Patent Application
20080067370
The invention will now be described in detail, by way of example only, with reference to the accompanying drawing in which
A detailed description will now be given of an embodiment of the calibration device with reference to
The device consists of a single crystal grown on a Gas substrate by a semiconductor crystal growth technique. The epitaxial structures are produced by introducing an additional material other than gallium or arsenic into the growth chamber, such as aluminum or indium, to produce epitaxial layers of AlxGa1-xAs (AlGaAs) or InyGa1-yAs (InGaAs). The epitaxial layers should not exceed the critical thickness for that material in relation to the GaAs layers, where the strain induced by the different crystal lattice spacing of the epitaxial layer causes breakdown of the single crystal structure with the formation of dislocations and other crystal defects.
The calibration sample consists of a plurality of groups of thin, epitaxial, atomically abrupt AlGaAs layers alternating with GaAs layers of nominally the same thickness. The period of each set of layers is kept constant. The initial set of layers (2) is grown on a GaAs substrate (1). The AlGaAs and GaAs layers have very similar lattice constants, so a variety of thicknesses of AlGaAs layers can be grown without epitaxial breakdown. Several additional groups of alternating AlGaAs and GaAs layers (3,4 and 5) are grown to allow calibrations over a broad range of lengths. The thickest AlGaAs layers may be comprised of thinner alternating AlGaAs and GaAs layers, to minimize the effects of aluminum oxidation. The thick 1-micrometer surface layer of GaAs contains ultra-thin InGaAs layers of a few atomic layers thickness (6) approximately every 35 nm. This additional superlattice generates a strong XRD signal, and is used as a cross-reference for verifying all other dimensions on the device. The InGaAs layers have a substantially different lattice constant than GaAs, so only very thin InGaAs layers can be grown to allow the InGaAs lattice spacing to be strained to match the lattice spacing of the GaAs layers, and to avoid epitaxial breakdown.
After the crystal growth is complete, the material is cut into appropriately sized pieces and two of these pieces are mounted surface-to-surface to provide a cross-sectional device comprised of two complete structures inverted in relation to each other. This cross-section is polished flat. For electron microscope samples, contrast in the image is provided by elemental contrast between the layers. For AFM samples, alternating layers are selectively etched or oxidized to provide topographic contrast. When viewed in an electron or scanning probe microscope, the sample appears as a series of groups of alternating bands of known thickness, with each group having a different, accurately known period. The lengths of the periods for the different groups are used as reference lengths in the microscopes. Using the periods as the reference lengths avoids inaccuracies due to uncertainties in defining the edges of individual layers.
The device can be prepared as a cross-sectional TEM sample for accurate layer thickness measurements. When the TEM sample is viewed down a crystal zone axis, a lattice image can be formed. By measuring the atomic lattice spacings of the substrate material, all layers in the sample can be very accurately calibrated against this known natural constant. This provides direct traceability to a constant of nature, with the reference unit length available on the sample itself. SEM or AFM calibration devices can then be made from regions on the wafer where the layer thicknesses are very accurately known through TEM measurements. The very accurate length measurements obtained by using the TEM thickness verification samples are thereby transferred to the SEM and AFM calibration devices.
The approximately 1 micrometer thick surface layer contains periodic ultra-thin highly strained InGaAs layers separated by much thicker GaAs layers. This surface periodic structure provides a strong XRD signal, which provides an additional, alternate reference length for verifying the dimensions of the other periodic structures. This surface periodic structure is visible by TEM, and can be used to confirm and verify the TEM length measurements obtained by reference to the crystal lattice spacing of the substrate.
When the device is aligned perpendicular to an electron beam or a scanning probe, the different periods of the layered structures incorporated into the device allow it to be used to calibrate microscope ranges from ranges displaying sub-nanometer lengths up to ranges displaying lengths of several micrometers. The most valuable attribute of the sample is that all period thickness values are very accurately known for many magnification ranges, and all period thicknesses are directly traceable to a constant of nature, the crystal lattice spacing of the substrate material of the device itself. Another valuable attribute is that by referencing to measurements of multiple periods rather than individual layer measurements, uncertainties in individual layer edge definition are eliminated.
There are several additional specialized functions that the device in accordance with the invention can perform in electron or scanning probe microscopes. When used in conjunction with an x-ray analyzer in an electron microscope, it can aid in the characterization of electron beam size and beam spreading. When used in an AFM, it can aid in the characterization of the scanning tip profile. Although the entire device is described as a GaAs-based single crystal, other suitable substrates may be used, and the multilayer structure grown on the substrate does not necessarily have to be single crystal or epitaxial.
