett., 62, 1993, pages 834-836, are one-sided clamped lever arms as grid probes off crystalline semiconductor material known. To those on Ra Lever arm microscope clamped on one side are at the free, unclamped end of the Lever arms, fine probe tips attached, the mate Rial surfaces and their topography Detect down to atomic resolution. In doing so physical interactions between the mate rial surface and the probe tip. The rela tiv is moved to the material surface. The reciprocal kung can be long-range and related to van-der-Waals- Forces, capillary forces, magnetic or electroma genetic forces, it can also short range be in physical and chemical absorption forces in elastic or plastic deformation forces animals. Each  Interaction at the probe tip leads to a Deflection of the lever arm relative to its on Scanning probe microscope clamped on one side Lever arm end and elastic bending of the Lever arm. The amount of deflection is that at the Probe tip attacking force proportional and at a movement of the probe tip over the material surface che depending on the location, so that from the lever arm the local topography structure can close the material surface. The amounts the deflection is generally evaluated digitally and converted into data images that enable the topography clearly, for example Screens.

It is next to to measure the deflection of the lever arm electron tunneling, optical interferometry, light beam deflection or capacitance measurement changes of state also known on the lever arm itself to install a signal generator, the electrical Sig nale proportional to the deflection (M. Tortonese et al, "Atomic resolution with an atomic microscope using piezoresistive detection ", Appl. Phys. Lett., 62; (1993), pp. 834-836). With this last mentioned Signal generator is made of single-crystal on a lever arm Silicon as a semiconductor material is an electrical one Conductor endowed that changes its resistance when with a deflection of the lever arm due to pull and Compressive stress in the semiconductor material strains are caused. For many applications purposes, especially when the probe tip is deflected in of the order of 0.1 nm, are signal generators with small drift and fewer interference signals required.

The object of the invention is on a single-crystal half  conductor material for indicating the elongation of the semiconductor materials a signal generator with high sensitivity, d. H. high signal component with a small interference component create.

This task is the one with a strain gauge gangs mentioned by the claim given characteristics solved. After that, on the half lead term material used as signal generator MOSFET as a measure of the elongation of the Semiconductor material the changing Source-drain voltage with constant source-drain current as the size proportional to the deflection of the lever arm utilized.  

Analogous to MOSFETs made of semiconductor material The lever arms for scanning probe microscopes are MOSFETs as sensors for strain measurement with the above Advantages generally suitable and everything for Lever arms has been discussed also applies to semiconductors strain gauges.

The invention and further refinements of the invention are nä using examples ago explained. The drawing shows schematically in a individual:

Fig. 1 MOSFETs with p-channel on n-doped semiconductor material;

FIG. 2 shows two MOSFETs arranged on one another at an angle of 90 ° to one another on semiconductor material (enlarged section II of FIG. 4);

Figure 3 is switched four MOSFETs as a Wheatstone bridge.

Fig. 4 probe holder for an atomic force microscope with a unilaterally clamped lever arm of semiconductor material and attached MOSFETs in FIG. 2.

In Fig. 1 is a schematic representation of n-doped semiconductor material 1, a patch MOSFET with MOSFET channel 2, in the exemplary embodiment a p-channel reproduced. If, in contrast to the exemplary embodiment, 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. The following is therefore assumed for all exemplary embodiments of 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: On the semiconductor material 1 , a metal layer 3 is applied, which 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 MOSFET channel 2 , the semiconductor layer 6 being arranged at the source and the semiconductor layer 7 at the drain. The MOSFET channel has a gate length l and a gate width w. The gate voltage U _G can be regulated.

MOSFETs behave piezoresistive, G. Dorda, J. Appl. Physics, 42, 1971, pp. 2053-2060, and from a publication by C. Canadi et al., J. Phys. D : Appl. Phys., 12 , 1979, 1973-1983, it follows that with an expansion D (δl / l) of the semiconductor material over the gate length l with a constant source-drain voltage U _D, a change in the source-drain current I _D (δI _D / I _D ) is measurable. The source-drain current I _D shows strong, anisotropic, piezoresistive effects. The magnitude of the current change δI _D / I _D can be treated analogously to a change in resistance of an electrical resistance during expansion,

δI _D / I _D = k _I · ∈ (1)

where the value k _I as a dimensionless gain factor for the elongation ∈ - comparable in magnitude to the dimensionless quantity k _R with an electrical resistance change - is in the range between ± 100. The dependence of k _I 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 _R on the same parameters.

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

In order to achieve a high sensitivity for an expansion ∈ 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. Is the semiconductor level of the single-crystal semiconductor material in the boundary layer between semiconductor material 1 and metal layer 3 z. B. a (001) -Kri stall area (z. B. with single crystal silicon), it is advantageous to place the MOSFET channel in a (110) direction, and the main direction of elongation in the semiconducting term material parallel or perpendicular to the MOSFET channel, in order to obtain the greatest possible value for k _I. For the MOSFET shown in Fig. 1 with semiconductor material made of single crystal silicon and the direction of the MOSFET channel in (110) direction, the k _I values for

• - strains running parallel to the MOSFET channel ∈ _parallel :
  k _{I / II} = 102
• - strains perpendicular to the MOSFET channel ∈ _vertical :
  k _{I / I} = -85,

where the gate voltage UG is much larger than that Select the threshold voltage. With other endowments or other crystal surfaces of the semiconductor material there are other excellent directions for that Orientation of the MOSFET channel.

In contrast to the strain measurement with a MOSFET by determining the change in the source-drain current I _D (.delta.I _D / I _D) results in a signal amplification if at constant I _D the change in the source-drain voltage V _D (.DELTA.u _D / U _D ) is measured as a function of the strain ∈,

δU _D / U _D = k _U · ∈ (2).

Because with R _i as a differential resistor (R _i = δU _D / δI _D ) and R _D as a nominal resistor (R _D = U _D / I _D ) of the MOSFET channel, (1) results in the gain factor k _U

k _U = k _I · R _i / R _D (3).

If, for example, as with a lever arm of a scanning probe microscope, deflection of the lever arm is to be measured, the value to be determined can be falsified by thermal drift, the disturbance variable k _T at the operating point of

k _T = δI _D / δT (4)

can be described. A current change due to thermal drift cannot be distinguished from a current change δI _D / I _D due to an expansion ∈ of the semiconductor material.

The temperature dependence of the source-drain current is generally somewhat smaller than the temperature dependency of semiconductor material resistors. In particular, an operating point of particularly small thermal drift can be set via the gate voltage U _G.

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 becomes 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, the thermal drift can be largely eliminated. For the difference δI _{D8 / 9} between the two source-drain currents I _D8 , I _D9 in the MOSFET channels applies namely with uniform expansion ∈ and the same temperature difference ΔT = T - T₀ in the semiconductor material

I _{D 8/9} = I ° ₈ [1 + k _I8 · ∈ + k _T8 (T-T₀)]
- I ° _D9a [1 + k _I9 · ∈ + k _T9 (T - T₀)] (5).

If one sets the same nominal source-drain current for both MOSFETs 8 , 9 , I ° _D8 = I ° _D9 and, in the case of structurally identical MOSFETs, k _T8 = k _T9 and the amplification number k _I8 = -k₁₉, the result is:

I _{D8 / 9} = ∈ (I ° _D8 · k _I8 + I ° _D9 · k _I9 ) (6).

The first aforementioned condition, to set an equally large source-drain current in both MOSFETs, can be met by correspondingly regulating the independently adjustable gate voltages U _G8 , U _{G9 of} both MOSFETs 8 , 9 . The second condition is fulfilled in structurally identical MOSFETs if they are attached in such a way that their MOSFET channels are arranged in crystal directions of the semiconductor materials which are equivalent to the strain to be measured (for example in the case of single-crystal silicon in the (110) and (110) directions) . The third of the aforementioned conditions can always be achieved due to the anisotropic behavior of k _I when the two MOSFETs 8 , 9 are oriented at a 90 ° angle to one another.

To eliminate additive disturbances is among the above conditions mentioned also a correlation method applicable. After that who for the measurement of strains due to the semi-egg compressive and tensile stresses attacking the term material two MOSFETs initially with no load on the semiconductor ma material the measured value deviations of both MOSFETs at one Change in environmental parameters determined and determined a calibration function, which then during the measurement carried out to correct the displayed measured values. When measuring yourself changing additive disturbances from environmental influences are eliminated in this way.

If only a 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 ΔI _D / I _D at a non-loaded semiconductor material is used to determine the calibration function Change of disturbance variables at two different gate voltages U _GI , U _{GII is} evaluated. The inversion in the MOSFET channel and thus the value of the respectively available gain factor k _I depends on the set gate voltage. The two gate voltages U _GI , U _GII are thus to be selected such that the third condition for the gain factors k _{I / I} and k _{I / II} required for the correlation method results, namely:

k _{I / I} = -k _{I / II} .

A calibration can also be carried out in this way determination function, which then for measured value correction is used.

In Fig. 3 is a Wheatston bridge circuit of four MOSFETs 10 , 11 , 12 , 13 given again. There are at least two MOSFETs on the semiconductor material whose elongation is to be measured, in the exemplary embodiment the MOSFETs 10 , 11 , for the other two MOSFETs there are no requirements for piezo behavior. For example, they can be integrated on the holder of the lever arm. The gate voltage of one of the four MOSFETs, in the exemplary embodiment of MOSFET 12 , is used to zero the bridge, the setting of the gate voltages of the other three MOSFETs is available to optimize their operating points, so that there are different manufacturing tolerances between the MOSFETs used have the gate voltage set individually for each MOSFET.

Another advantage of using MOSFETs as strain gauges on semiconductor material is the possibility of superimposing the measurement signal I _D or U _{D of} a MOSFET with the aid of an appropriately frequency-modulated gate voltage U _G with a fundamental frequency and the modulated measurement signal in a known manner by means of a lock -in-technique to reinforce. The thermal drift is then eliminated in the amplifier chain used here.

An important area of application of MOSFETs on semi-conductors The term material for strain measurement is the Rasterson denmicroscopy. With scanning probe microscopes determine the topography of surfaces, and levels and terraces on the surface can go all the way up measured atomic resolution. The interdependency too mechanisms between the scanning probe and the sample top areas are examined.

In atomic force microscopes, semiconductor ma Material lever arms used, their deflection due to the action of surface forces at Grid of the surface is to be measured. The one on the grid Force microscope lever arms clamped on one side by waiters attacking at their free lever arm end surface forces deflected. The result of Tensile and compressive stresses in the semiconductor material during Bending the lever arm will result in elongations measured. The elongation values are a measure of that Deflection of the lever arm.

FIG. 4 schematically shows a holder 14 which is fixedly attached to a scanning force microscope and has a lever arm 15 made of single-crystal silicon clamped on one side. A section II of the lever arm 15 is shown enlarged in FIG. 2.

On the lever arm 15 , two MOSFETs 8 , 9 are attached, of which a MOSFET with its MOSFET channel in the main direction of expansion when the lever arm is deflected, the other MOSFET being used at an angle of 90 ° thereto.

The lever arm 15 has the usual dimensions for Ra stersonde of this type: lever arm length L = 400 microns, width B = 40 microns, thickness S = 4 microns. The two MOSFETs 8 , 9 and their conductor tracks 16 on the lever arm are correspondingly small, see FIG. 2. The MOSFET channel lengths l 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
• 3. Manufacture of the gate electrode 4 with a conductor strand.

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

In the exemplary embodiment according to FIG. 4, for reasons of position, 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 atomic force microscopy. 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. 4.

MOSFETs on semiconductor material are used for strain measurement not only on lever arms of scanning probe microscopes applicable. They can be generalized Semiconductor strain gauges (semiconductor strain gauges) for Er averaging the elongation of the semiconductor material or Attach material on which the semiconductor strain gauge is based is set. The way in which MOSFETs Semiconductor strain gauges to be installed correspond to the previous ones described embodiments. You can get involved or more MOSFETs on the semiconductor strain gauge ren, the MOSFET channels in selected crystalline lographic directions on the n- or p-doped Semiconductor materials are arranged. With einkri stable silicon, the MOSFET channels are preferred 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 are also more arbitrary Suitable for orientation.

Claims (1)

Strain gauge for measuring the strain of a single-crystalline semiconductor material used as a semiconductor strain gauge with a signal transmitter formed on the semiconductor material for indicating the strain, the signal transmitter being at least one MOSFET with a source, a drain and a gate electrode, and that of the source and The electrical signals emitted by the drain electrode serve as a measure of the elongation of the semiconductor material, characterized in that the electrical signals are the source-drain voltage with a constant source-drain current.