Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
  • G01Q30/00—Auxiliary means serving to assist or improve the scanning probe techniques or apparatus, e.g. display or data processing devices
  • G01Q30/20—Sample handling devices or methods
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
  • G01Q60/00—Particular types of SPM [Scanning Probe Microscopy] or microscopes; Essential components thereof
  • G01Q60/24—AFM [Atomic Force Microscopy] or apparatus therefor, e.g. AFM probes
  • G01Q60/30—Scanning potential microscopy
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/30—Measuring arrangements characterised by the use of optical techniques for measuring roughness or irregularity of surfaces

Definitions

• the present invention relates to the field of polarized electronic devices.
• the invention more particularly relates to a method for measuring a potential on electronic devices that are biased through an external voltage source.
• the present method makes it possible to obtain a nanoscale potential mapping that can notably be applied to semiconductor devices.
• AFM Atomic Force Microscopy
• An atomic force microscope corresponds to a type of microscope with a local probe, the latter being in the form of a tapered tip. Such a microscope makes it possible to analyze areas having dimensions ranging from a few nanometers to a few micrometers and to measure nanonewton forces.
• Said probe of the AFM type microscope is disposed at the free end of a resilient micro-lever, also called "cantilever". This lever is able to move in all directions of space through a piezoelectric tube with which it is associated.
• Deviations from the lever are traditionally measured by laser reflection.
• the probe is mounted on a micro-lever on the surface of which a laser beam is reflected.
• the reflected laser beam is deflected, it also corresponds to a deviation of the lever in one direction or the other, which makes it possible to reveal an interaction between the probe and the surface of the sample analyzed, this interaction possibly corresponding to either a attraction, or to a repulsion between the two elements, probe and analyzed surface.
• An atomic force microscope can also be used to measure the value of the electrostatic potential at the surface of the sample to be analyzed, so as to map the surface potential of said sample.
• the measurement of the potential of a surface is performed by two successive passes of the probe at the same location of said surface.
• the first pass is made with the tapered tip of the probe in contact (or intermittent contact) of the surface to be analyzed and allows a measurement of the topographic profile of the latter.
• the device uses this topographic measurement to place and maintain the probe at a constant height above the analyzed surface, for example at a distance of between 20 and 100 nm, so as to allow performing electrostatic potential measurements of said surface.
• JP 2002 214 113 a method for topographic measurement and potential measurement, the two measurements being made in "contact" mode, that is to say with a very small distance between the tip of the probe and the surface of the sample, this distance being for example of the order of a few Angstroms.
• Two measurements are made, for each of the points of the surface, and it is sought to minimize the interactions between the tip of the probe and the surface of the sample, so that the effect of electrostatic charge is canceled.
• the electrostatic charge between the tip and the sample is then detected, and a "bias potential" is returned to the micro-lever in order to minimize the effect of the electrostatic force on said lever.
• the imprecise measurements of the topography of a surface are mainly due to the fact that, in the case of a polarized sample, a charge density can be created either on the surface of this sample, ie at a very small distance from it.
• the presence of this charge density will cause a change in the interaction between the tip of the probe and the surface of the sample, adding an additional electrical force, the Coulomb force.
• This additional force will cause an attraction or repulsion of the micro-lever, as would a change in the height of the surface.
• the measuring device does not make it possible to differentiate between an actual modification of the topographic profile of the surface and the presence of additional interaction forces, due to the polarization of the sample.
• the topographic profile obtained is likely to be distorted. Consequently, the measurement of the surface potential of the probe is also imprecise, because the height at which this probe is placed during its second passage is set according to the topographic measurements taken during the first pass.
• the invention offers the possibility of overcoming the various drawbacks of the state of the art by proposing a method making it possible to perform particularly precise measurements of the topography of the surface of a polarized electronic device, in particular a semiconductor device, so the accuracy of subsequent potential measurements is also optimal.
• the present invention relates to a method for measuring the surface potential of a polarized sample comprising the following steps:
• the topographic profile of said sample is measured by scanning the surface of the latter with the aid of a tapered tip connected to a microlevel activated at its resonance frequency by a piezoelectric activator;
• said method being characterized in that said sample is not polarized during the step of measuring the topographic profile and in that said sample is polarized during the measurement of the potential profile.
• said sample is biased through an external voltage source applying a voltage of between 0 and ⁇ 10V.
• the present invention also relates to a device comprising a topography measuring means and a potential measuring means using the results of the topography measurement, said device further comprising a switch designed to allow the application of a voltage to said sample in position closed and to cancel the application of said voltage in the open position, and a module of synchronization configured to synchronize the opening and closing of said switch so that the voltage is not applied to the sample during the topographic measurement, and is applied to the sample during the measurement of the potential.
• said device comprises a tapered tip adapted to scan the surface of a polarized sample via an external source of voltage, said tapered tip being connected to a micro-lever adapted to be activated at its resonant frequency by a piezoelectric activator and a first generator, said device further comprising a piezoelectric scanner capable of controlling the positioning of the tapered tip and means for detecting variations in the amplitude of oscillation of the micro-lever, these detection means being connected to a signal amplifier device, itself connected to a box having as reference the signal of the first generator, said box being connected to a device able to compare the data obtained with the reference data, said device for comparison being able to transmit the data to a return loop connected to the scanner piezoelectric, said loop controlling the position of the tip through said scanner, said comparison device being further connected to a second generator adapted to deliver a voltage to said micro-lever, said synchronization module being connected on the one hand to the feedback loop and secondly to said external voltage source via said
• the device according to the invention further comprises an amplifier, connected to the second generator and able to amplify the voltage delivered by it to the microlevier.
• the present invention has many advantages. In particular, it makes it possible to eliminate the topographic artefacts from which a false measurement of the surface potential of a polarized material results. In addition, the present invention is relatively easy to implement; indeed, it is only necessary to add, to the existing devices, a module for synchronizing the passages of the tapered tip with the polarization of the polarized material.
• FIG. 1 diagrammatically represents an embodiment of a device for implementing the method according to the invention
• FIG. 2 diagrammatically represents a measurement of the topographic profile and of the electrostatic potential of the surface of a sample to be analyzed
• FIGS. 3A and 3B schematically illustrate a sectional view of a polarized transistor, consisting of a (thin-film transistor of organic material), and the voltages applied to this transistor;
• FIGS. 4A and 4B are graphs representing respectively the height (in nm) and the potential (in V) as a function of the position (in ⁇ m) of the tip on the sample; these graphs make it possible to illustrate and compare the topography and the potential of a sample when it is not polarized during the first measurement step with respect to the same sample which remains polarized throughout the process.
• FIG. 1 firstly makes it possible to schematically illustrate a method currently implemented in the state of the art by means of a device 1 to make it possible to measure the topographic profile and the potential of a surface of a sample 2. More particularly, the diagram of FIG. 1 represents the state of the art if the synchronization module 18 is removed and switch S3, and when the switches referenced SI and S2 are respectively closed and open during the measurement of topographic profile and vice versa during the measurement of potential.
• a probe consisting of a tapered tip 3 scans point by point the surface of said sample 2.
• Said tapered tip 3 is attached to the end of a micro-lever 4 adapted to be activated by a piezoelectric activator 5.
• the micro-lever 4 Via a first generator 10 applying a signal to the piezoelectric activator 5 (switch S1 closed), the micro-lever 4 can be activated at its resonant frequency, and then oscillates according to a certain amplitude; the tapered tip 3 of the micro-lever 4 is then brought into contact with the surface of the sample 2 to be analyzed and a piezoelectric scanner 16 makes it possible to control the positioning of said micro-lever 4 and thus of the tapered tip 3.
• a laser 6 is preferably used to detect a variation of the oscillation amplitude of the micro-lever 4.
• the position of the laser beam reflected by said micro-lever 4 is identified by means of a detector 7. having a plurality of quadrants.
• said detector 7 may in particular consist of a split photodiode.
• the signals picked up by the different quadrants of this detector 7 are then amplified by an amplifier device 8 and then sent back to a box 9 which knows as a reference the signal of the first generator 10 applied to the piezoelectric activator 5.
• the data concerning on the one hand the reflection of the laser beam, reflecting the oscillation of the micro-lever 4, and on the other hand the signal of the first generator 10, corresponding to the reference oscillating signal applied to the micro-lever 4, are then transmitted to a device 15 allowing a comparison of these data. Any variation in the amplitude of the oscillation of the micro-lever 4 is thus detected.
• the tapered tip 3 follows the topography of the surface of the sample 2; this results in the detection of a topographic profile 11, shown on the right part of Figure 2, and this profile 11 is then recorded.
• said tapered tip 3 is raised relative to the surface of the sample 2 and is maintained at a distance d constant from this surface, d being typically of the order of 100 nm.
• said tip 3 passes through the sample 2 and the system records the electrostatic potentials 12 of the surface so as to also obtain a profile 13 of these potentials. differences between the potential of this tip 3 and the potential of the surface of the sample 2, and to cancel the oscillation of the micro-lever 4 and the tapered tip 3, a voltage is applied to said micro-lever 4 (switch S2 closed) .
• this voltage is applied via a second generator 19, and possibly an amplifier 20, the latter allowing an amplification of the voltage delivered by the second generator.
• a return loop 14 will allow a control of the tapered tip 3 of the micro-lever 4.
• this feedback loop 14 is connected in particular to the data comparison device 15 and to the piezoelectric scanner 16. More particularly, during the first step of measuring the topography of the sample 2 to be analyzed, the feedback loop 14 uses the information on the amplitude of oscillation of the micro-lever 4 sent by the box 9 to the device of FIG. Comparing the data 15.
• the feedback loop 14 will then generate a response proportional to the difference in amplitude between the reference amplitude and that detected, so that the piezoelectric scanner 16 extends or retracts so that the tapered tip 3 away from or near the surface of the sample 2, in order to maintain a constant force of interaction between said tip 3 and the sample 2.
• the micro-lever 4 is grounded, and it There is no return on the applied voltage.
• the tapered tip 3 follows the pre-recorded topographic profile 11 and, consequently, there is no return on the positioning in height of said tip 3.
• the latter is not polarized during the measurement step of the topographic profile 11 of the surface of the sample 2. This advantageously makes it possible to avoid a charge accumulation on the surface of the sample 2 during the measurement of the topography of this surface.
• the second phase in which the potential of said surface is measured, is performed in the presence of the external voltages applied to the sample 2.
• the external voltages produce a polarization of said sample 2, and the measurement of the surface potential profile allows to identify certain characteristics of the sample.
• polarizable electronic devices such as for example, but not limited to, semiconductor devices.
• a polarizable device corresponds to a device which is polarized in operation, via a voltage source, in particular external.
• the polarization of the sample 2 at the time of the potential measurement is obtained via an external voltage source 17, shown schematically in Figure 1 attached.
• the external voltages applied to said sample 2 through said source 17 allow to put it in its operating state, which can reveal a large amount of electrical charges near its surface, these charges being incompatible with a precise measurement topography.
• the external source 17 is advantageously synchronized to the control loop 14 via a synchronization module 18.
• the connection between said control loop 14 and said module 18 is preferably obtained via a TTL output (transistor-transistor logic) 21.
• a switch S3 visible in FIG. 1 attached allows loop synchronization 14 / voltage source 17 S3 is then open at the time of the topography measurement, and, on the contrary, at the time of potential measurements, switch S3 is closed.
• Such synchronization between the control loop 14 and the external voltage source 17 makes it possible to ensure that the analyzed sample 2 is not polarized at the time of the topographic measurement step, and polarized at the time of the measurement of the potential profile.
• the topography of a sample 2 is measured line by line. Such a measure usually takes on the order of a second.
• the tapered tip 3 is raised and placed at a constant distance from the topography of the sample 2, the switch S3 is closed and the potential of the line is measured.
• the device 1 goes to the next line, and the switch S3 is opened. So, about all the seconds, it is appropriate to apply or not a voltage to polarize or not said sample 2.
• the external voltage source 17 consists of a generator capable of delivering voltages to be included in an operating range compatible with the second generator 19 and with the possible amplifier 20; Indeed, the latter must be able to apply a voltage to cancel the oscillation of the tapered tip 3 at the time of potential measurements.
• the voltages that can be delivered by a second generator 19 are between 0 and ⁇ 10V.
• this voltage range can be extended by means of an amplifier 20, up to a hundred or so of volts.
• Other technical solutions must if necessary be implemented, for example by working under vacuum, to make possible the extension of this voltage range.
• Example 1 The demonstration of the efficiency of the process according to the present invention is detailed in Example 1 below and corresponding Figures 3 and 4.
• This example intended to illustrate the interest of the invention, is in no way limiting as to the scope of the invention described and claimed herein.
• the invention also relates to a device 1 for implementing the method described above.
• the device 1 essentially comprises a topography measuring means and a potential measuring means. The latter uses the results obtained by means of the measurement of topography.
• the device 1 according to the invention further comprises a switch S3. The latter is designed to allow the application of a voltage to said sample 2 in the closed position and to cancel the application of said voltage in the open position.
• the device 1 also comprises a synchronization module 18 configured to synchronize the opening and closing said switch S3 so that the voltage is not applied to the sample 2 during the topographic measurement, and is applied to the sample 2 during the measurement of the potential.
• the synchronization module 18 is connected on the one hand to the feedback loop 14 and on the other hand to said external voltage source 17, said synchronization module being able to control the switch S3 for its opening or its closure.
• such a device 1 comprises at least:
• a tapered tip 3 able to scan the surface of a sample 2
• micro-lever 4 connected to said tip 3;
• a piezoelectric activator 5 connected to a first generator 10 for activating the micro-lever 4 at its resonant frequency, which oscillates at a certain amplitude;
• a piezoelectric scanner 16 adapted to control the positioning of the tapered tip 3;
• these means preferably consist of a laser housing 6 and a detector 7, in particular a split photodiode;
• an amplifier device 8 connected to the signal detection means
• a housing 9 connected on the one hand to the amplifier device 8 and on the other hand to the first generator 10, the housing 9 having, as a result, the signal of said first generator 10 applied to the micro-lever 4;
• a device 15 connected to the housing 9, and able to compare the data obtained with the reference data;
• a feedback loop 14 connected to the device 15 and the piezoelectric scanner 16;
• a second generator 19 connected to the device 15, and possibly to an amplifier 20 of the voltage delivered by said generator 19, this voltage being preferentially between 0 and ⁇ 10V and being applied to the micro-lever 4 so as to counterbalance its oscillation and to allow the measurement of potential;
• the device 1 according to the invention advantageously makes it possible to synchronize the application or not of a voltage by the source 17 to the measurement of potential and topography.
• the synchronization module 18 makes it possible not to polarize the sample 2 during the measurement of the topographic profile.
• Example 1 Demonstration of the Method According to the Invention on a Polarized Electronic Device of OFET Type
• the present method has been implemented on a polarized electronic device, and more particularly on an OFET transistor (Organic Field Effect Transistor) 21, shown in the appended FIG. 3A, and in which the semiconductor material 22 consists of poly (3). - hexylthiophene) or P3HT, deposited on a structure of electrodes forming a transistor.
• OFET transistor Organic Field Effect Transistor
• transistors of this type can be subjected to applied voltages that can reach ⁇ 100V, which poses problems when measuring their topographic profile.
• Said OFET transistor 21 comprises in particular three zones of interest 23, 24 and 25.
• the zone 23 corresponds to a "drain” electrode with constant potential
• the intermediate zone 24 corresponds to the channel of the transistor 21
• the zone 25 corresponds to a "source” electrode with constant potential
• the bias of the transistor 21 is external and the voltages applied at the zones of interest 23, 24 and 25 are not traditionally synchronized with the passage of the tip 2; accordingly, the step of measuring the topography of the transistor 21 is performed while the latter is polarized.
• F V d, F ei and F capi i respectively correspond to the interaction force of Van der Waals, to the electrostatic force due to the presence of surface charges, and the capillary force due to the humidity of the air. This force can notably be present when the method is implemented under ambient conditions.
• the topography measurement is carried out without external polarization of the component, by canceling the component F ei .
• the means of measuring the value of the component F ei is to move the tapered tip 3 more than 10 nm from the surface. Indeed, the Van der Waals forces F vdw and the capillary forces F capillile decrease significantly when the distance between the tapered tip 3 and the sample 21 is greater than 10 nm. Generally, said tip is placed at a distance of the order of 100 nm from the surface, which makes negligible the two components F Vd and F capi i. External voltages are applied to the component during this second phase.
• the topographic profile and the potential of a polarized transistor 21 were first measured by the application of a voltage equal to -15 V between the source 25 and the drain 23 (Uds in FIG. 3B) and, on the other hand, by the application of a voltage equal to -15V between the gate 26 and the source 25 (Ugs in FIG. 3B). These measurements were also performed on a transistor 21 whose polarization is synchronized according to the step of the method that is implemented, that is to say that the voltages applied to the transistor 21 are equal to 0V during the measurement of the topographic profile.
• FIG. 4A makes it possible to compare the topographic measurement of a continuously biased transistor 21 (dashed curve) during the two measurement steps and the same transistor 21 when the polarization thereof is synchronized (continuous curve), unpolarized during topographic measurement, and polarized during potential measurement.
• the potential measurements obtained are shown in FIG. 4B; the dotted curve represents the potentials measured on a transistor 21 which is polarized continuously and the continuous black curve represents the potentials of a transistor 21 whose polarization is alternated, said transistor 21 being depolarized at the time of the topographic measurements and polarized during the measurements of potential.

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• 2012
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