Patent Grant
7365016
This invention relates to a method of manufacturing of Micro-Electro-Mechanical-Systems (MEMS) such as micro-gyroscopes, micro-accelerometers, resonant accelerometers, micro-mirrors, micro-motors, micro-actuators and other such micro-devices.
In the manufacture of MEMs device, the integration of very sensitive moving mechanical parts causes a very serious challenge because:
Liquid HF chemistries are not suitable for stiction-free releases of sensitive MEMS devices. Vapor HF has been used to release such devices without stiction. Unfortunately, vapor HF also attacks the underlying silicon nitride, producing an undesirable fluorinated silicon nitride compound with a rough surface. Fortunately, it has been reported that this fluorinated compound can be evaporated at high temperature using atmospheric pressure ovens operated under a nitrogen or inert ambient and/or at high temperature using vacuum ovens operated under a vacuum as to leave a clean surface under the removed sacrificial layer. Unfortunately, the fluorinated compound present onto the MEMS devices prior to such an ex-situ evaporation is indeed toxic and its manipulation involves the exposure of operators to this toxic fluorinated silicon nitride compound and/or to toxic vapors of this fluorinated silicon nitride compound resulting from its evaporation. More, the fluorinated compound is indeed unstable in presence of moist air and result in non-evaporable residues that cannot be evaporated in the ex-situ vacuum oven
The design of a custom anhydrous HF chamber capable of anhydrous HF release at a high enough temperature and at a good enough vacuum can result in a residue-free release of the micro-devices integrating very sensitive moving mechanical parts. Similarily, such a custom anhydrous HF chamber capable of anhydrous HF release at a high enough temperature but at a vacuum level which is not yet low enough to prevent the formation of the toxic fluorinated silicon nitride residues but which is indeed capable of an in-situ evaporation of the toxic fluorinated silicon nitride residues can result in a safe operation and in a residue-free release of the micro-devices integrating very sensitive moving mechanical parts.
Stiction Issues with Liquid Buffered HF and Non-Buffered HF Solutions
Liquid buffered HF and non-buffered HF solutions have been used to mechanically release the sacrificial oxides underlying the silicon-based (polysilicon-based or silicon-germanium-based) structures, such as the ones shown in
The above references show that HF solutions buffered with 40 wt % ammonium fluoride, NH4F, can be used to remove the sacrificial layer of silicon dioxide, SiO2, because the ammonium fluoride buffer maintains a stable pH and a stable release rate over time. Since ammonium fluoride has a melting point of 993° C. and a boiling point of 1700° C. it is then solid at room temperature. Since ammonium fluoride has a solubility limit of 40 grams/liter of water @ 15° C., it is typically used as a water-based ammonium fluoride solution, NH4F(aq.), at a concentration of 40 wt % NH4F in water. Since a 40 wt % NH4F has a pH of 6.0 and boiling point of 106° C., it is then liquid at room temperature. These buffered liquid HF solutions remove the sacrificial layer of silicon oxide, SiO2, by producing the ammonium fluorosilicate, (NH4)2SiF6, and more water solvent to dissolve the ammonium fluorosilicate by-product:
SiO2(s)+4HF(aq.)+2NH4F(aq.)→(NH4)2SiF6(aq.)+2H2O (I)
The ammonium fluorosilicate (also called ammonium silicofluoride, ammonium fluosilicate, ammonium hexafluorosilicate or bararite) is a solid white cubic (2.011 g/cm3) or triclinic (2.152 g/cm3) crystal at room temperature. It has a high solubility of 250 grams/liter in water @ 20° C.:
(NH4)2SiF6(aq.)⇄2NH4+(aq.)+SiF6−2(aq.)
This means that improperly rinsed BHF released wafers will result in an undesirable precipitation of solid ammonium fluorosilicate, (NH4)2SiF6, crystals under the released mechanical parts. This clearly undesirable effect related to the use of BHF solutions is to be prevented in the manufacturing of MEMS devices.
The ammonium fluorosilicate decomposes at 100° C. and results in the formation of volatile ammonia, NH3, of volatile silicon tetrafluoride, SiF4, and of another undesirable white solid by-product, ammonium bifluoride, NH4HF2:
(NH4)2SiF6(aq.)→NH4HF2(aq.)+SiF4(g)+NH3(g)
Fortunately, ammonium bifluoride (also called ammonium hydrogendifluoride) has a solubility of 630 grams/liter in water @ 20° C.:
NH4HF2(aq.)⇄NH4+(aq.)+HF2−(aq.)
Ammonium bifluoride has a melting point of 125° C. and a decomposition temperature of 238° C.:
NH4HF2(aq.)→NH4F(aq.)+HF(aq.)
Because ammonium fluoride, NH4F, has a melting point of 993° C., improperly rinsed mechanical parts released with BHF solutions and heated to high temperatures will also result in undesirable refractory solid residues accumulating under the improperly rinsed mechanical parts. Because advanced MEMS devices incorporating deep cavities and narrow access openings that cannot easily be rinsed, the use of BHF solutions in the manufacturing of such advanced MEMS devices gets much more complicated than the use of non-buffered HF solutions which do NOT result in the formation of such solid by-products.
The cited references also show that the non-buffered 49 wt % HF (in water) solution is used to remove the sacrificial layer of silicon oxide, SiO2. This 49 wt % HF non-buffered solution (without NH4F) has a boiling point of 106° C. and is then liquid at room temperature. The 49 wt % HF liquid solution removes the sacrificial layer of silicon oxide by producing the fluorosilicic acid, H2SiF6, and more water solvent to dissolve the fluorosilicic acid by-product:
SiO2(s)+6HF(aq.)→H2SiF6(aq.)+2H2O (I)
Since the fluorosilicic acid (also called hexafluorosilicic acid, hydrogen hexafluorosilicate, hydrofluorosilicic acid, dihydrogen hexafluorosilicate, hexafluorosilicate (2-) dihydrogen, hydrofluorosilicic acid, hydrogen hexafluorosilicate, hydrogen hexafluorosilic acid, sand acid, silicate (2-) hexafluorodihydrogen, silicic acid, silicofluoric acid, silicofluoride or silicon hexafluoride dihydride) has a melting point of −15.5° C. and a boiling point of 105° C., it is then liquid at room temperature. The fluorosilicic acid, H2SiF6, is very soluble in water:
H2SiF6(aq.)⇄2H+(aq.)+SiF6−2(aq.)
As shown in
Unfortunately, the surface tension of liquid 49 wt % HF solution causes a very undesirable stiction effect on more flexible and more fragile mechanical parts than such rigid silicon blocks.
The Physics of the Undesirable Stiction Effect Observed with Liquid HF Solutions
WA=γLA+γSA−γLS
where γLA and γSA are the specific surface energies of the liquid and the solid in ambient air respectively and where γLS is the interfacial energy between the solid and the liquid. The work of adhesion is often determined by contact angle measurements using a goniometer. If the tested liquid droplet is in thermodynamic equilibrium with the solid, then:
γSA=γLS+γLA cos Θ
where Θ is the contact angle between the liquid droplet and the solid surface. The work of adhesion now can be expressed with the Young-Dupré equation:
WA=γLA+γLS+γLA cos Θ−γLS=γLA(1+cos Θ)
The work of adhesion depends on the liquid surface tension (γLA) and the contact angle (Θ). The following two cited references:
The 3D interfometric image of
The chemicals used to remove the sacrificial oxide layer over the underlying surface of silicon, silicon nitride or other non-removed layer dictate the behavior of the various exposed surfaces in liquid water i.e. dictates if these become hydrophilic or hydrophobic surfaces.
When the mechanical release of the mechanical parts is done in a non-buffered 49 wt % HF solution, a final rinse in de-ionized water is performed to stop the release and to eliminate the residual H+ and F− ions from the exposed surfaces. The following reference:
Lower surface tension liquids have been used to try minimizing the stiction effect. The following reference:
In order to verify the effect of the surface tension, γLA, on the detachment length, Ld:
Ld=[2E/(9γLA cos Θ)]1/4[h2t3/(1+t/w)]1/4
four types of SOI substrates have been used to machine cantilevers of various length:
This reference shows that the various cantilevers are machined in the top silicon region (Young modulus of 170 GPa) as to reach the bottom sacrificial oxide. Following proper resist strip, the cantilevers are released from their underlying oxide using a non-buffered 49 wt % HF solution followed by a rapid displacement of the 49 wt % HF releasing solution using either:
This
Such small differences of detachment length, Ld, of sensitive mechanical parts using smaller surface tension drying liquids do not fulfill the requirements for most advanced MEMS devices integrating a large number of sensitive mechanical parts and does not safely prevent the stiction issues associated with such type of devices.
Another technique is to be used to prevent stiction of such advanced MEMS devices.
Stiction Reduction using Super-Critical CO2 Drying
Super-critical CO2 drying has been used to try minimizing the stiction effect. The following references:
Because the liquid-to-vapor transition occurs in the supercritical region, there are no attractive capillary forces to cause stiction during the drying phase.
This supercritical CO2 drying technique is not that well suited for mass production because the high pressure of about 1200 psi (required to dry the released structures by passing from the liquid phase to the gas phase through the supercritical region and above the supercritical point of 31.1° C. and 1073 psi) imposes the use of bulky and thick walls mechanical chambers to manually load and dry, one-by-one, the solvent-soaking substrates integrating advanced MEMS devices having their sensitive mechanical parts already released and yet dripping solvent. The most advanced supercritical CO2 dryer currently available is the “Automegasamdri®-915 Series C” from Tousamis:
Incompatibility of Standard CMOS Metals to Liquid HF
The above-mentioned release techniques involving liquid HF suffer from either stiction complications or lack of automation, thus limiting the mass production of advanced MEMS devices integrating sensitive mechanical parts.
Over these very serious problems, liquid-based HF solutions also suffer from metal incompatibility issues. These incompatibility issues are due to the fact that advanced MEMS devices typically integrate digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions. Since this CMOS electronics needs to be exposed to the chemicals used for the removal of the oxide sacrificial material to allow the release the mechanical parts of these advanced MEMS devices, the liquid-based HF solutions should not chemically attack the metal-based interconnections of this CMOS electronics. Unfortunately almost all CMOS interconnect materials, namely aluminum alloys (such as aluminum-silicon, aluminum-copper or aluminum-silicon-copper alloys), titanium (and titanium compounds such as titanium nitride) and copper are rapidly attacked by these liquid-based HF solutions. The following documents are used as prior art references to demonstrate these undesirable limitations:
This clearly limits the use of liquid-based HF solutions for the mass production of advanced MEMS devices integrating digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions.
Embodiments of the invention provide an integrated process involving the use of anhydrous HF and either a methanol, isopropanol, acetone, 1-buthanol or 2-buthanol additive to release MEMS devices at a temperature of less than 100° C. and at vacuum level greater than the ammonium bifluoride, NH4HF2, triple point pressure of about 40 Torr followed by an in-situ vacuum evaporation at a temperature of more than 100° C. and at vacuum level lower than the ammonium bifluoride, NH4HF2, triple point pressure of about 40 Torr as a way to achieve an operator-safe, residue-free and stiction-free release of MEMS micro-devices integrating very sensitive moving mechanical parts, digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions.
According to one aspect of the invention there is provided a method of etching a sacrificial oxide layer covering an etch-stop silicon nitride underlayer, comprising exposing the sacrificial oxide to anhydrous HF at a temperature of less than about 100° C. and at vacuum level greater than 40 Torr; and subsequently performing an in-situ vacuum evaporation of etch by-products at a temperature of more than about 100° C. and at vacuum level lower than the 40 Torr without exposure to ambient air.
The invention offers an improvement over the vapor HF presently used to protect against stiction complications.
Since the mechanical deformation and resulting stiction of sensitive microstructures are caused by the pulling action (surface tension) of the liquids as they are dried from the surfaces of these microstructures, an alternate approach using vapor HF (gas phase H2O/HF solutions) was proposed in the Prior Art. The following eight documents are cited as prior art references for the stiction-free release of advanced MEMS devices integrating sensitive mechanical parts prone to stiction:
These eight references and in particular A. Witvrouw”s and M. A. Lester's references show that the metal incompatibility is still a problem with the vapor HF process (mostly performed using FSI International's Excalibur® 2000 Vapor HF Etching System of FIG. 11—http://www.fsa.org/directories/suppliers/profile.asp?id=617): the reported experiments show that although vapor HF process conditions can be adjusted to achieve selective removal of oxide sacrificial layers films without stiction and collapse of polysilicon structures, the aluminum, titanium and copper layers are indeed seriously attacked during the removal of the sacrificial oxide layers using non-anhydrous vapor HF.
This incompatibility of the vapor HF process and the typical CMOS interconnects metals again clearly limits the use of vapor HF solutions for the mass production of advanced MEMS devices integrating digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions.
The invention is an improvement over the anhydrous HF mixed with solvent or alcohol additives presently used to protect against stiction and CMOS metal interconnects attack complications
The removal of a solid phase sacrificial oxide layer, SiO2(s), using a gas phase anhydrous vapor hydrofluoric acid, HF(g), produces a gas phase silicon tetrafluoride, SiF4(g)↑, and gas phase water vapor, H2O(g)↑:
SiO2(s)+4HF(g)→SiF4(g)↑+2H2O(g)↑
Silicon tetrafluoride, SiF4, also called silicon (IV) fluoride or silicon fluoride or tetrafluorosilane has a melting point of −90° C. and a boiling point of −86° C. It is then a gas at room temperature. The water vapor reaction by-product needs to be eliminated from the surrounding of the sacrificial layer as to prevent the undesirable metal incompatibility effects described in the previous section. The elimination of the water vapor by-product is possible via the use of a solvent (or alcohol) additive that offers a way to prevent the condensation of the water vapor by-product of the upper chemical reaction onto the released mechanical surfaces and the surrounding surfaces i.e. a way to prevent against the initial stage of a possible stiction complication:
The resulting is a good gas phase release process free from stiction complications.
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:
The inventors have clearly demonstrated the CMOS metal compatibility of the anhydrous HF & solvent (or alcohol) additive process of the above mentioned U.S. Pat. No. 5,439,553 by releasing 800 μm diameter photonics mirrors composed of a 2.5 μm thick doped silicon structure directly coated with a very thin layer (75 nm) of an Al/1.0 wt % Si/0.5 wt % Cu infrared mirror (as per our granted U.S. Pat. No. 6,656,528) and perforated with an array of release holes spaced about 50 μm apart (thus ensuring complete mirror release when laterally removing 100 μm of the underlying 1.0 μm thick sacrificial oxide). Following an extended exposure to this anhydrous HF & solvent (or alcohol) additive process which allowed the removal of 100 μm of sacrificial oxide, the 75 nm thick top coating of aluminum alloy remained untouched and perfectly intact and allowed the excellent infrared reflectance disclosed in
The above chemical reaction involving anhydrous HF and the sacrificial oxide layer to be removed tends to indicate that this anhydrous HF & solvent (or alcohol) additive release process might be ideal for the mechanical release of advanced MEMS devices integrating sensitive mechanical parts prone to stiction as well as digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions.
Unfortunately, the silicon nitride dielectric layer which is typically used as an etch-stop layer underneath the oxide to be removed as to limit the penetration of the anhydrous HF and as to prevent the attack of other oxide layers underneath this silicon nitride etch-stop layer does not resist very well when exposed to this anhydrous HF & solvent (or alcohol) additive release process since the exposure of this silicon nitride to anhydrous HF results in an undesirable chemical attack:
Si3N4(s)+16HF(g)→2(NH4)2SiF6(s)+SiF4(g)↑
Ammonium fluorosilicate is also known as: ammonium fluosilicate, ammonium hexafluorosilicate, ammonium silicofluoride, bararite, cryptohalite, cryptophthalite, and diammonium hexafluorosilicate crystals. These toxic ammonium fluorosilicate crystals have two crystalline phases: a cubic α phase with a 2.011 g/cm3 density and a trigonal β phase with a 2.152 g/cm3 density. Since both crystalline phases are highly soluble in water (solubility of 250 grams/liter in water @ 20° C.) or alcohol, they could be removed by a water or alcohol rinse but such a liquid exposure would result in stiction. Another elimination technique than a water rinse should be found to remove them without induced stiction. It is known that ammonium fluorosilicate crystals are thermally unstable and experience a thermal decomposition at a temperature of more than 100° C.:
2(NH4)2SiF6(s)→2NH4HF2(s)+2SiF4(g)↑+2NH3(g)↑
The chemical compound resulting from this thermal decomposition, ammonium bifluoride, NH4HF2, shown in
These toxic ammonium bifluoride crystals are also known as: acid ammonium fluoride, ammonium acid fluoride, ammoniumbifluoride, ammonium difluoride, ammonium fluoride, ammonium hydrofluoride, ammonium hydrogen bifluoride, ammonium hydrogen difluoride, ammonium hydrogen fluoride, ammonium monohydrogen difluoride, and flammon crystals. Since these toxic ammonium fluorosilicate crystals are soluble in water or alcohol, they could be removed by a water or alcohol rinse but such a liquid exposure would result in stiction. Another elimination technique than a water rinse should be found to remove them without induced stiction. The following document shows that ammonium bifluoride has a melting point of 124.6° C. and a boiling point of 240° C. at atmospheric pressure:
The elimination of the toxic ammonium bifluoride crystals at a temperature exceeding 240° C. at atmospheric pressure is potentially an excellent technique that could ensure proper elimination of these crystals without the use of any liquids and any stiction issues.
Unfortunately, the use of an ex-situ evaporation into an ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure is a very serious hazard which causes very serious health and safety concerns since the ex-situ thermal decomposition of the toxic ammonium fluorosilicate crystals, (NH4)2SiF6(s), into toxic ammonium bifluoride crystals, NH4HF2(s), sublimating as a toxic ammonium bifluoride vapor, NH4HF2(g), implies a direct exposure of operators to this toxic ammonium bifluoride vapor when they un-load the evaporated wafers from the oven:
2(NH4)2SiF6(s)→2NH4HF2(s)+2SiF4(g)↑+2NH3(g)↑
2NH4HF2(s)→2NH4HF2(g)↑
Moreover, the use of an ex-situ evaporation into an ex-situ oven in a typical production environment systematically results in a direct exposure of ammonium fluorosilicate to moisten ambient atmosphere (a modern semiconductor fabrication facility is typically maintained at a fixed 40% relative humidity as to protect against electro-static discharges, ESD) and in an uncontrolled delay between the first moments of direct exposure of ammonium fluorosilicate to moisten ambient atmosphere and the ex-situ evaporation of ammonium fluorosilicate into the oven operated under a nitrogen ambient at atmospheric pressure. It is known from the glass frosting industry that ammonium fluorosilicate to crystals, (NH4)2SiF6(s), are deliquescent salts which are so hygroscopic that they readily attract moisture from this direct exposure to moisten ambient atmosphere and readily form liquid solutions:
It has recently been observed that deliquescence of thin film salts is a nucleation phenomenon:
This implies that the expected very uniform film of ammonium fluorosilicate resulting from the exposure of a uniform film of silicon nitride to the upper-mentioned patented anhydrous HF & solvent (or alcohol) additive release process will gradually result in the transformation of this expected uniform film of ammonium fluorosilicate into a surface covered by series of isolated droplets of ammonium fluorosilicate solution following a direct exposure to moisten ambient atmosphere. If this exposure is prolonged some of these isolated liquid droplets may coalesce and form larger liquid droplets, thus resulting in a random size distribution of liquid droplets. These liquid droplets are indeed chemically active liquid solutions that may result in modifications of their chemical content. As previously discussed in the upper section 2.1 “Stiction issues with liquid buffered HF and non-buffered HF solutions”, the following physical and chemical reactions result from the exposure of ammonium fluorosilicate to the adsorbed and absorbed water vapor in the non-uniform liquefied droplets:
2(NH4)2SiF6(s) in moist ambient→2(NH4)2SiF6(droplet)
2(NH4)2SiF6(droplet)⇄4NH4+(droplet)+4OH−(droplet)+2SiF6−2(droplet)+4H+(droplet)
The resulting random size distribution of liquid droplets will be transformed into a random size distribution of crystals when the surface covered by these liquid droplets will be investigated using a Scanning Electron Microscope, SEM, operating under vacuum. This phase transformation is provoked by the vacuum evaporation of water from the liquid droplets and results in the formation of a random size distribution of isolated crystals. The chemical composition of the individual crystals depends upon the chemical reactions that have occurred in the isolated liquid droplets in direct contact with the moisten atmosphere prior to water evaporation and SEM investigation.
The hygroscopic nature of the expected uniform surface of ammonium fluorosilicate, the deliquescent nature of the ammonium fluorosilicate surface provoking the liquefaction of this expected uniform surface into a uniform distribution of liquid droplets, the coalescence of this uniform distribution of liquid droplets into a random distribution of droplets that can achieve larger heights than the deposited thickness of silicon nitride and the crystallisation of this random distribution of droplets into a random distribution of isolated crystals of various chemical compositions that result from the exposure of silicon nitride to the upper-mentioned patented anhydrous HF & solvent (or alcohol) additive release process followed by exposure to moisten atmosphere prior to an ex-situ evaporation in an oven under a nitrogen ambient at a temperature of more than 240° C. at atmospheric pressure is here below demonstrated using two vacuum investigation techniques: 1) Electron X-Ray Diffraction, EDX, investigation & 2) Secondary Electron Microscopy, SEM, investigation.
The Electron X-Ray Diffraction investigation of
It is then clear that the ex-situ evaporation of ammonium fluorosilicate crystals into an ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure in a typical production environment implying a direct exposure to moisten ambient atmosphere and an uncontrolled delay between the first moments of this direct exposure to moisten ambient atmosphere and the ex-situ evaporation of ammonium fluorosilicate crystals into an ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure result in hydrated ammonium fluorosilicate crystals from the adsorbed and absorbed water vapor from the moist ambient atmosphere.
The deliquescence nature of ammonium fluorosilicate results in an initial uniform size distribution of liquefied ammonium fluorosilicate droplets that gradually becomes a random size distribution of coalesced ammonium fluorosilicate droplets with the individual droplet dimensions and water content being dictated by water water surface tension effects:
2(NH4)2SiF6(s) in moist ambient→2(NH4)2SiF6(droplet)
2(NH4)2SiF6(droplet)⇄4NH4+(droplet)+4OH−(droplet)+2SiF6−2(droplet)+4H+(droplet)
This surface tension induced differenciation in the water content of the individual liquid droplets is a first step in the chemical differentiation of the individual ammonium fluorosilicate solution droplets.
It is well known that ammonium cations in water solutions are very unstable and readily convert to ammonia by a proton exchange from the ammonium cation to the hydroxyl anion formed from the condensing water vapor:
2NH4+(droplet)+2OH−(droplet)⇄2NH3(droplet)+2H2O(droplet)
This mechanism is shown in
2NH4+(droplet)+2OH−(droplet)⇄2NH3(droplet)+2H2O (droplet)→NH3(g)↑+NH3(droplet)+2H2O(droplet)
The resulting irreversible decrease of pH of the individual liquefied droplet of various dimensions and water content which experience the irreversible elimination of more or less of their ammonium cations by irreversible evaporation of ammonia, NH3(g)↑, is a second step toward the chemical differenciation of these individual droplets (the island solution becomes more acidic):
2(NH4)2SiF6(droplet)→2NH3(g)↑+2NH4+(droplet)+2OH−(droplet)+2H2O(droplet)+2SiF6−2(droplet)+4H+(droplet)
A similar proton exchange mechanism than the one shown in
2SiF6−2(droplet)+4H+(droplet)→2SiF4(droplet)+4HF(droplet)
Because the resulting silicon tetrafluoride in the liquified droplet, SiF4(droplet), is also a gas at atmospheric pressure and temperature, some of the formed silicon tetrafluoride in the individual liquid droplet, SiF4(droplet), do evaporate. This partial and irreversible silicon tetrafluoride evaporation, 2SiF4(g)↑, results in the irreversible elimination of fluorine anions from the individual liquid droplet solutions:
2SiF6−2(droplet)+4H+(droplet)⇄2SiF4(droplet)+4HF(droplet)→SiF4(g)↑+SiF4(droplet)+4HF(droplet)
The resulting decrease of fluorine of the individual liquefied droplet of various dimensions and water content which experience the elimination of more or less of their fluorine by evaporation of silicon tetrafluoride is a third step toward the chemical differenciation of these individual droplets:
2(NH4)2SiF6(droplet)→2NH3(g)↑+2NH4+(droplet)+2OH−(droplet)+2H2O(droplet)+SiF4(g)↑+SiF6−2(droplet)+2H+(droplet)+2HF(droplet)
Depending upon relative humidity conditions, exposure duration to moisten ambient atmosphere, ambient temperature and ambient atmospheric pressure, the net and relative proportion of evaporated ammonia, NH3(g)↑, or silicon tetrafluoride, SiF4(g)↑, and the surface tension effect of the individual droplets will dictate the residual pH of the individual droplets by dictating the residual water content, H2O(droplet), as well as the residual proportion of ammonium solution, 2NH4+(droplet)+2OH−(droplet), and the residual proportion of fluorosililic acid solution, SiF6−2(droplet)+2H+(droplet), within the various dimension liquid droplets.
Some of these individual droplets will have a relative chemical composition of ammonium solution, 2NH4+(droplet)+2OH−(droplet), and fluorhydric acid, HF(droplet), which will provoke a fourth and undesirable step: the chemical formation of ammonium bifluoride (also called ammonium hydrogendifluoride) in some of the droplets, NH4HF2(droplet):
NH4+(droplet)+OH−(droplet)+2HF(droplet)⇄NH4HF2(droplet)+H2O(droplet)
The resulting undesirable chemical reaction between the ammonium solution, 2NH4+(droplet)+2OH−(droplet), and hydrofluoric acid, HF(droplet), is a fourth undesible step toward the chemical differenciation of these individual droplet and is to be avoided as to ensure a residue-free complete evaporation of these various liquid droplets during the ex-situ evaporation in the ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure:
2(NH4)2SiF6(droplet)→2NH3(g)→+NH4+(droplet)+OH−(droplet)+3H2O(droplet)+SiF4(g)↑+SiF6−2(droplet)+2H+(droplet)+NH4HF2(droplet)
The ammonium bifluoride now in solution in some specific droplets are indeed deliquesced ammonium bifluoride crystals:
A large proportion of the formed droplets could result in the formation of these undesirable deliquesced ammonium bifluoride crystals because ammonium bifluoride has a high solubility of 630 grams/liter in water @ 20° C.
Some of these individual droplets having a pH and a proportion of ammonium bifluoride crystals, NH4HF2(droplet), prone to ammonium bifluoride decomposition will trig a fifth and undesirable step: the chemical formation of ammonium fluoride, NH4F(droplet):
NH4HF2(droplet)→NH4F(droplet)+HF(droplet)
The resulting undesirable decomposition of deliquesced ammonium bifluoride crystals, NH4HF2(droplet), into deliquesced ammonium fluoride crystals, NH4F(droplet), is a fifth undesible step toward the chemical differenciation of these individual droplet and is to be avoided as to ensure a residue-free complete evaporation of these various liquid droplets during the ex-situ evaporation in the ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure:
2(NH4)2SiF6(droplet)→2NH3(g)↑+NH4+(droplet)+OH−(droplet)+3H2O(droplet)+SiF4(g)↑+SiF6−2(droplet)+2H+(droplet)+NH4F(droplet)+HF(droplet)
Since ammonium fluoride, NH4F, has a melting point of 993° C. and a boiling point of 1700° C. it is then solid at room temperature. This net transformation of deliquesced ammonium fluorosilicate crystals, (NH4)2SiF6 (deliquesced), into deliquesced ammonium bifluoride crystals, NH4HF2(deliquesced), will result in a random distribution of residual ammonium fluoride crystals, NH4F(s), on the silicon nitride surface following the ex-situ evaporation in the ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure in which:
It is clear that the ex-situ evaporation of ammonium fluorosilicate, (NH4)2SiF6, into an ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure in a typical production environment implying a direct exposure to moisten ambient atmosphere and an uncontrolled delay between the first moments of this direct exposure to moisten ambient atmosphere and the ex-situ evaporation of ammonium fluorosilicate, (NH4)2SiF6, into an ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure results in a undesirable random distribution of residual ammonium fluoride crystals which presence causes very serious yield problems as an alternate way to achieve operator-safe residue-free release of anhydrous HF to release MEMS micro-devices integrating very sensitive moving mechanical parts, digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions:
2(NH4)2SiF6(s)→3NH3(g)↑+2SiF4(g)↑+3HF(g)↑+NH4F(s)
The presence of this random distribution of solid ammonium fluoride crystals residues that CANNOT be evaporated anymore using the ex-situ evaporation of ammonium fluorosilicate crystals into an ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure (if the evaporation temperature is maintained lower than ammonium fluoride's, NH4F, boiling point of 1700° C.) is a very serious problem that needs to be eliminated.
The formation of these non-evaporated ammonium fluoride residues has been observed for air-exposed ammonium fluorosilicate crystals that we tried to evaporate using ex-situ evaporation of these ammonium fluorosilicate crystals into an ex-situ oven operated at a temperature of more than 240° C. under nitrogen ambient at atmospheric pressure.
The hygroscopic nature of the expected uniform surface of ammonium fluorosilicate, the deliquescent nature of the ammonium fluorosilicate surface provoking the liquefaction of this expected uniform surface into a uniform series of liquid droplet, the coalescence of this series of uniform liquid droplet into a random distribution of isolated droplets that can achieve larger heights than the deposited thickness of silicon nitride, the subtle chemical reactions resulting in ammonia and silicon tetrafluoride evaporation from some of these formed droplets and the random formation of deliquesced ammonium fluoride within this random distribution of liquid droplets that result from the exposure of silicon nitride to the upper-mentioned patented anhydrous HF & solvent (or alcohol) additive release process to a moisten ambient atmosphere is clearly demonstrated using the Secondary Electron Microscopy investigation of
Unfortunately, this coalescence mechanism results in the formation of residues that are non-sublimable at low temperature, such as ammonium fluoride, NH4F(s). These residues are undesirable because they can block the mechanisms of the released mechanical parts.
It is clear that the health and safety hazards associated with the operator exposure to the toxic ammonium fluorosilicate crystals, (NH4)2SiF6(s), covering the surface of the wafers following the exposure of the wafers to a anhydrous HF & solvent (or alcohol) additive process of the upper mentioned U.S. Pat. No. 5,439,553 is NOT a desirable technique to safely evaporate these residues because of the presence of hazardous volatile ammonia, silicon tetrafluoride and hydrofluoric acid by-products:
2(NH4)2SiF6(s)→3NH3(g)↑+2SiF4(g)↑+3HF(g)↑+NH4F(s)
It is also clear that the use of an ex-situ evaporation of ammonium fluorosilicate crystals exposed to a moisten ambient atmosphere prior to an evaporation into an ex-situ oven operated at a temperature of more than 240° C. under a nitrogen ambient at atmospheric pressure results in a random distribution of solid crystals of very important dimensions causing serious mechanical issued is not a desirable technique to reliably release mechanical parts as to achieve stiction-free and residue-free release of MEMS micro-devices integrating very sensitive moving mechanical parts, digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions:
2(NH4)2SiF6(s)→3NH3(g)↑+2SiF4(g)↑+3HF(g)↑+NH4F(s)
The removal of the solid phase sacrificial oxide layer, SiO2(s), using a gas phase anhydrous vapor hydrofluoric acid HF(g), produces a clean residue-free elimination of oxide by simple evaporation of gas phase silicon tetrafluoride, SiF4(g)↑, and gas phase water vapor, H2O(g)↑:
SiO2(s)+4HF(g)→SiF4(g)↑+2H2O(g)↑
Unfortunately, the silicon nitride dielectric layer, Si3N4(s), which is typically used as an underlying protective etch-stop layer underneath the oxide to be removed as to limit the penetration of the anhydrous HF and as to prevent the attack of other oxide layers underneath this silicon nitride etch-stop layer does not resist very well when exposed to this anhydrous HF & solvent (or alcohol) additive release process since the exposure of this silicon nitride to anhydrous HF results in an undesirable chemical attack resulting the formation of solid surface of toxic ammonium fluorosilicate, (NH4)2SiF6(s):
Si3N4(s)+16HF(g)→SiF4(g)↑+2(NH4)2SiF6(s)
It is known from the Material Safety Data Sheet of ammonium fluorosilicate, (NH4)2SiF6(s):
It is also known from the Material Safety Data Sheet of ammonium bifluoride, NH4HF2(s):
2(NH4)2SiF6(s)→2SiF4(g)↑+2NH3(g)↑+2NH4HF2(g)↑
SiO2(s)+4HF(g)→SiF4(g)↑+2H2O(g)↑
Si3N4(s)+16HF(g)→SiF4(g)↑+2SiF4(g)↑+2NH3(g)↑+2NH4HF2(g)↑
Unfortunately,
The surface ratio, ν/νm of the actual surface coverage of anhydrous HF adsorbed onto the oxide sacrificial layer, ν, to the maximum amount of anhydrous HF that can be adsorbed as to form a complete monolayer, νm, at an absolute temperature, T, follows the Brunauer-Emmet-Teller (BET) equation
described in the following publication:
C. S. Lee, J. T. Baek, H. J. Yoo, S. E. Woo, “Modeling and Characterization of Gas-Phase Etching of Thermal Oxide and TEOS Oxide Using Anhydrous HF and CH3OH”, J. Electrochem Soc., Vol. 143, No. 3, March 1996. pp. 1099-1103;
It is clear from this
SiO2(s)+4HF(g)→SiF4(g)↑+2H2O(g)↑
Si3N4(s)+16HF(g)→SiF4(g)↑+2SiF4(g)↑+2NH3(g)↑+2NH4HF2(g)↑
such release conditions of the sacrificial oxide results in a logarithmic reduction of the surface coverage of anhydrous HF raw material onto the surface of sacrificial oxide to be released; i.e. the BET plot of
This undesirable reduction of release rate due to the poor surface coverage of HF when operating the anhydrous HF release at a process pressure less than the ammonium bifluoride, NH4HF2, triple point pressure of about 40 Torr and at a process temperature within the highlighted grey zone of
These very undesirable release durations show that it is not acceptable to perform the anhydrous HF release process at a pressure of less than 40 Torr and at a temperature of more than 100° C. to achieve a residue-free removal of sacrificial oxides at a pressure of less than the ammonium bifluoride, NH4HF2, triple point pressure of about 40 Torr and at the temperature of the highlighted grey zone of
A custom-made batch release chamber capable of releasing up to 24 wafers simultaneously was constructed. This custom-made batch release chamber can be delivered attached to a cluster tool similar to the one shown in
This batch chamber design allows the release of up to 24 wafers simultaneously and then relaxes the need for a fast release process. This batch chamber design allows the release at temperatures lower than 100° C. and/or pressures of more than 40 Torr to achieve faster release rates (
This combination of a batch anhydrous HF release process (with or without solvents or alcohols) at temperatures lower than 100° C. and/or pressures of more than 40 Torr as to achieve faster release rates followed by an in-situ sublimation of undesirable reaction by-products at a temperature higher than 100° C. and at a pressure of less than 40 Torr as to achieve a residue-free removal of sacrificial oxides at a pressure of less than the ammonium bifluoride, NH4HF2, triple point pressure of about 40 Torr and at the temperature of the highlighted grey zone of
The use of such a batch release chamber and of such a cluster tool configuration allows the anhydrous HF release to be performed at temperatures lower than 100° C. and/or pressures of more than 40 Torr as to achieve a release rate of at least 0.1 μm/minute:
SiO2(s)+4HF(g)→SiF4(g)↑+2H2O(g)↑
Si3N4(s)+16HF(g)→2(NH4)2SiF6(s)+SiF4(g)↑
This minimum release rate of 0.1 μm/minute in such a custom-made batch chamber and in such a cluster tool equipped with five (5) anhydrous HF batch release chambers each capable of releasing up to 24 MEMS wafers simultaneously (for a total of 120 MEMS wafers that can be released simultaneously) is indeed acceptable for the mass production of advanced MEMS devices because the sacrificial oxide release over the same lateral distances ranging between 5 μm and 200 μm underneath the suspended mechanical part as to free its movement would imply acceptable release durations:
Such a spectacular increase of process throughput allows the cluster tool to be used for a controlled (without exposure to the moisten ambient atmosphere) in-situ sublimation of the undesirable reaction by-products at a temperature higher than 100° C. and at a pressure of less than 40 Torr as to achieve a residue-free removal of sacrificial oxides at a pressure of less than the ammonium bifluoride, NH4HF2, triple point pressure of about 40 Torr and at the temperature of the highlighted grey zone of
2(NH4)2SiF6(s)→2NH4HF2(g)↑+2SiF4(g)↑+2NH3(g)↑
This very important feature is key to the present invention because it allows the prevention of the indesirable effects of exposure of the silicon nitride reaction by-products to moisten ambient atmosphere and allows a clean release of the sacrificial oxide layer and an operator-safe and clean sublimation of the silicon nitride reaction by-products:
SiO2(s)+4HF(g)→SiF4(g)↑+2H2O(g)↑
Si3N4(s)+16HF(g)→2NH4HF2(g)↑+3SiF4(g)↑+2NH3(g)↑
Embodiments of the invention offer a number of advantages over the prior art. The in-situ evaporation allows a residue-free etch of the sacrificial oxide layer covering an etch-stop silicon nitride underlayer, a residue-free release of the structural layer of a micro-electro-mechanical system (MEMS), an operator-safe etch the sacrificial oxide layer covering an etch-stop silicon nitride underlayer, an operator-safe release of the structural layer of a micro-electro-mechanical system (MEMS), a stiction-free etch of the sacrificial oxide layer covering an etch-stop silicon nitride underlayer, and a stiction-free release of the structural layer of a micro-electro-mechanical system (MEMS).
The method in accordance with the invention can be used to fabricate electro-mechanical systems integrating very sensitive moving mechanical parts, digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing actuation functions. The digital and/or analog CMOS control logic and/or high voltage CMOS drivers may integrate metal interconnects composed, for example, of aluminum, aluminum alloys, aluminium compounds, titanium, titanium alloys, titanium compounds, copper, gold, tungsten, tungsten alloys, tungsten compounds, molybdenum, molybdenum alloys, molybdenum compounds, or combinations of these.
This application claims the benefit under 35 USCC 119(e) of prior U.S. provisional application No. 60/638,377 filed Dec. 27, 2004, the contents of which are herein incorporated by reference.
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|---|---|---|---|
| 5439553 | Grant et al. | Aug 1995 | A |
| 5814562 | Green et al. | Sep 1998 | A |
| 6576151 | Vereecke et al. | Jun 2003 | B1 |
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| 20030080082 | Chinn et al. | May 2003 | A1 |
| 20050002079 | Novotny et al. | Jan 2005 | A1 |
| Number | Date | Country |
|---|---|---|
| 03055791 | Jul 2003 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20060211163 A1 | Sep 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60638377 | Dec 2004 | US |
