Patent Description:
<CIT> relates to an apparatus for performing atomic force microscopy comprising an AFM measurement unit operating in a first controlled atmosphere including an inert gas and a pretreatment unit operating in a vacuum atmosphere. The pretreatment unit may be located within the AFM measurement unit.

Aspects of the invention are defined in the appended independent claims.

Other aspects and advantages of embodiments of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.

The Q (also known as quality factor) of a vibrating cantilever of the type used in a force microscope can be increased by reducing pressure, or by using an atmosphere of a low viscosity gas such as helium (He). Furthermore, helium has a higher thermal conductivity at pressures near atmospheric pressure than ambient air. Examples of references that support these findings are:.

However, the advantages of using a helium atmosphere at reduced pressure to accomplish both goals simultaneously have not been previously disclosed. The pressure range where this works is higher than most people would use for enhanced Q (not realizing that with helium, the pressure does not have to approach vacuum).

A scanning force microscope in accordance with embodiments of the invention is operated in a helium environment at pressures ranging from a fraction of an atmosphere up to (but not including) <NUM> kPa (one (<NUM>) atmosphere). The microscope is operated in an AC mode (continuous vibration of the cantilever), and the resulting increased cantilever Q provides enhanced sensitivity.

<FIG> is a diagram an atomic force microscope (AFM) <NUM> in accordance with an embodiment of the invention. The AFM <NUM> includes a sample <NUM> to be examined mounted on a sample scanner <NUM>, which may be designed to be controlled by one or more piezo motion devices for X, Y, and Z motion. The AFM <NUM> also includes a sharp probe tip <NUM> on a flexible microcantilever <NUM> that interacts with the sample <NUM>, and nm-scale deflection of the cantilever <NUM> is sensed by a suitable deflection sensor <NUM>, which is usually an optical lever system. For AC mode operation, a dither drive <NUM> vibrates the cantilever, usually at one of its mechanical resonance frequencies. Forces or force gradients acting between the sample <NUM> and the sharp tip <NUM> change the vibration amplitude, frequency, or phase of the vibrating cantilever <NUM>, and these changes are interpreted by a controller (not shown) to generate an image of the sample surface to indicate the spatial variation of some property of the sample. In its simplest operation mode, an AFM maps the topography of the sample by maintaining a constant gap between the tip and sample surface as the sample is raster scanned under the tip. By tracking and recording the control signals necessary to maintain a constant gap, a topographic image can be generated. If a tunable light source <NUM> is added as shown in <FIG>, and the light from this source is focused on the tip-sample interface, absorption of light by the sample gives rise to force and force gradients acting between the tip and sample. Such forces and force gradients may also affect the amplitude, frequency, or phase of vibration of the cantilever. By recording these changes as a function of light wavelength and position on the sample, spectrographic images of the wavelength-dependent optical absorption of the sample may be generated, and operating in this mode is referred to as photo-induced force microscopy (PiFM). A more detailed description of PiFM, its theory of operation, and description of apparatus can be found in the publication "<NPL>).

As shown in <FIG>, the tip <NUM> and the sample <NUM> of the AFM <NUM> are enclosed in an air-tight enclosed chamber <NUM>, which may optionally include other elements of the AFM such as the scanner <NUM>, the deflection sensor <NUM>, and the tunable light source <NUM> (or light may be brought in through a transparent window or optical fiber). After evacuating most of the air from the chamber <NUM> using a vacuum pump <NUM> connected to the chamber via a valve <NUM>, sufficient helium gas is introduced into the chamber from a He source <NUM> (e.g., a tank of He) using another valve <NUM> to achieve a target pressure in the chamber using a pressure sensor <NUM> attached to the chamber. Thus, a reduced damping environment with mostly helium at less than one atmospheric pressure is created within the chamber to measure interactions between the probe tip <NUM> and the sample <NUM>.

By maintaining at least a fraction of an atmosphere of He pressure (as opposed to vacuum) in the chamber <NUM>, cooling of system components in the helium environment is similar to or better than operating in ambient air. Operating at reduced pressure in He is particularly beneficial for PiFM for increasing sensitivity and resolution, and mitigating thermal drift which could be caused by equipment heating in vacuum, since it may be necessary to examine a particular location on a sample for an extended period of time for the purpose of recording the photo-induced force at many wavelengths of light.

Operating in low pressure (e.g., <NUM> kPa to <NUM> kPa (<NUM> - <NUM> atm)) helium requires a simpler apparatus than operation in vacuum requires. For vacuum operation, a more elaborate vacuum pump is needed, and the chamber <NUM> must be sealed well enough to hold a suitable vacuum level for an extended period, since operation of the pump is generally incompatible with operation of the microscope because of vibrations. Furthermore, to mitigate thermal drift caused by self-heating of the microscope components, operating in vacuum may require the use of active temperature control, such as a thermoelectric cooler, or a heater with temperature controller, to keep temperatures sufficiently constant to avoid excessive thermal drift. Such temperature control systems may have long stabilization times, lowering the productivity of the user while waiting for temperature stabilization.

In an embodiment of the invention, the AFM <NUM> is operated at a helium pressure which provides the same cooling as the instrument has in ambient air. Since the thermal conductivity of He is six times higher than air at full atmospheric pressure (<NUM> kPa (<NUM> atm)), the pressure at which He provides the same cooling as air at <NUM> kPa (<NUM> atm) would typically be in the range of <NUM> kPa to <NUM> kPa (<NUM> to <NUM> atm). By matching the cooling as ambient air, the temperature of microscope elements in the chamber will tend to stay stable when the chamber is closed and the microscope is operated versus when the chamber is opened for sample exchange, idle time when not in use, etc. This choice of He pressure eliminates temperature changes when switching from air atmosphere to He atmosphere and the associated temperature stabilization time that would be needed to reduce thermal drift to an acceptable level. When operating at this He pressure, the instrument may be used immediately after changing to He atmosphere, without down time waiting for temperature stabilization. Although this might be viewed as a rather mild vacuum, because of the reduced damping effect of He gas on a vibrating cantilever, Q is significantly enhanced compared to operation at full atmospheric pressure in air.

In an embodiment of the invention, the cantilever <NUM> of the AFM <NUM> is driven to vibrate by providing a fixed frequency and amplitude of dither vibration from the dither drive <NUM>. The drive frequency is typically at one of the free-space resonances (or "eigenmodes") of the cantilever (resonant frequency when it is not close to or interacting with the sample) or slightly above this resonance, but still well within the width of the resonance peak. When the tip <NUM> is brought close to the sample <NUM>, force gradients acting between the sample and tip typically lower the resonant frequency, causing the fixed drive frequency to deviate further from the center of the resonance peak, which result in a decrease in vibration amplitude. Changes in amplitude can be used to track changes in the tip-sample interaction. This mode is conventionally known as the amplitude modulation (AM) mode or the slope detection mode. In this mode, the high Q of the cantilever <NUM> reduces the bandwidth of the system, so that it takes about Q cycles of oscillation for the amplitude to change when the interaction force gradient changes. By choosing to operate at a pressure where Q is not exceedingly high (i.e., at a fraction of an atmosphere of pressure rather than high vacuum) and by choosing a resonant mode with a sufficiently high frequency (such as the second vibrational mode rather than the first vibrational mode) sufficient bandwidth can be achieved with the AM mode, which is generally the simplest mode of operation.

Another option is to detect the tip-sample interaction force gradients by measuring changes in the phase of the cantilever vibration with respect to the fixed driving signal. This mode is also a well-known mode of operation for AFM.

Alternatively, as is also well-known for conventional AFM, a frequency modulation (FM) mode may be used, which overcomes the bandwidth concern, but is more complex to implement. A particular implementation of the FM mode is to drive the cantilever <NUM> with a variable frequency oscillator, and adjust the frequency so that the phase of the cantilever vibration is held constant with respect to the cantilever drive signal using a phase locked loop (PLL). An alternative FM mode is to self-oscillate the cantilever at one of its vibrational resonant frequencies by amplifying and filtering the output of the deflection sensor and using this signal as the dither drive source.

For PiFM, two vibrational modes of the cantilever <NUM> are typically employed. One vibrational mode is for tracking topography and maintaining a constant tip-sample spacing, and a second vibrational mode is for sensing photo-induced force or force gradient. The tunable light source <NUM> is typically modulated at a frequency which is the difference frequency of the two vibrational cantilever modes employed. This is referred to as the sideband mode of operation for PiFM. Alternatively, the light source <NUM> may be modulated at a resonant frequency of the cantilever <NUM>. This is known as the direct drive mode for PiFM.

Although the AFM <NUM> has been described with respect to helium atmosphere, in other embodiments, another damping-reducing gas or a combination of damping-reducing gas may be used instead of helium. As used herein, a damping-reducing gas is gas exhibiting less damping on a vibrating cantilever than air.

Turning now to <FIG>, a process flow diagram of a method for operating a scanning force microscope, e.g., the atomic force microscope <NUM>, in accordance with an embodiment of the invention is shown. At block <NUM>, at least some air from an enclosed chamber of the scanning force microscope is evacuated using a vacuum pump. The enclosed chamber of the scanning force microscope contains at least a probe tip and a sample scanner of the scanning force microscope. Next, at block <NUM>, a damping-reducing gas is introduced into the enclosed chamber after evacuation of air from the enclosed chamber without increasing pressure within the enclosed chamber to one atmospheric pressure. Thus, the pressure in the enclosed chamber is maintained at below one atmosphere. The damping-reducing gas is any gas that exhibits less damping on a vibrating cantilever than air: according to the invention, this gas is helium. In an embodiment, helium is introduced or allowed to enter into the enclosed chamber until the enclosed chamber includes at least <NUM>% helium with the remainder being other gases. In another embodiment, helium is introduced or allowed to enter into the enclosed chamber until the enclosed chamber includes at least <NUM>% helium with the remainder being other gases. Helium is introduced or allowed to enter into the enclosed chamber until the pressure in the enclosed chamber is at a predefined pressure that is selected so that the temperature of microscope elements within the enclosed chamber is substantially +/- <NUM> (five degrees Celsius) of the temperature of the microscope elements when operated in ambient environment, i.e., air environment at ambient temperature and pressure. The predefined pressure is between <NUM> kPa and <NUM> kPa (<NUM> and <NUM> atmosphere). Next, at block <NUM>, a sample on the sample scanner of the scanning force microscope is engaged using the probe tip in a reduced damping environment with the damping-reducing gas within the enclosed chamber at a pressure below one atmospheric pressure to measure properties of the sample. In an embodiment, force gradients between the probe tip and the sample are detected by a change in amplitude of a vibrating cantilever of the scanning force microscope that is driven to oscillate at a frequency near a mechanical resonance of the cantilever. The resonance of a cantilever can be either sharp or broad. The sharpness of the resonance is described by the so-called "Quality Factor" Q of the resonance. The width of the resonance is roughly equal to the resonant frequency (at the center of the resonance) divided by the Q. So for example, if a cantilever has a resonant frequency of <NUM> and a Q factor of <NUM>, then its resonance is about <NUM> wide. If a cantilever has a resonance of <NUM> and a Q of <NUM>,<NUM>, then its resonance is only <NUM> wide. Thus, as used herein, "near" a resonance means that the frequency is within the peak width of the resonance, or in other words, it is within approximately Fr/Q cycles of the center of the resonance, where Fr is the resonant frequency, and Q is the quality factor. The Q goes up as viscous drag from the atmosphere is reduced. For example, a cantilever that has a Q of <NUM> in ambient air for a particular resonance will see the Q value increase as the pressure is reduced (pumping down in a chamber). Likewise, if the air is replaced with helium, Q will increase relative to what it was in air at the same pressure.

In another embodiment, the force gradients are detected by a change in phase of the vibrating cantilever driven to oscillate at a frequency near a mechanical resonance of the cantilever. In another embodiment, the force gradients are detected by a change in frequency of the vibrating cantilever driven to self-oscillate at a frequency near a mechanical resonance of the cantilever. In this embodiment, the oscillation frequency of the cantilever may be controlled by a phase locked loop (not shown) which includes a detector measuring the phase of the cantilever oscillation relative to a driving signal. In an embodiment, the tip-sample junction is illuminated by a modulated light source operating at a specific wavelength or over a specific band of wavelengths, wherein the modulated light source causes a time varying force or force gradient acting on the tip, and the time varying force or force gradient is detected and recorded by the microscope. In this embodiment, surface topography of the sample may be sensed using one vibrational eigenmode of the cantilever, and the force or force gradient generated by the interaction of the sample with the modulated light source is sensed using a second vibration eigenmode of the cantilever. The laser modulation frequency may be the same as one of the eigenmode frequencies of the cantilever. Alternatively, the laser modulation frequency may be a difference or sum frequency of the frequencies of two vibrational eigenmodes of the cantilever.

The components of the embodiments as generally described in this document and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations.

The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims.

It should also be noted that at least some of the operations for the methods may be implemented using software instructions stored on a computer useable storage medium for execution by a computer. As an example, an embodiment of a computer program product includes a computer useable storage medium to store a computer readable program that, when executed on a computer, causes the computer to perform operations, as described herein.

Furthermore, embodiments of at least portions of the invention can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer readable medium can be any apparatus that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.

The computer-useable or computer-readable medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device), or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, non-volatile memory, NVMe device, persistent memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disc, and an optical disc. Current examples of optical discs include a compact disc with read only memory (CD-ROM), a compact disc with read/write (CD-R/W), a digital video disc (DVD), and a Blu-ray disc.

Claim 1:
A method of operating a scanning force microscope (<NUM>), the method comprising:
evacuating (<NUM>) at least some air from an enclosed chamber (<NUM>), the enclosed chamber containing at least a probe tip (<NUM>) and a sample scanner (<NUM>) of the scanning force microscope;
introducing (<NUM>) a damping-reducing gas into the enclosed chamber after evacuation of air from the enclosed chamber without increasing pressure within the enclosed chamber to one atmospheric pressure, the damping-reducing gas being gas exhibiting less damping on a vibrating cantilever than air, wherein introducing the damping-reducing gas into the enclosed chamber includes introducing helium into the enclosed chamber until the pressure in the enclosed chamber is between <NUM> kPa and <NUM> kPa (<NUM> and <NUM> atmosphere) so that a temperature of microscope elements within the enclosed chamber is +/- <NUM> (+/- <NUM>) of a temperature of the microscope elements when operated in ambient environment; and
engaging (<NUM>) a sample (<NUM>) on the sample scanner using the probe tip in a reduced damping environment with the damping-reducing gas within the enclosed chamber at a pressure below one atmospheric pressure to measure properties of the sample.