Patent Application
20090078674
This invention relates to a method of etching metals. It has been developed primarily to provide a method for etching metals that cannot be etched using conventional metal etching methods.
The following application has been filed by the applicant simultaneously with the present application:
The disclosure of this copending application is incorporated herein by reference. The above application has been identified by its filing docket number, which will be substituted with the corresponding application number once assigned.
Various methods, systems and apparatus relating to the present invention are disclosed in the following US patents/patent applications filed by the applicant or assignee of the present invention:
CMOS and MEMS processes typically require one or more metal etching steps. Conventionally, reactive ion etching (RIE) of metals uses a plasma of BCl3 and/or Cl2. Indeed, the present Applicant has employed a gas chemistry of BCl3 and Cl2 for etching inkjet heater materials, such as titanium nitride and titanium aluminium nitride. This gas chemistry provides excellent etch rates and selectivity in most RIE processes for metals (including metal nitrides and the like).
In the development of improved heater elements for thermal inkjet (bubblejet) nozzles, the present Applicant has conducted considerable research effort in evaluating suitable material candidates. The resistive heaters in inkjet printheads operate in an extremely harsh environment. They must heat and cool in rapid succession to form bubbles in the ejectable liquid—usually a water soluble ink with a superheat limit of approximately 300° C. Under these conditions of cyclic stress, in the presence of hot ink, water vapor, dissolved oxygen and possibly other corrosive species, the heaters will increase in resistance and ultimately fail via a combination of oxidation, cracking and fatigue.
To protect against the effects of oxidation, corrosion and cavitation on the heater material, some inkjet manufacturers use protective layers, typically made from Si3N4, SiC and/or Ta. However, protective layers affect heat transfer from the heater element to the ink, and therefore reduce the overall efficiency of the printhead.
To this end, the Applicant developed inkjet heater elements formed of superalloys (See U.S. application Ser. Nos. 11/482,953 (Docket No. MTD001US) and 11/482,953 (Docket No. MTD002US) both filed Jul. 10, 2006. Superalloys, such as Inconel® offer high temperature strength, corrosion and oxidation resistance far exceeding that of conventional thin film heaters used in known thermal inkjet printheads. The primary advantage of superalloys is that they can provide sufficient strength, oxidation- and corrosion-resistance to allow heater operation without protective coatings. Thus, the energy wasted in heating a protective coating is removed from the design. As a result, the input energy required to form a bubble with a particular impulse is reduced, lowering the level of residual heat in the printhead and improving overall efficiency.
Inconel™ is a hard, nickel-based superalloy that has been shown by the present Applicant to have excellent efficacy in inkjet nozzles. However, whilst superalloys and other metals may have excellent properties for use in inkjet nozzles, the inherent hardness of such materials presents its own manufacturing problems. In particular, conventional metal RIE processes using BCl3/Cl2 fail to etch such materials, or etch at rates that are too slow to be practicable.
An alternative metal etch process has been described by Nakatani and co-workers. U.S. Pat. No. 6,391,216 describes an RIE process employing a gas chemistry of CO and NH3. This gas chemistry is reported to be highly effective for etching hard metal alloys, such as Fe—Ni and Co—Cr alloys, which cannot be etched by conventional methods. Nakatani provides a rationale for etching transition metal alloys using a CO/NH3 gas chemistry. It is understood that the formation of volatile transition metal carbonyl compounds, such as Fe(CO)5, Ni(CO)4, Mn2(CO)10, Cr(CO)6, V(CO)6, Mo(CO)6 and W(CO)6, drives the etch. Further, NH3 is believed to play the role of inhibiting disproportionation of CO into C and CO2 in the plasma.
Whilst Nakatani has developed one method of etching hard, metal alloys, considerable problems still remain. Carbon monoxide, in its pure form, is a highly toxic gas requiring special safety precautions. In general, MEMS fabs are not equipped to plumb carbon monoxide into their plasma etching tools.
Accordingly, it would be desirable to provide a process for etching metals, which is suitable for etching hard, metal alloys, and which does not employ highly toxic gases, such as carbon monoxide, requiring special safety provisions.
It would be further desirable to explore other uses of any new etching conditions, which may replace or complement existing MEMS processes.
In a first aspect, there is provided a method of etching a metal by a reactive ion etching process, wherein an etchant gas chemistry for said reactive ion etching process consists essentially of NH3. This method provides superior etching rates whilst avoiding the use of highly toxic gases, such as CO.
Optionally, the metal is an alloy or a superalloy, which are particularly difficult to etch by conventional methods.
Optionally, the metal has a microstructure with a grain size of less than 100 nanometres.
Optionally, the metal has a Fe content of up to 60% by weight.
Optionally, the metal has a Ni content of between 25% by weight and 70% by weight.
Optionally, the metal has a Co content of between 35% by weight and 65% by weight.
Optionally, the metal is a superalloy comprising at least one metal selected from the group comprising: Cr, Al, Mo, Nb, Ta, Y, La, Ti, Fe, Ni and Co
Optionally, the superalloy is of formula MCrAlX, where M is one or more of Ni, Co, Fe with M contributing at least 50% by weight, Cr contributing 8% and 35% by weight, Al contributing up to 8% by weight, and X contributing from 0 to 25% by weight, with X being selected from at least one of: Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y and Hf.
Optionally, the superalloy comprises Ni, Fe, Cr and Al.
Optionally, the metal is selected from:
Optionally, the gas chemistry additionally consists of an inert gas, selected from helium, argon and xenon. Alternatively, the etchant gas chemistry consists of NH3 only.
Optionally, etch regions of the metal are defined by an etch mask.
Optionally, the etch mask is a hard mask comprised of at least one of: silicon dioxide, silicon nitride, tantalum, tungsten, zirconium and hafnium.
Optionally, the etch mask comprises a layer of silicon nitride and a layer silicon nitride.
Optionally, the method of the invention provides an etch rate of at least 200 Angstroms per minute.
Optionally, the method is a step of a MEMS fabrication process.
Optionally, the metal is a heater element for an inkjet nozzle assembly, and the method is a step of a printhead fabrication process.
Optional embodiments of the present invention will now be described, by way of example only with reference to the accompanying drawings in which:
The invention will now be described in the context of fabrication of thermal inkjet nozzles. Two types of thermal inkjet nozzles, and their corresponding fabrication processes, will now be described.
With reference to
The printhead also includes, with respect to each nozzle 3, side walls 6 on which the nozzle plate is supported, a chamber 7 defined by the walls and the nozzle plate 2, a multi-layer substrate 8 and an inlet passage 9 extending through the multi-layer substrate to the far side (not shown) of the substrate. A looped, elongate heater element 10 is suspended within the chamber 7, so that the element is in the form of a suspended beam. The printhead as shown is a microelectromechanical system (MEMS) structure, which is formed by a lithographic process which is described in more detail below.
When the printhead is in use, ink 11 from a reservoir (not shown) enters the chamber 7 via the inlet passage 9, so that the chamber fills to the level as shown in
Turning briefly to
When the element 10 is heated as described above, the bubble 12 forms along the length of the element, this bubble appearing, in the cross-sectional view of
The bubble 12, once generated, causes an increase in pressure within the chamber 7, which in turn causes the ejection of a drop 16 of the ink 11 through the nozzle 3. The rim 4 assists in directing the drop 16 as it is ejected, so as to minimize the chance of drop misdirection.
The reason that there is only one nozzle 3 and chamber 7 per inlet passage 9 is so that the pressure wave generated within the chamber, on heating of the element 10 and forming of a bubble 12, does not affect adjacent chambers and their corresponding nozzles. However, it is possible to feed ink to several chambers via a single inlet passage as long as pressure pulse diffusing structures are positioned between chambers. The embodiment shown in
The advantages of the heater element 10 being suspended rather than embedded in any solid material, are discussed below. However, there are also advantages to bonding the heater element to the internal surfaces of the chamber. These are discussed below with reference to
The increase in pressure within the chamber 7 not only pushes ink 11 out through the nozzle 3, but also pushes some ink back through the inlet passage 9. However, the inlet passage 9 is approximately 200 to 300 microns in length, and is only about 16 microns in diameter. Hence there is a substantial inertia and viscous drag limiting back flow. As a result, the predominant effect of the pressure rise in the chamber 7 is to force ink out through the nozzle 3 as an ejected drop 16, rather than back through the inlet passage 9.
Turning now to
The collapsing of the bubble 12 towards the point of collapse 17 causes some ink 11 to be drawn from within the nozzle 3 (from the sides 18 of the drop), and some to be drawn from the inlet passage 9, towards the point of collapse. Most of the ink 11 drawn in this manner is drawn from the nozzle 3, forming an annular neck 19 at the base of the drop 16 prior to its breaking off.
The drop 16 requires a certain amount of momentum to overcome surface tension forces, in order to break off. As ink 11 is drawn from the nozzle 3 by the collapse of the bubble 12, the diameter of the neck 19 reduces thereby reducing the amount of total surface tension holding the drop, so that the momentum of the drop as it is ejected out of the nozzle is sufficient to allow the drop to break off.
When the drop 16 breaks off, cavitation forces are caused as reflected by the arrows 20, as the bubble 12 collapses to the point of collapse 17. It will be noted that there are no solid surfaces in the vicinity of the point of collapse 17 on which the cavitation can have an effect.
Relevant parts of the manufacturing process of a printhead according to embodiments of the invention are now described with reference to
Referring to
Guard rings 26 are formed in the metallization of the interconnect layers 23 to prevent ink 11 from diffusing from the region, designated 27, where the nozzle of the unit cell 1 will be formed, through the substrate portion 21 to the region containing the wiring 25, and corroding the CMOS circuitry disposed in the region designated 22.
The first stage after the completion of the CMOS fabrication process consists of etching a portion of the passivation layer 24 to form the passivation recesses 29.
If, instead, the hole 32 were to be etched all the way to the interconnect layers 23, then to avoid the hole 32 being etched so as to destroy the transistors in the region 22, the hole 32 would have to be etched a greater distance away from that region so as to leave a suitable margin (indicated by the arrow 34) for etching inaccuracies. But the etching of the hole 31 from the top of the substrate portion 21, and the resultant shortened depth of the hole 32, means that a lesser margin 34 need be left, and that a substantially higher packing density of nozzles can thus be achieved.
The skilled person will appreciate that optimal etch conditions may be achieved by varying standard etch parameters, such as ICP power, bias, chamber pressure, temperature etc. Specific examples of suitable etch parameters, which have been used to etch Inconel™ 718, are detailed in Table 1 below. These etch parameters are readily achievable in any standard etching tool.
A hard mask is typically used for the NH3 etch. Suitable hard mask materials include silicon dioxide, silicon nitride, tantalum, tungsten, zirconium and hafnium. Optionally, the hard mask comprises a layer of silicon nitride and a layer of silicon oxide. The hard mask may be etched to define etch regions of the underlying metal using any standard hard mask etch conditions. For example, a silicon nitride/silicon oxide hard mask may be etched using a standard C4F8/O2 etchant gas chemistry.
Then, the sacrificial resist of each of the resist layers 35, 39, 42 and 48, is removed using an oxidative plasma, to form the structure shown in
Referring firstly to
The unit cells shown in
The passivation layer may be a single silicon dioxide layer is deposited over the interconnect layers 23. Optionally, the passivation layer 24 can be a silicon nitride layer between two silicon dioxide layers (referred to as an “ONO” stack). The passivation layer 24 is planarised such that its thickness on the M4 layers 50 is preferably 0.5 microns. The passivation layer separates the CMOS layers from the MEMS structures and is also used as a hard mask for the ink inlet etch described below.
After this step, a layer photoresist 42 is again spun onto the wafer 8 as shown in
Once the photoresist layer 42 is removed, another layer of photoresist 35 is spun onto the wafer as shown in
With the photoresist 35 defining the chamber roof and support walls, a layer of roof material, such as silicon nitride, is deposited onto the sacrificial scaffolding. In the embodiment shown in
The wafer 8 is then turned over so that the ‘backside’ 70 (see
In use, ink is fed from the backside 70 into the channel 32 and into the front side inlet 31. Gas bubbles are prone to form in the ink supply lines to the printhead. This is due to outgassing where dissolved gasses come out of solution and collect as bubbles. If the bubbles are fed into the chambers 7 with the ink, they can prevent ink ejection from the nozzles. The compressible bubbles absorb the pressure generated by the nucleating bubbles on the heater elements 10 and so the pressure pulse is insufficient to eject ink from the aperture 3. As the ink primes the chambers 7, any entrained bubbles will tend to follow the columnar features on either side of the ink inlet 31 and be pushed toward the bubble vent 66. Bubble vent 66 is sized such that the surface tension of the ink will prevent ink leakage, but trapped gas bubbles can vent. Each heater element 10 is enclosed on three sides by chamber walls and by additional columnar features on the fourth side. These columnar features diffuse the radiating pressure pulse to lower cross-talk between chambers 7.
Superalloys are a class of materials developed for use at elevated temperatures. They are usually based on elements from Group VIIA of the Periodic Table and predominantly used in applications requiring high temperature material stability such as jet engines, power station turbines and the like. Their suitability in the thermal inkjet realm has until now gone unrecognized. Superalloys can offer high temperature strength, corrosion and oxidation resistance far exceeding that of conventional thin film heaters (such as tantalum aluminium, tantalum nitride or hafnium diboride) used in known thermal inkjet printheads. The primary advantage of superalloys is that they can have sufficient strength, oxidation and corrosion resistance to allow heater operation without protective coatings, so that the energy wasted in heating the coatings is removed from the design.
Testing has indicated that superalloys can in some cases have far superior lifetimes compared to conventional thin film materials when tested without protective layers.
The applicant's prior work indicates that oxidation resistance is strongly correlated with heater lifetime. Adding Al to TiN to produce TiAlN greatly increased the heater's oxidation resistance (measured by Auger depth profiling of oxygen content after furnace treatment) and also greatly increased heater lifetime. The Al diffused to the surface of the heater and formed a thin oxide scale with a very low diffusivity for further penetration of oxygen. It is this oxide scale which passivates the heater, protecting it from further attack by an oxidative or corrosive environment, permitting operation without protective layers. Sputtered Inconel 718 also exhibits this form of protection and also contains Al, but has two other advantageous properties that further enhance oxidation resistance; the presence of Cr, and a nanocrystalline structure.
Chromium behaves in a similar fashion to aluminium as an additive, in that it provides self passivating properties by forming a protective scale of chromium oxide. The combination of Cr and Al in a material is thought to be better than either in isolation because the alumina scale grows more slowly than the chromia scale, but ultimately provides better protection The Cr addition is beneficial because the chromia scale provides short term protection while the alumina scale is growing, allowing the concentration of Al in the material required for short term protection to be reduced. Reducing the Al concentration is beneficial because high Al concentrations intended for enhanced oxidation protection can jeopardize the phase stability of the material.
X-ray diffraction and electron microscope studies of the sputtered Inconel 718 showed a crystalline microstructure, with a grain size less than 100 nm (a “nanocrystalline” microstructure). The nanocrystalline microstructure of Inconel 718 is beneficial in that it provides good material strength yet has a high density of grain boundaries. Compared to a material with much larger crystals and a lower density of grain boundaries, the nanocrystalline structure provides higher diffusivity for the protective scale forming elements Cr and Al (more rapid formation of the scale) and a more even growth of the scale over the heater surface, so the protection is provided more rapidly and more effectively. The protective scales adhere better to the nanocrystalline structure, which results in reduced spalling. Further improvement in the mechanical stability and adherence of the scale is possible using additives, such as rare earth metals e.g. yttrium, lanthanum etc.
It should be noted that superalloys are typically cast or wrought and this does not yield a nanocrystalline microstructure: the benefits provided by the nanocrystalline structure are specific to the sputtering technique used in the MEMS heater fabrication of this application. It should also be noted that the benefits of superalloys as heater materials are not solely related to oxidation resistance: their microstructure is carefully engineered with additives to encourage the formation of phases that impart high temperature strength and fatigue resistance. Potential additions comprise the addition of aluminium, titanium, niobium, tantalum, hafnium or vandium to form the gamma prime phase of Ni based superalloys; the addition of iron, cobalt, chrome, tungsten, molybdenum, rhenium or ruthenium to form the gamma phase or the addition of C, Cr, Mo, W, Nb, Ta, Ti to form carbides at the grain boundaries. Zr and B may also be added to strengthen grain boundaries. Controlling these additives, and the material fabrication process, can also act to suppress undesirable age-induced Topologically Close Packed (TCP) phases, such as sigma, eta, mu phases which can cause embrittlement, reducing the mechanical stability and ductility of the material. Such phases are avoided as they may also act to consume elements that would otherwise be available for the favoured gamma and gamma prime phase formation. Thus, while the presence of Cr and Al to provide oxidation protection is preferred for the heater materials, superalloys in general can be considered a superior class of materials from which selection of heater material candidates may be made, since considerably more effort has been put into designing them for high temperature strength, oxidation and corrosion resistance than has been put into improving the conventional thin film heater materials used in MEMS.
The Applicant's results indicate that superalloys
a Cr content between 2% by weight and 35% by weight;
a Al content of between 0.1% by weight and 8% by weight;
a Mo content of between 1% by weight and 17% by weight;
a Nb+Ta content of between 0.25% by weight and 8.0% by weight;
a Ti content of between 0.1% by weight and 5.0% by weight;
a Fe content of up to 60% by weight;
a Ni content of between 26% by weight and 70% by weight; and or,
a Co content of between 35% by weight and 65% by weight;
are suitable for use as a thin film heater element within a MEMS bubble generator (e.g. suspended heater element, bonded heater element and so on).
Superalloy's having the generic formula MCrAlX where:
M is one or more of Ni, Co, Fe with M contributing at least 50% by weight;
Cr contributing between 8% and 35% by weight;
Al contributing more than zero but less than 8% by weight; and,
X contributing less than 25% by weight, with X consisting of zero or more of Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y, Hf;
provide good results in open pool testing (described above).
In particular, superalloys with Ni, Fe, Cr and Al together with additives comprising zero or more of Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y, or Hf, show superior results.
Using these criteria, suitable superalloy materials for thermal inkjet printhead heaters may be selected from:
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