A Method of Machining Brittle Materials

Abstract

A method of cutting (e.g. micro-milling) brittle materials where a protective ductile layer is present on the brittle material during the cutting step. The method offers an improved edge quality of the machined profile relative to previous cutting (e.g. micro-milling) techniques. A machined product obtained by or obtainable by this method is disclosed.

Claims

1. A method of cutting a channel in a brittle material comprising: providing a composite material in which a layer of ductile material is arranged on a surface of a brittle material; and cutting a channel through the layer of ductile material, removing the portion of the ductile material that is arranged over the portion of the brittle material that is to be cut, and into the brittle material to provide a machined composite.

2. The method of claim 1, wherein the layer of ductile material is in direct contact with the surface of the brittle material.

3. The method of claim 1, wherein providing the composite material comprises applying a layer of a ductile material to the surface of a brittle material to provide the composite material.

4. The method of claim 1 further comprising removing the ductile material from the machined composite to provide a machined sample of the brittle material.

5. The method of claim 1, wherein at least a portion of the channel is arcuate in a plane parallel to the surface of the brittle material.

6. The method of claim 1, wherein the channel comprises a plurality of curves and/or angles in a plane parallel to the surface of the brittle material.

7. The method of claim 1, wherein the ductile material is selected from the group consisting of aluminum, chromium, copper and alloys thereof.

8. The method of claim 1, wherein the ductile material is an acrylic polymer.

9. The method of claim 1, wherein the ductile material is an epoxy resin.

10. The method of claim 1, wherein the brittle material is from 50 m to 100 mm thick.

11. The method of claim 1, wherein the channel that is cut into the brittle material is from 5 m to 500 m deep.

12. The method of claim 11, wherein the channel that is cut into the brittle material is from 100 m to 200 m deep.

13. The method of claim 1, wherein the channel that is cut into the brittle material is from 50 m to 1000 m wide.

14. The method of claim 1, wherein the layer of ductile material is from 25 to 500 m thick.

15. The method of claim 1, wherein the brittle material is selected from the group consisting of silicon, germanium, sapphire and gallium nitride.

16. The method of claim 15, wherein the brittle material is monocrystalline silicon.

17. The method of claim 16, wherein the channel is cut along the <100> direction of the crystal.

18. A method of milling a channel in a brittle material comprising: providing a composite material in which a layer of ductile material is arranged on a surface of a brittle material; and milling a channel through the layer of ductile material, removing the portion of the ductile material that is arranged over the portion of the brittle material that is to be milled, and into the brittle material to provide a machined composite.

19. A machined product obtained by by the methods of claim 1.

20. The method of claim 18, wherein: providing the composite material in which the layer of ductile material is arranged on the surface of the brittle material comprises providing the composite material in which the layer of ductile material from 25 m to 500 m thick selected from the group consisting of aluminum, chromium, copper and alloys thereof is arranged in direct contact on the surface of the brittle material selected from the group consisting of silicon, germanium, sapphire and gallium nitride; and milling the channel through the layer of ductile material, removing the portion of the ductile material that is arranged over the portion of the brittle material that is to be milled, and into the brittle material to provide the machined composite comprises milling the channel through the layer of ductile material, removing the portion of the ductile material that is arranged over the portion of the brittle material that is to be milled, and into the brittle material to provide the machined composite having the portion of the channel milled into the brittle material being from at least 5 m deep, and from 50 m to 1000 m wide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

[0070] FIG. 1 shows a schematic of an exemplary method of the invention.

[0071] FIG. 2 shows an exemplary micro-milling system that can be used to carry out the methods of the invention.

[0072] FIG. 3 shows annotated scanning electron microscopy (SEM) images of machined profiles for the micro-milling of silicon using i) a copper ductile material on a titanium adhesive layer; ii) no ductile material; and iii) an acrylic paint ductile layer.

[0073] FIG. 4 shows the surface morphologies of machined samples obtained using various ductile materials and cutting tools.

[0074] FIG. 5 shows the surface morphologies of machined silicon samples obtained using a PMMA ductile layer with various coating thicknesses.

[0075] FIG. 6 shows the surface morphologies of machined silicon samples obtained using PMMA ductile layer under various machining orientations and total machined depths.

DETAILED DESCRIPTION

[0076] A brittle material as referred to in this specification is one having a fracture toughness (K.sub.IC), the ability of a material to resist crack propagation, of up to 10 MPam. Certain materials of particular interest have a fracture toughness (K.sub.IC) of up to 5 MPam or even up to 3 MPam.

[0077] A ductile material, as referred to in this specification, is one having an elongation ratio in 50 mm of 5% or greater (i.e. the % increase in length of a 50 mm sample that can be tolerated when the sample is subjected to strain). Certain materials of particular interest have an elongation in 50 mm of 10% or greater or even 25% or greater.

[0078] Where a material is described as an alloy of a metallic element, e.g. copper, it is intended to mean that the alloy comprises that metallic element. It may mean that the alloy contains greater than 50% by weight of the indicated metallic element.

[0079] Exemplary milling machines that can be used to perform the methods of the invention include: Nanoware MTS5R, Minitech Mini-Mill/GX, Kern Evo and Kugler Micromaster3/5X.

[0080] Throughout the description and claims of this specification, the words comprise and contain and variations of them mean including but not limited to, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0081] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0082] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Example 1: Initial Study on Reduction of Edge Chipping

[0083] Experimental Method

[0084] Summary of Edge Chipping Reduction Strategy

[0085] Reducing the generation of edge chipping is important in silicon micro-milling. Chipping presence may lead to functional failure, as edge chipping is formed by a surface micro pitting and cracking. The inventors have deposited ductile materials onto the top surface of silicon to reduce edge cracking during machining. This may help to create an energy buffer that absorbs the impact energy between the cutting tool and the workpiece surface during contact. It is expected that similar results will be observed for other brittle materials.

[0086] Ductile materials, such as copper and acrylic based paint were chosen. Both materials can be easily coated onto silicon. FIG. 1 shows the schematic of an exemplary edge chipping reduction method, illustrating various processing stages in the methods of the invention.

[0087] Prior to machining, layers of copper or acrylic based paint of approximately 90 m in thickness were respectively electroplated or manually coated as an emulsion paint onto the surface of a (001) surface silicon wafer (Step 1). For the copper on silicon workpiece, a thin sheet of titanium (not shown) at approximately 50 nm thick was used to adhere the copper onto the silicon surface to prevent any delamination. Upon completion of the respective surface ductile layer coating processes, slot milling was conducted on the surfaces of each workpiece to create a channel (Step 2). Machining begins on the ductile layer and continues into the silicon substrate (Step 3). The cutting force was constantly monitored during the milling process. A small change in the cutting force was noticed when the depth of the cutting tool reached the interface between the ductile layer and silicon. With the presence of this phenomenon, the location of the surface of the silicon substrate was determined and this marked the starting point for determining the depth of channel to be milled into the silicon. Finally, nitric or hydrofluoric acid solutions were used to remove the electroplated copper layer and the titanium adhesive layer, while acetone was used for the removal of acrylic based paint after machining (Steps 4 and 5).

[0088] Set-Up of Machining System

[0089] The experiments described above were conducted on the Nanowave MTSSR micro-milling system shown in FIG. 2. It comprises of a three axes motion stage with feed precision of 0.1 m, an air bearing spindle, a workpiece holder and Kistler dynamometer. A 0.5 mm diameter single crystal diamond end-mill with single flute was fitted onto the spindle. The spindle was mounted along the Z axis (perpendicular to the surface of the brittle material) of the motion stage. Furthermore, movement of the motion stage along the X or Y axes (parallel to the surface of the brittle material), actuates the workpiece holder and dynamometer for cutting force measurement during machining.

[0090] Experimental Procedure

[0091] Conditions to achieve good machining quality include the choice of main machining parameters namely the channel depth, feed rate and spindle speed, the tool rotation direction (i.e. up-milling or down-milling, in which the spindle axis of the milling cutter is moved respectively in the same or the opposite direction from the cutting edges that contact the brittle material). Machining along <100> directions on a {100} surface silicon wafer was preferred. This was due to the <100> directions having higher fracture toughness as compared to other orientations. Table 1 shows the optimal range and also the employed machining parameters for the comparative examples described below. The optimal ranges were not used for the comparative examples described below in order to demonstrate the efficacy of the methods of the invention at reducing edge chipping even in sub-optimal conditions.

TABLE-US-00001 TABLE 1 Machining conditions for silicon micro-milling, investigations Cutting Speed Feed per Tooth V.sub.c, fz, (m/tooth) (m/min) Orientation Ideal Machining Parameters <0.2 31.42 <100> 78.54 Optimal Machining Parameters 0.075 31.42 <100> 78.54 Machining Parameters Employed in Comparative Examples 0.15 78.54 <100>

[0092] Machining was carried out on the two workpieces with ductile layers to achieve several different depths of channel, ranging between 30 and 150 m. Similar machining was also conducted on an uncoated silicon workpiece for comparison purposes. The depth of channel mentioned above refers to the distance from the base of the machined channel to the silicon surface. For the coated workpieces, machining began on the copper or polymer surface and continued into the silicon substrate. The cutting force was constantly monitored during the milling process. A small change in the cutting force was noticed when the depth of the cutting tool reaches the interfacing layer between the ductile material and silicon. Thus, the location of the surface of the silicon substrate was determined and this marked the starting point for determining the depth of channel to be milled into the silicon.

[0093] After machining, the respective workpieces were detached from the grounded metal backing and cleaned with acetone and by exposing to ultrasonic water bath. For example, Nitric and hydrofluoric acid solutions were used to remove the deposited copper and the titanium adhesive sheet from the copper coated workpiece. The acetone wash removed the acrylic paint from the paint coated workpiece. Finally, the scale of edge chipping generated on the final machined surfaces were quantitatively and qualitatively characterized using a table top Hitachi TM3030 SEM system.

[0094] Results and Discussion

[0095] FIG. 3 shows the SEM images of the final machined surfaces on three silicon workpieces: i) one which had been coated with a copper ductile material on a titanium adhesive layer during the machining; ii) one coated with no ductile material during the machining; and iii) one which had been coated with an acrylic paint (PMMA) ductile layer during the machining.

[0096] The amount of edge chipping that can be measured is an average length. The measurement is performed on the scanning electron microscopy (SEM) images of each machined profile. Measurement was taken on the up-milling side of each machined profile.

[0097] Edge chipping was quantitatively characterized by taking an average length measurement on the chipped surface across fifty equally spaced data points along the vertical image view of the machined channel. The results are provided in Table 2.

TABLE-US-00002 TABLE 2 Average length of edge chipping for milling of copper/adhesive coated and acrylic coated silicon Channel Average Length of Edge Average Length of Edge Depth Chipping for Copper Coated Chipping for (m) Workpiece (m) Acrylic Coated Workpiece (m) 30 0 0 50 0 0 100 80 10

[0098] Thus, with both acrylic paint and the copper/titanium adhesive system an absence of edge chipping was observed at a channel depth of 50 m. In contrast, the silicon that had not had a ductile layer present during machining showed significant amounts of edge chipping for 30 m and 50 m deep channels. Additionally, the edge chipping generated for a 100 m deep channel for the silicon workpiece machined with an acrylic based paint layer was significantly smaller than the workpiece machined with a copper coating and a titanium adhesive layer.

Example 2 Further Study on the Reduction of Edge Chipping

[0099] In a further machinability study, a copper coated silicon sample (copper was electroplated onto silicon as in Example 1), alongside PMMA and SU-8 coated silicon samples were machined and compared with uncoated silicon sample. In this study both PMMA and SU-8 were spin coated onto silicon.

[0100] Machining was conducted with both single crystal diamond (SCD) and TiAlN coated tools respectively. FIG. 4 depicts the comparison on the capability of different sacrificial materials and the cutting tool. At such a deep machining depth of 150 m, it was evident from FIG. 4 that minimal edge chipping and surface defects can be achieved when machined with SCD alongside with either PMMA or SU-8 as the sacrificial layer.

[0101] The coating thickness was varied between 20 m to 80 m. FIG. 5 shows the surface morphologies of post-machined micro-grooves of PMMA coated silicon previously deposited with different coating thickness. It was also evident that with the surface edges protected by the sacrificial layer material, variation on the size of generated edge chipping was minimal with the increment of coating thickness.

[0102] Previous work has shown that the orientation of the silicon when being machined can affect the amount of edge chipping. FIG. 6 shows the surface morphologies of post-machined silicon micro-grooves under different machining orientation at various total machining depths. In fact, minimal variation and generation of edge chipping was observed at different machining orientation and total machining depths. Furthermore, similar behaviour was also observed under the grooves machined by both single crystal diamond (SCD) and TiAlN coated tools, although the SCD tool was shown to be considerably better than the TiAlN tool.

[0103] The results of the study are summarised quantitatively in Table 3.

TABLE-US-00003 TABLE 3 Measured edge chipping length of various workpiece machined by SCD and TiAlN coated tools Channel Single Crystal Diamond Tool TiAlN Coated Tool Depth Copper PMMA SU-8 Uncoated Copper PMMA SU-8 Uncoated (m) (m) (m) (m) (m) (m) (m) (m) (m) 10 0 0 0 10 0 0 0 5 50 0 0 0 55 35 5 20 30 150 30 0 0 120 20 10 55 90

Example 3Work of Adhesion

[0104] There are countless methods to measure the adhesion strength of a coating, or to be specific in this context, the adhesion strength between the ductile sacrificial material and the brittle substrate material. In one method, adhesion strength was quantatively characterized through the measurement of surface wettability. Wettability, oftenly defined as the tendency on the spreading or adhesion of one fluid to a solid surface in the presence of other immiscible fluids. Such measurement can be conducted through a process known as contact angle measurement. The process involves depositing a droplet of liquid onto the surface of interest, while using a goniometer to measure the contact angle of the liquid droplet as it spreads along the surface. At least two liquid with known surface tension are commonly used in such studies. In the current study, de-ionized water (DI-H.sub.2O) and 100% glycerine were chosen. Measurements were respectively conducted on the surfaces of copper with titanium seed layer (Cu/Ti), PMMA, SU-8 and monocrystalline silicon substrate.

[0105] Upon obtaining the relevant contact angles with respect to each tested surfaces, both the work of adhesion and interfacial tension indicating the work required to separate two surfaces and the adhesion strength respectively can be calculated by a harmonic mean model as follows:

[0106] The harmonic mean model used to approximate the interaction of low energetic phases with the polar and non-polar parts of the interfacial energy and therefore evaluate the work of adhesion W.sub.Is:

[00001]$\begin{array}{cc}{W}_{\mathrm{ls}}=4\left(\frac{{}_l^d{}_{\text{?}}^d}{{}_l^d+{}_s^d}+\frac{{}_l^p{}_{\text{?}}^p}{{}_l^p+{}_s^p}\right).Math..Math.\text{?}.Math.\text{indicates text missing or illegible when filed}.Math.& \left(1\right)\end{array}$

[0107] where:

[0108] .sub.l.sup.d.fwdarw.non-polar part of the layer surface tension

[0109] .sub.l.sup.d.fwdarw.polar part of the layer surface tension

[0110] .sub.S.sup.d.fwdarw.non-polar part of the substrate surface tension

[0111] .sub.S.sup.p.fwdarw.polar part of the substrate surface tension

[0112] Furthermore, the work of adhesion necessary to separate a liquid layer from a solid substrate in an air environment can also be deduced by the Dupre equation:

W.sub.Is=(1+cos ).sub.l(2)

[0113] where:

[0114] .fwdarw.measured contact angle between the liquid layer and solid substrate

[0115] .sub.l total surface tension of layer comprising the non-polar (.sub.l.sup.d) and polar part (.sub.l.sup.p)

[0116] The contact angle of the two materials can be measured using deionized water (DI-H2O) and glycerine respectively. A number of data points are taken and the mean contact angle across those data points was determined.

[0117] Thus, the mean contact angle for copper-titanium seed layer, PMMA, SU-8 and silicon surface respectively were determined under both deionized-water and glycerine conditions. After which, the work of adhesion and interfacial tension in air environment were computed using the combination of Equation 1 and 2. Tables 3, 4 and 5 shows the mean contact angle, computed superficial surface tension of each material and the computed work of adhesion alongside with the interfacial tension respectively. Table 5 provides the calculated work of adhesion for SU-8, PMMA and copper.

[0118] Such phenomenon corroborates with the superior mechanical properties of SU-8 over both PMMA and copper used in the study.

TABLE-US-00004 TABLE 4 Mean of measured contact angle Mean Contact Angle () Mean Contact Angle () Materials (DI-H2O) (Glycerine) Cu/Ti 80.2 70.8 PMMA 75.4 65.4 SU-8 64.6 54.4 Silicon 70.6 45.4

TABLE-US-00005 TABLE 5 Calculated superficial surface tension of each materials Superficial Tension, Non-Polar, .sup.d Polar, .sup.p Materials (mJ/m.sup.2) Component (mJ/m.sup.2) Component (mJ/m.sup.2) Cu/Ti 31.8 16.0 15.8 PMMA 35.1 17.5 17.6 SU-8 42.4 19.9 22.5 Silicon 48.6 34.8 13.8 Dl-H.sub.2O 72.8 21.8 51 Glycerine 63.4 37 26.4

TABLE-US-00006 TABLE 6 Calculated work of adhesion and interfacial tension in air environment Work Materials in Contact of Adhesion (mJ/m.sup.2) Interfacial Tension (mJ/m.sup.2) Cu/Ti on Silicon 73.3 7.1 PMMA on Silicon 77.5 6.2 SU-8 on Silicon 84.9 6.1