HK1230135B - Method of marking a solid state material, and solid state materials marked according to such a method - Google Patents

Description

Method for marking solid state material and solid state material marked according to the method

Technical Field

The present invention relates to solid state materials including gemstones and, more particularly, to the marking thereof.

Background Art

Natural diamonds have been used in jewelry for centuries, each diamond being formed throughout the earth's evolution and therefore all unique in nature.

While there are many rules or grading systems for judging and assessing the quality of a particular piece or gemstone, it can be difficult to assess the difference between two diamonds or gemstones with similar gradings. Therefore, it is important to mark diamond gemstones to provide each diamond or gemstone with a unique marking to allow for easy identification and therefore tracking.

When identifying gemstones and grading and analyzing diamond quality, the top view, perpendicular to the top surface of the gemstone or diamond, is used to observe and evaluate the clarity and cut, providing relevant evidence and information. The clarity and cut are certified by international standard laboratory reports, including those from GIA (Gemological Institute of America Inc.), IGI (International Gemological Institute), Gem-A (The Gemmeological Association of Great Britain), and NGTC (National Gemstone Testing Center, China).

From a consumer's perspective, parameters such as a diamond's scintillation brightness are often utilized, such as "brilliance" (the amount of light reflected by the diamond) or "sparkle" (the spreading of light into different colors), which are typically observed or viewed from the top surface of the diamond, as well as the top table.

For both commercial and security purposes, it is important that parameters of a gemstone or diamond (such as parameters of the gemstone or diamond that characterize quality, grading, cut, origin) are associated with the gemstone or diamond.

Due to the wide variation in the value of diamonds or precious stones, and due to incidents of theft and counterfeiting, marking should be performed in such a way that the diamond or precious stone can be identified by a unique identification mark that characterizes the diamond or stone. This identification can be exploited in conjunction with known parameters of the diamond or stone.

Within the prior art, there are two main techniques for marking gemstones and diamonds, which are:

(i) laser marking, and

(ii) FIB (focused ion beam) marking.

Laser marking is limited by the laser spot size, which is typically not suitable for producing fine patterns on diamond surfaces. Laser marking works by absorbing the laser energy when the laser beam reaches the diamond surface, ablating a portion of the diamond and leaving a mark that follows the path of the laser beam. Due to the large heat-affected zone (HAZ) of the laser spot, laser damage to the diamond can occur. While ultrafast lasers have been developed to deliver low pulse energies and high pulse energy densities, resulting in smaller HAZs, such methods still pose a risk of damage when marking diamonds. Furthermore, laser marking techniques typically do not leave a clean surface on the diamond, as ablation can lead to graphite formation, regardless of whether the laser source is an excimer, picosecond, or femtosecond laser. Furthermore, given the relatively large marks lasers can create, the resulting darkened mark on the diamond is highly visible, even to the naked eye.

FIB marking offers several advantages over laser marking. The spot size is 1,000 times smaller than a laser beam, allowing for the marking of surfaces with larger amounts of data. Laser beam marking is typically limited to creating certain letters, characters, and simple logos. Using FIB allows for the engraving of images, logos, Chinese characters, and high-resolution trademarks.

Summary of the Invention

In a first aspect, the present invention provides a method of forming a non-optically detectable identifiable mark on an outer surface of an article formed of a solid material, the method comprising the steps of:

(i) forming a plurality of recesses in predetermined areas of a photoresist applied to an outer surface of an article formed of a solid material, wherein the plurality of recesses are formed by two-photon absorption lithography, and wherein the one or more recesses extend at least partially through the photoresist, from the outer surface of the photoresist, toward the outer surface of the article formed of a solid material;

(ii) applying an etching process such that at least a portion of the outer surface of the article is exposed and etched to form a plurality of etched portions extending from the outer surface of the article into the article and corresponding to the plurality of recesses;

wherein the predetermined area of the photoresist defines an identifiable mark to be applied to the outer surface of the article; wherein the plurality of etched portions form the non-optically identifiable mark on the outer surface of the article; and wherein the maximum width of the etched portions is less than 200 nm, such that the identifiable mark is non-optically detectable in the visible light spectrum.

In an embodiment of the invention, one or more of the recesses of the plurality of recesses extend through the photoresist and provide one or more holes therethrough and provide one or more exposed portions of the outer surface of the article prior to application of the etching process, such that the etched portions corresponding to the one or more holes have substantially the same depth in the article.

In another embodiment of the invention, prior to applying said etching process, said recesses extend through said photoresist at depths that vary relative to each other, such that said etched portions have different depths in said article.

Preferably, the photoresist has a thickness in the range from 10 nm to 500 μm, and the recess has a maximum width in the range from 10 nm to 200 nm or less.

Preferably, the etched portion has a depth in the range of about 5 nm to about 30 nm.

In the predetermined area of the photoresist, the recesses of the plurality of recesses may be arranged in a non-periodic and non-uniform arrangement relative to each other.

The photoresist may have a uniform thickness, or alternatively, the photoresist may have a non-uniform thickness.

The recesses in the plurality of recesses may have the same width, or alternatively, the recesses in the plurality of recesses may have non-uniform widths.One or more recesses are formed from a plurality of adjacent recesses.

The etching process may be a plasma etching process, and may be an aspect ratio determining etch (ARDE) microwave plasma etching process, wherein a radio frequency (RF) bias is applied during the etching process.

The etching process may alternatively be a reactive ion etching (RIE) process, an inductively coupled plasma (ICP) etching process, a focused ion beam (FIB) etching process, or a helium ion microscope (HIM) etching process.

The solid material may be selected from the group consisting of gemstones and may be diamond.

Alternatively, the solid state material may include pearl, silicon, synthetic sapphire, etc.

The solid state material may be a sapphire-based material, and the etching process includes the presence of chlorine gas, boron trichloride (BCl 3 ) gas, or a combination thereof.

In an embodiment of the present invention, the non-optically identifiable markings may be formed on the outer surface of the article in a predetermined spatial arrangement relative to the optically identifiable markings formed on the outer surface of the article, wherein detection of the optically detectable markings allows subsequent detection of the non-optical markings by reference to the predetermined spatial arrangement.

The non-optically identifiable marking may be formed on an outer surface of the article in a predetermined spatial arrangement of optically identifiable properties of the article, wherein the spatial arrangement of the optically identifiable properties of the article allows subsequent detection of the non-optical marking by reference to the predetermined spatial arrangement of the optically identifiable properties of the article.

The identifiable mark is non-optically detectable in the visible light spectrum and observable in the ultraviolet (UV) spectrum, and the identifiable mark may be observable by differential interference contrast (DIC) microscopy, scanning electron microscopy, or the like.

In a second aspect, the present invention provides an article formed of a solid state material having a non-optically detectable identifiable mark thereon, wherein the non-optically detectable identifiable mark is applied to the solid state material by a method according to the first aspect.

The solid material is selected from the group consisting of gemstones and may be diamond.

Optionally, the solid material includes pearl, silicon, synthetic sapphire, etc.

The non-optically detectable identifiable marker is non-optically detectable in the visible light spectrum and observable in the ultraviolet (UV) spectrum, and may be observable by differential interference contrast (DIC) microscopy, scanning electron microscopy (SEM), and the like.

The non-optically identifiable markings may be formed on the outer surface of the article in a predetermined spatial arrangement relative to the optically identifiable markings formed on the outer surface of the article, wherein detection of the optically detectable markings allows subsequent detection of the non-optically identifiable markings by reference to the predetermined spatial arrangement.

The non-optically identifiable marking is formed on an outer surface of the article in a predetermined spatial arrangement of optically identifiable properties of the article, wherein the spatial arrangement of the optically identifiable properties of the article allows subsequent detection of the non-optical marking by reference to the predetermined spatial arrangement of the optically identifiable properties of the article.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments and details of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

FIG1a schematically depicts the effect of aspect ratio determined etching (ARDE) showing the variation of etch depth with aspect ratio;

FIG1 b shows the apparent etch rate variation with aspect ratio;

FIG2 is an SEM image of a photoresist having holes formed by two-photon absorption;

FIG3 is a schematic diagram of a combination of an optically visible mark and a non-optically visible mark according to the present invention;

FIG4 a is a cross-sectional view of a support supporting a diamond, the top surface of which is coated with photoresist and to be marked with an optically identifiable mark and a non-optically identifiable mark, as described with reference to FIG3 , according to the present invention, the cross-sectional view being through line A-A of FIG4 d viewed in the direction of arrow B;

FIG4 b is a cross-sectional view of FIG4 a through line A-A of FIG4 d when a hole is formed through the photoresist to expose a portion of the top surface of the diamond;

FIG4c is a cross-sectional view of FIG4b through line A-A of FIG4d when a recess is formed in the top surface of the diamond;

FIG4d is a top view of the diamond and support of FIG4a, FIG4b and FIG4c after removing the photoresist to reveal the optically identifiable indicia and the non-optically identifiable indicia in accordance with the present invention;

FIG5 a depicts a top view of a diamond and a support according to another embodiment of the present invention;

FIG5 b depicts an enlarged view of a marked portion of the diamond in the embodiment shown in FIG5 a ;

Figure 5c depicts a cross-sectional view along line C-C in the direction E as shown in Figures 5a and 5b using the photoresist after development and before removal; and

FIG5d depicts the cross-sectional view of FIG5c after the photoresist has been removed following etching.

DETAILED DESCRIPTION

The present invention seeks a method of combining two-photon absorption lithography and plasma etching to produce a mark on a solid material including gemstones and these solid materials include solid materials such as diamond, pearl, silicon, sapphire, synthetic sapphire, sapphire-based materials, etc., the mark consisting of a mark that is invisible to visible light, the method being referred to as "invisible marking".

The present invention allows marking an article formed of a solid state material with a non-optically detectable identifiable mark by forming a plurality of recesses in predetermined areas of a photoresist applied to the outer surface of the article using two-photon absorption lithography.

A plurality of recesses extend from the outer surface of the photoresist, and the recesses may extend all the way through the photoresist to the surface of the article to be marked.

In an embodiment of the present invention, a plurality of recesses formed by two-photon absorption lithography may extend through the photoresist to provide holes therethrough and provide corresponding exposed portions of the surface of the solid object to which the marking is to be applied.

In other embodiments of the present invention, a combination of recesses and holes may be provided.

After forming the recesses and/or holes, an etching process is utilized to form a plurality of etched portions on the outer surface of the article extending from the outer surface of the article into the article.

Different etching processes may be utilized, including plasma etching processes such as ARDE microwave plasma etching.

The present invention may be used to mark solid state materials including gemstones, diamonds, pearls, sapphires, synthetic sapphires, silicon or silicon-based materials, and the like.

The predetermined area of the photoresist defines an identifiable mark to be applied to an outer surface of the article, and the plurality of etched portions form a non-optically identifiable mark on the outer surface of the article; and wherein a maximum width of the etched portions is so small that the identifiable mark is non-optically detectable in the visible light spectrum.

Since the labels are not optically detectable in the visible light spectrum, they can be detected by applying ultraviolet (UV) light and observed by differential interference contrast (DIC) microscopy, scanning electron microscopy (SEM), and the like.

Due to the invisibility and small size of the identifiable mark and the associated difficulty in detection, it may be necessary for the reference base surface to be arranged in a known spatial relationship with the invisible mark. This can be provided by other optically detectable marks that do not adversely affect the article to which they are applied. The invisible mark can be separated from, partially overlap, or completely overlap the optically detectable mark. Due to the known relationship between the optically and non-optically identifiable marks, the recognition of the optically identifiable mark allows the non-optically identifiable visible mark to be located and observed using techniques including those discussed above. Alternatively, another example of such a base surface can be a known physical landmark of the article, with the invisible mark being in a known spatial relationship with the known physical landmark for detection and observation.

The marking according to the present invention applied to solid materials including precious stones, gemstones, diamonds, etc. can be used for secure identification, which can include a unique identification of the marked item or a trademark-type mark. This can be used for applications such as anti-counterfeiting purposes, identification purposes for determining whether an item is what it claims to be, and identification purposes in the event of theft or fraud.

The invisible mark according to the present invention, when applied to a solid object such as a precious gemstone, does not necessarily interfere with the optical properties of the object, and the treatment used to apply the mark does not necessarily damage the object or affect the visible properties of the object, thereby having no impact on the value or quality.

By way of background on marking such solid objects, there are generally two types of markings with which solid objects are marked for identification purposes, such as authentication and anti-counterfeiting markings, and these are:

(a) a “visible mark” which can be seen with a magnifying glass or microscope, and

(b) “Invisible markings,” which can be considered to contain hidden information or a hidden message and require detection and observation through other techniques, such as differential interference contrast (DIC) microscopy, in order to be visible. Invisible markings consisting of shadows and small features that can be observed under UV light using DIC microscopy, scanning electron microscopy (SEM), etc., can be utilized.

By way of explanation, DIC microscopy is an optical microscopy illumination technique used to enhance contrast in unstained, transparent specimens or articles. DIC works on the principle of interferometry to obtain information about small optical path length differences in a specimen having such markings, allowing otherwise invisible features to be seen.

For invisible marking designs such as those in the present invention, the Rayleigh criterion needs to be considered. In the case of providing markings that are invisible under a microscope using visible light, the feature size must be smaller than the microscope resolution R. As is known in the art, for maximum resolution, the resolution R follows the rule that the numerical aperture (NA) between the objective lens and the condenser should be as high as possible.

When both NAs are equal, the resolution can be simplified to R = 0.61λ/NA, where λ is the wavelength of the light source. In the visible light spectrum, the best resolution of a microscope is approximately 200nm. Therefore, the feature size of an invisible mark should typically be less than 200nm. In other words, the maximum width of any recess forming an invisible mark should be less than 200nm. To observe these invisible features under a microscope, UV illumination is required.

1a and 1b illustrate an illustrative example of the mechanism of aspect ratio-determined etching (ARDE). Aspect ratio-determined etching (ARDE) refers to a phenomenon whereby etching rates are not proportional to absolute feature size, but rather to aspect ratio. Generally, increasing aspect ratio reduces etching rate due to reduced transport of reactant species in deep and narrow structures.

As shown in Figure 1a, the effect of ARDE is demonstrated and Figure 1b shows the relationship between the apparent etch rate and aspect ratio. It has been shown that this phenomenon is particularly pronounced when the size of the feature is in the range of 0.4 to 20 μm, whereby the etch rate changes by approximately 40%. Therefore, as will be understood, trenches with wide notches have higher etch rates than those with narrow notches.

2 , an example of a photoresist 200 is shown having a plurality of holes 210. In accordance with the present invention, the holes 210 in this example of the present invention are formed by two-photon absorption extending through the photoresist 200. The diameter of the circular holes is approximately 200 nm, a size applicable to the needs of the present invention as claimed and described.

In this example, a Nanoscribe device was used to provide two-photon absorption according to the present invention as shown in FIG2 . This device from Nanoscribe (www.nanoscribe.de) can provide 2D feature sizes as small as 100 nm. Although the holes are shown in FIG2 as being in a uniform arrangement, the present invention does not require periodicity or uniformity of the holes, and thus does not require a concave distribution to generate the mark, and non-uniform or randomly distributed holes are equally applicable.

Features can have a maximum width in the range of 10 nm to 200 nm, and typically, features with a maximum width of 50 nm can be achieved.

3 , there is depicted a schematic diagram of a layout of a combination of optically visible markings 300 and non-optically visible markings 320 that may be applied to the surface of an article such as a gemstone in accordance with the present invention.

In this schematic diagram, the dimensions of the marked patterns are not to scale with respect to the dimensions of the following detailed description of the invention.

Note that this representation of a marking layout is not of photoresist material or marking that has been applied to an article, but rather is a schematic layout for applying both optically and non-optically identifiable markings to an article in accordance with the present embodiment.

The optically visible indicia 300 is shown as being represented by a line of uniform width, while the non-optically visible indicia 320 is represented by multiple or randomly distributed dots.

The letter "M" 300 represents an optically visible mark 300 that is not filled with any pattern etc. In this example, the line width of the logo 300 of the optically visible mark is 5 μm. In an alternative embodiment of the present invention, the optically visible mark may be formed by a periodic structure of small feature size.

The non-optically visible marking 320 is represented by a random distribution of dots in the form of the letter "D" 320, whereby the dots have a diameter of less than 200 nm. The dots filling the letter "D" are invisible to visible light but visible to UV light, making it an invisible marking.

As will be appreciated, although the randomly distributed pattern may be any shape other than circular dots, circular shapes are a preferred shape as the pattern can be easily written by a laser beam.

In this example, to facilitate forming holes through the photoresist by two-photon absorption lithography (such as by a Nanoscribe device), the diameters of the dots are the same, as only one fixed parameter needs to be utilized for writing the holes. As will be appreciated by those skilled in the art, in other or alternative embodiments, the diameters of the holes may be configured to be different from one another, provided the diameters are less than 200 nm.

It should be noted that in this example, both the optically visible marking 300 and the non-optically visible marking 320 are formed in association. Because the non-optically visible marking 320 is effectively "invisible" and extremely small relative to the surface of the article to which it is applied, locating the non-optically visible marking 320 without any reference point would be very difficult.

Thus, in this example, the optically visible marker 300 is used in conjunction with the non-optically visible marker 320 , and detection of the optically visible marker 300 may indicate the location of the non-optically visible marker 320 due to the known spatial relationship between the two markers.

As mentioned above, the proportions and sizes of the optically visible indicia 300 and the non-optically visible indicia 320 are not to scale and are shown as being of the same size and immediately adjacent to one another as arbitrary illustrative parameters. As should be understood, in other and alternative embodiments, the optically visible indicia 300 and the non-optically visible indicia 320 may be of different sizes, may be spaced differently, and in some cases, may overlap one another.

Additionally, as must be understood, although letters have been used in this example of optically visible marking 300 and non-optically visible marking 320, other symbols representing data may be utilized. In this example, optically visible marking 300 is shown as an example of how non-optically visible marking 320 may be located on an article.

In other examples, the non-optically identifiable markings 320 can be positioned on an exterior surface of an article in a predetermined spatial arrangement with optically identifiable properties of the article, and the spatial arrangement with the optically identifiable properties of the article can allow subsequent detection of the non-optical markings 320 by reference to the predetermined spatial arrangement with respect to the optically identifiable properties of the article.

For example, an optically identifiable attribute may be a feature or landmark on an item (such as a corner, protrusion, or vertex of a facet of a gemstone, etc.) However, as will be appreciated and understood by those skilled in the art, there are many such identifiable attributes, and the predetermined spatial arrangement may be associated with two or more attributes.

4a, 4b, 4c and 4d, schematic diagrams of applying optically identifiable markings and non-optically identifiable markings to an article are depicted in accordance with the present invention.

In the examples depicted and described with reference to Figures 4a, 4b, 4c, and 4d, a combination of the optically visible marking 300 and the non-optically visible marking 320 of Figure 3 is applied to an article.

4a, 4b and 4c are cross-sectional views through line A-A of Fig. 4d viewed in the direction of arrow B. Reference is made to the optically visible marking 300 and the non-optically visible marking 320 as described with reference to the references, whereby the representation of Fig. 3 extends into the page.

4a, a schematic representation is shown in cross-section, whereby a support 400 is depicted on which an article is supported, in the case where the optically visible marking 300 and the non-optically visible marking 320 as described with reference to FIG. 3 are to be applied to a diamond 410.

As shown, before forming holes as discussed above according to the present invention, diamond 410 is fixed in a holder 400 by performing pattern writing using two-photon absorption lithography using an apparatus such as the Nanoscribe apparatus described above, after which the surface of diamond 410 is coated with a thick photoresist 420 having a thickness of, for example, 2 μm. In the following description, 2 μm is used instead of 10 μm, although 100 μm can also be used for pattern writing according to the present invention.

For such a process of providing a hole through the photoresist, in order to utilize the relevant parameters for the correct exposure dose, in this example, there are 5 main parameters to adjust, including:

(i) Scanning speed.

(ii) laser power,

(iii) 3D pixel distance,

(iv) the number of voxels, and

(v) z offset.

For a 2 μm thick photoresist, typically one voxel is sufficient for writing through the photoresist.In this example, the number of voxels and the voxel distance do not have to be considered.

Furthermore, because the photoresist is thin enough, the voxel z-offset can be set to zero. Thus, in this example of the invention, only two parameters need to be considered, and thus the optimal combination of scanning speed and laser power can be easily determined.

In this example, the cross-sectional views of FIG. 4 b and FIG. 4 c are in an orientation such that the combined representation of the optically visible indicia 300 and non-optically visible indicia 320 of FIG. 3 extending into the page extends into the surface 412 of the diamond 410 .

As shown in FIG4 b , for example, when diamond 410 coated with photoresist 420 is processed using two-photon absorption lithography and developed in a developer, diamond surface 412 is exposed in the region of “M” 422 and the hole of “D” 424 indicated in FIG3 . In this example, the aspect ratio of “M” 422 is 2 μm/10 μm=0.5, and for the hole of “D” 424, it is 2 μm/200 nm=10.

4 c , the aspect ratio difference is high enough to produce a difference in etch depth in the upper surface 412 of the diamond 410 adjacent to the hole 422 and the hole 424 during the following plasma etching process, so as to form an optically visible mark relative to M and a non-optically visible mark relative to D, thereby forming a recess 414 in the surface 412 of the diamond 410 adjacent to the hole 422, and thereby forming a recess 416 in the surface 412 of the diamond 410 adjacent to the hole 424.

During two-photon absorption lithography, a laser is focused into the photoresist and scanned layer by layer. A shutter is opened and the laser beam is held for a sufficient time, typically hundreds of microseconds, in the area of the photoresist to be exposed.

5a to 5d , other embodiments of the present invention are shown. As shown, a diamond 510 coated with photoresist 520 is disposed in a holder 500 with reference to the embodiment marked in region “M”.

In this embodiment, the processing and description with reference to Figures 4a to 4d may be considered applicable to mark "D", however, the processing and description with reference to Figures 4a to 4d may not be considered applicable to the present embodiment described in this article with reference to Figures 5a to 5d.

In this embodiment, two-photon absorption lithography is used to generate a 3D pattern in photoresist 520, as shown in Figure 5c. As shown, this adjustable aspect ratio is achieved throughout the pattern design by varying the etch depth and trench depth 522 in photoresist 520 along the C-C line of the "M" pattern.

In this embodiment, in order to have a sufficiently high aspect ratio difference, the diamond surface 512 is coated with a photoresist with a thickness of, for example, 10 μm.

During the two-photon absorption lithography process, the laser focus will be focused into the photoresist 520 and scanned layer by layer. In the area of the photoresist to be exposed, the shutter will open and the laser beam will stay for a long enough time, usually hundreds of microseconds.

Thus, it can be appreciated from the present invention that the thickness of the exposed photoresist varies according to the design, resulting in different depths of the recesses in the photoresist to provide different aspect ratios in the photoresist 520. As shown, the line width is the same along "M", but the trench depths of the recesses are different, and therefore, the aspect ratios along "M" are different, as depicted in the figures.

After development as shown in FIG. 5 c , the aspect ratio of “M” changes from 2 μm/5 μm=0.4 to 10 μm/5 μm=2 in this example.

As shown in FIG5 d , a cross-sectional view of a localized region of an etched diamond 510 after etching along a cross-sectional line C-C viewed in direction E is shown, and the varying heights of the pattern of etched portions 514, corresponding to recesses in the photoresist 520 extending into the surface 512 of the diamond 510, can be observed using suitable observation methods and equipment (e.g., under a DIC microscope, SEM, etc.). This provides an invisible marking or hidden message that is undetectable to visible light. Depending on the depth of the etching, observation under UV light may require a DIC microscope to enhance contrast.

With reference to plasma etching, types of etching include RIE or ICP etching. With reference to this example of the invention and as used with reference to Figure 2, the preferred etching process is microwave plasma etching, and with reference to suitable techniques utilized in the invention above, this technique includes that from Muegge (www.muegge.de).

This technology consists of a microwave plasma source and an RF power source. The RF power source is used to accelerate the ions generated by the microwave plasma source. The plasma generated by this technology is cold plasma, with a temperature significantly lower than the reaction temperature between ions and diamond. However, the ions can react with photoresist.

This technique provides the following advantages: In embodiments of the present invention, the exposed diamond surface can be cleaned in the machine before RF power is turned on to etch the diamond. Any possible photoresist residue on the exposed areas of the diamond surface or from subsequent etching after development and hole formation can be completely removed without damaging the diamond itself, thereby mitigating any defects caused by the photolithographic process.

The cleaning process uses nitrogen, oxygen, or CF4 at pressures exceeding 150 mT. Once the photoresist residue on the exposed areas has been completely removed, RF power may be subsequently turned on, after which the accelerated ions begin attacking the diamond surface, creating the necessary recesses and thus forming the mark. Typically, a 30nm etch depth is achieved within 4 minutes, with variations of less than 10nm due to design-dependent aspect ratio differences.

When the diamond is etched to the necessary depth, the RF power is turned off again and the cold plasma removes all photoresist on the diamond surface, resulting in a clean marking surface after the entire process, as shown and described with reference to above.

According to the present invention, for feature sizes of 200 nm, UV lithography is impractical and can only be produced by other methods including electron beam lithography, X-ray lithography, laser interference lithography, or two-photon absorption lithography. Electron beam lithography is generally considered too expensive and too slow for industry. As for X-ray lithography, it relies too much on limited X-ray sources that can only be produced by synchrotron radiation, and is therefore also too expensive.

Laser interference lithography generally does not allow for easy casting of the desired pattern into the photoresist due to the need for a shadow mask, which increases the complexity of the manufacturing process. Furthermore, the patterns generated by laser interference are always periodic, providing a pattern that appears as a grating structure, regardless of the period. This, in turn, results in marks that diffract visible light, rendering the pattern itself visible, provided the illumination is long enough. Thus, in contrast to the present invention, laser interference lithography does not allow for the creation of optically invisible marks.

Compared to the laser interference lithography and two-photon absorption lithography utilized in the present invention, a two-photon polymerization process is used that can write 3D features into both positive and negative photoresists. Therefore, it is practical to write small patterns such as dots or other shapes that are smaller than 200 nm and randomly distributed in a predetermined area, allowing the formation of invisible marks according to the present invention.

Two-photon absorption techniques, such as those used by Nanoscribe (www.nanoscribe.de), allow for the formation of features down to 100 nm in size, providing a suitable method for transferring invisible marking patterns to photoresist. Furthermore, the two-photon absorption utilized in the present invention provides write speeds of up to several cm/s, such as those provided by Nanoscribe devices. This high write speed allows for the direct writing of visible markings using two-photon absorption lithography according to the present invention. Thus, depending on the complexity of the overall pattern, the entire pattern can be written directly onto the photoresist layer by layer with a single laser beam scan in a time range of seconds to minutes.

While plasma etching may be considered relatively mature due to developments in the IC industry, as demonstrated with respect to the present invention, this technique may be used with diamond as well as silicon and other polymers.

Plasma etching is aspect ratio determined etching (ARDE). ARDE refers to the phenomenon where the etch rate is not proportional to the absolute feature size, but rather to the aspect ratio. Increasing the aspect ratio generally decreases the etch rate due to reduced transport of reactants in deep and narrow structures.

In addition to providing general information about the item itself, in embodiments of the present invention, the introduction of optically visible markings also provides an aid in locating the small invisible markings of the present invention on the surface of a relatively large diamond. Without the visible markings, locating the invisible markings would be very difficult.

In addition to the advantages provided by the present invention, the present invention provides the following:

(i) a mark that is aesthetically pleasing and cannot be readily observed without knowing the specific parameters of observation or the location of the mark;

(ii) markings which, when applied to articles such as precious stones or gemstones, allow identification for security purposes and tracking and tracing of the articles;

(iii) security purposes, which may be used to mitigate or detect counterfeiting and misconduct, including theft;

(iv) marking solid state materials without the disadvantages associated with destructive and aggressive methods of marking (such as etching, ablation, grinding, engraving, etc.);

(v) methods and products thereof that do not alter the optical properties or characteristics of the solid state material and are not detrimental to the clarity or color of the solid state material;

(vi) methods and products thereof that do not introduce contamination or impurities into the solid state material;

(vii) methods and products thereof that do not require substantial removal of material from the surface of the solid state material; and

(viii) Methods and products thereof, which have no associated chemical residues.

Plasma etching is a well-known technique due to the evolution of the integrated circuit (IC) industry. In a typical reactive ion etching (RIE) process, a large number of ions are generated and accelerated toward a target, resulting in the physical removal of material through sputtering and related processes. This process is known to have low selectivity.

Compared to RIE, inductively coupled plasma (ICP) etching is a large-scale chemical process in which a plasma is used to break down the etching gas into a mixture of radicals (i.e., neutral species) and ions (i.e., charged species). ICP etching is a large-scale chemical etching process, rather than a physical ablation process like RIE, and therefore can be considered to offer higher selectivity.

While RIE and ICP are distinct etching technologies, they share the concept of aspect ratio-determined etching (ARDE). ARDE refers to the concept where the etch rate scales not with absolute feature size but with aspect ratio. Increasing aspect ratio generally decreases etch rate due to reduced transport of reactants in deep, narrow structures.

For diamond marking, the etching depth is usually very small, usually in the range of 10nm to 50nm. In order to provide a fine etching pattern, a protective layer is usually generated by using UV lithography using a photoresist.

There are two primary methods for applying photoresist to diamond surfaces: spin coating and spray coating. Regardless of the method used, the thickness of the photoresist layer is generally uniform across the diamond surface, due to the relatively small surface area of diamond, typically only a few square millimeters. Photoresist thickness typically ranges from a few microns, hundreds of times the range of etch depth. Thus, for pattern designs with marks of varying line widths, aspect ratio variations are primarily a result of the photoresist thickness across the entire pattern line width.

According to experimental results related to the present invention, for a given design with line widths ranging from 200nm to 10μm, the etch depth difference during plasma etching with 2μm photoresist protection is quite high. When the 10μm feature is etched to a depth of 30nm, the etch depth of the 200nm feature is only about 5nm.

Claims (36)

1. A method for forming a non-optically detectable identifiable mark on the outer surface of an article formed of a solid material, the method comprising the steps of: (i) Forming a plurality of recesses within a predetermined area of a photoresist applied to the outer surface of an article formed of a solid material. The plurality of recesses are formed using two-photon absorption lithography, and One or more recesses extend at least partially through the photoresist and from the outer surface of the photoresist and toward the outer surface of the article; and (ii) Applying an etching process to the photoresist, such that at least a portion of the outer surface of the article is exposed and further etched to form a plurality of etched portions extending from the outer surface of the article into the article, wherein the plurality of etched portions correspond to the plurality of recesses formed in the predetermined region of the photoresist; The plurality of recesses formed within the predetermined area of the photoresist define non-optically detectable identifiable marks to be applied to the outer surface of the article; The plurality of etched portions extending into the article form the non-optically detectable identifiable mark on the outer surface of the article after the application; and The maximum width of the etched portion is less than 200 nm, such that the non-optically detectable identifiable mark is not optically detectable in the visible light spectrum. 2. The method according to claim 1, wherein, One or more of the plurality of recesses extend through the photoresist, thereby providing one or more through-holes therethrough. One or more exposed portions of the outer surface of the article are provided prior to the application of the etching process, such that the etched portions corresponding to the one or more holes have approximately the same depth in the article. 3. The method according to claim 1, wherein, Prior to the application of the etching process, the recesses extend through the photoresist at different depths relative to each other, such that the etched portions have different depths in the article. 4. The method according to any one of claims 1 to 3, wherein, The photoresist has a thickness ranging from 10 nm to 500 μm. 5. The method according to claim 1, wherein, The recess has a maximum width ranging from 10 nm to 200 nm. 6. The method according to claim 1, wherein, The etched portion has a depth in the range of 5 nm to 30 nm. 7. The method according to claim 1, wherein, Within the predetermined area of the photoresist, the recesses among the plurality of recesses are arranged in an aperiodic and non-uniform manner relative to each other. 8. The method according to claim 1, wherein, The photoresist has a uniform thickness. 9. The method according to claim 1, wherein, The photoresist has a non-uniform thickness. 10. The method according to claim 1, wherein, The recesses in the plurality of recesses are of the same width. 11. The method according to claim 1, wherein, The recesses among the plurality of recesses have non-uniform widths. 12. The method according to claim 1, wherein, One or more recesses are formed by multiple adjacent recesses. 13. The method according to claim 1, wherein, The etching process is a plasma etching process. 14. The method according to claim 1, wherein, The etching process is aspect ratio determined etching (ARDE) microwave plasma etching. 15. The method according to claim 14, wherein, A radio frequency (RF) bias is applied during the etching process. 16. The method according to claim 1, wherein, The etching process is reactive ion etching (RIE). 17. The method according to claim 1, wherein, The etching process is an inductively coupled plasma (ICP) etching process. 18. The method according to claim 1, wherein, The etching process is a focused ion beam (FIB) etching process. 19. The method according to claim 1, wherein, The etching process is a helium ion microscopy (HIM) etching process. 20. The method of claim 1, wherein, The solid material is selected from the group including gemstones. 21. The method according to claim 1, wherein, The solid material is diamond. 22. The method according to claim 1, wherein, The solid material includes pearl, silicon, or synthetic sapphire. 23. The method according to claim 1, wherein, The solid material is based on sapphire. 24. The method according to claim 23, wherein, The etching process includes the presence of chlorine gas, boron trichloride ( _BCl3 ) gas, or a combination thereof. 25. The method according to claim 1, In this context, in contrast to optically detectable identifiable marks formed on the outer surface of the article, non-optically detectable identifiable marks are arranged in a predetermined space on the outer surface of the article. The detection of the optically detectable identifiable markers allows for the subsequent detection of the non-optically detectable identifiable markers by referring to the predetermined spatial arrangement. 26. The method according to claim 1, Specifically, on the outer surface of the article, the non-optically detectable identifiable mark is formed by arranging the marks in a predetermined space, each having optically identifiable properties of the article. The spatial arrangement of the optically identifiable attributes of the article allows for subsequent detection of the non-optically detectable identifiable mark by referring to the predetermined spatial arrangement of the optically identifiable attributes of the article. 27. The method according to claim 1, wherein, The non-optically detectable identifiable marker is non-optically detectable in the visible light spectrum and observable in the ultraviolet (UV) spectrum. 28. The method according to claim 27, wherein, The non-optically detectable identifiable markings can be observed using differential interference contrast (DIC) microscopy or scanning electron microscopy (SEM). 29. An article formed of a solid material, wherein the article has a non-optically detectable identifiable mark, wherein, The non-optically detectable identifiable mark is applied to the solid material by the method according to any one of claims 1 to 20. 30. The article according to claim 29, wherein, The solid material is selected from the group including gemstones. 31. The article according to claim 29 or claim 30, wherein, The solid material is diamond. 32. The article according to claim 29 or claim 30, wherein, The solid material includes pearl, silicon, or synthetic sapphire. 33. The article according to claim 29, wherein, The non-optically detectable identifiable marker is non-optically detectable in the visible light spectrum and observable in the ultraviolet (UV) spectrum. 34. The article according to claim 33, wherein, The non-optically detectable identifiable markings can be observed by differential interference contrast (DIC) microscopy or by scanning electron microscopy (SEM). 35. The article according to claim 29, In contrast to optically detectable identifiable marks formed on the outer surface of the article, non-optically identifiable marks are formed by arranging them in a predetermined space on the outer surface of the article. The detection of the optically detectable identifiable markers allows for the subsequent detection of the non-optically detectable identifiable markers by referring to the predetermined spatial arrangement. 36. The article according to claim 29, Specifically, on the outer surface of the article, the non-optically detectable identifiable mark is formed by arranging the marks in a predetermined space, each having optically identifiable properties of the article. The spatial arrangement of the optically identifiable attributes of the article allows for subsequent detection of the non-optically detectable identifiable mark by referring to the predetermined spatial arrangement of the optically identifiable attributes of the article.