l Fluoride in Semiconductor Application

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Fluorinated Compounds for Semiconductor Industry “The rapid growth in fluorinated compound use is due to: Increased worldwide demand for semiconductor devices, The increase in semiconductor device complexity, and The lack of viable alternatives to fluorinated compounds.

June 28, 2011 Biniyam Jemal (755002)

Approved by: Proff. Navarrini Walter

Politecnico di Milano

POLITECNICO DI MILANO

Fluorinated Compounds for Semiconductor

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Contents Page

Abstract ii

1. Introduction 1

2. Semiconductor Process Gases 2

2.1. The use of Fluorinated Compounds 2

3. Fluorinated Compounds and Environmental Effects 4

3.1. Perfluoromethane 4

3.2. Perfluoroethane 5

3.3. Perfluoropropane 5

3.4. Trifluoromethane 6

3.5. Nitrogen Trifluoride 6

3.6. Sulfur Hexafluoride 7

Apendix: Gas Properties of Fluorinated Compounds 8

Reference 13



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Abstract

Fluorinated compounds are widely used by the semiconductor industry: tetrafluoromethane (CF4), hexafluoroethane (C2F6), octafluororopane (C3F8), octafluorobutane (c-C4F8), sulfur hexafluoride (SF6) nitrogen trifluoride (NF3) and trifluoromethane (CHF3)1. Collectively. Semiconductor manufacture uses fluorinated compounds in two critical processes: for cleaning chemical vapour deposition (CVD) chambers and for dry etching. Depending on the complexity of the product, the manufacturing process may require up to 100 fluorinated compounds using steps and up to 3 months process time.



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1. Introduction

luorine is the 12th and one of the most reactive elements in the periodic table. With an abundance of 0.65% in the Earth’s crust, it is found in more than 300 minerals and is widely distributed in the biosphere. With its small size, weight and closed-

shell electron configuration .it is used extensively for instance in Semiconductor industries, Pharmaceutical/health, energy storage and various other industries.

The electronics industry uses multiple long-lived fluorinated greenhouse gases (fluorinated GHGs). Semiconductors or integrated circuits act as the brains for advanced electronic controls and devices, which have become increasingly prevalent in consumer and industrial equipment in recent years. Computers, for example, contain information controllers, memory and processing devices, and they are also found in vehicles, appliances, buildings, and industrial processes. The use of fluorinated compounds in the semiconductor industry first began to occur in the late 1970’s; however, their use did not become widespread until the late 1980s or early 1990s, as a result of the need to create increasingly smaller more complex devices. As the industry continued its practice of exponentially increasing the functionality of semiconductor devices by making them simultaneously smaller and more powerful, new manufacturing processes were needed.

F



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2. Semiconductor Process Gases

The processes that allowed manufacturers to create smaller, more complex features rely on fluorinated compounds for both cleaning and etching functions. Six fully fluorinated compounds and one hydrofluorocarbon are currently widely used by the semiconductor industry: tetrafluoromethane (CF4), hexafluoroethane (C2F6), octafluororopane (C3F8), octafluorobutane (c-C4F8), suflur hexafluoride (SF6) nitrogen trifluoride (NF3) and trifluoromethane (CHF3)2. Information on these chemicals is summarized in Table 1.

Table 1. Semiconductor Process Gasses

Species Chemical Formula Atmospheric Lifetime (years)

Global Warming Potential (100 year time horizon)

Perfluoromethane CF4 50,000 6,500 Perfluoroethane C2F6 10,000 9,200

Perfluoropropane C3F8 2,600 7,000 Perfluorocyclobutane c-C4F8 3,200 8,700

Trifluoromethane (HFC-23) CHF3 264 11,700 Nitrogen Trifluoride NF3 740 8,000 Sulfur Hexafluoride SF6 3,200 23,900

2.1. The use of Fluorinated Compounds Fluorinated compounds are used for plasma cleaning of Chemical vapor deposition (CVD) chambers and plasma (dry) etching of the thin insulating and metal layers. The fluorinated compounds allow manufacturers to accurately etch the submicron scale patterns on these metal and dielectric layers and perform rapid chemical cleaning of CVD tool chambers. The carbon and fluorine that these compounds deliver in plasma are essential when etching advanced integrated circuits because, in addition to etching, they form polymers, which allow for highly selective and anisotropic (directional) film removal. CVD is a process where one or more volatile inorganic, metal-organic, or organometallic precursors are transported in the vapor phase, often in a carrier gas, to the reactor chamber where they decompose on a heated substrate and subsequently deposit a solid film along with volatile byproducts. Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch species, can be either charged (ions) or neutral (atoms and radicals). During the process, the plasma will generate volatile etch products at room temperature from the chemical reactions between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the physical properties of the target.



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Chemical vapor deposition (CVD) chambers – often called CVD tools − are the industry workhorses for depositing materials that will act as insulators and wires. Commonly deposited CVD films include polycrystalline silicon, silicon nitride, silicon dioxide, and metals such as tungsten. To eliminate buildup of these materials on internal chamber parts and the concomitant risk of contamination, periodic chamber cleans must be performed. CF4, C2F6, C3F8 and NF3 are the usual fluorinated compounds used in chamber cleaning.

During the clean cycle, the fluorinated compound is converted to F atoms in a plasma, which chemically etches away residual material from chamber walls, electrodes, and chamber hardware. However, due to the low destruction efficiency (high dissociation energy) of FFCs, a portion of the gas flowing into the chamber during the clean is not dissociated into F-atoms and, therefore, not used to clean the chamber: that unreacted portion flows through the chamber and, unless emission abatement technologies are used, eventually into the atmosphere. Of the total fluorinated compound emissions from semiconductor manufacturing, roughly >60 percent (million metric tons of carbon equivalent or MMTCE basis) results from chamber cleaning steps.

Once the electronic components (e.g., transistors) have been fabricated in the silicon, thin conducting material is added – the minute wires − to interconnect individual circuit components. In complex devices the length of this wiring will exceed 4 km per cm2 of device area. The pathways for these wires are etched into the insulating layers using another industry workhorse – etch chambers or etch tools. These etch tools also use FFCs in a plasma. In etch tools, both F-atoms and polyatomic species such as CF2 are created and react at the film surface (following prescribed patterns) to selectively remove (etch) substrate material. Etch processes are used to form, for example, trenches that are subsequently filled with metal to form the wires. CF4, CHF3, C2F6, C3F8, c-C4F8, NF3 and SF6 are etching gases. In some etching processes it is important that certain CF-containing polymers are formed on surfaces, which permits highly selective and directional removal of film material. So, depending on the etch process, either CF-containing fluorinated compounds are used or simply SF6. In etch processes, on average, more of the fluorinated compounds is utilized, so less of the original fluorinated compound exits the chamber unreacted and is emitted into the atmosphere. Of the total fluorinated compound emissions from semiconductor manufacturing, roughly <40 percent (MMTCE basis) result from plasma etching steps.

In addition to being directly used in manufacturing processes, fluorinated compounds can also be transformed during cleaning and etch processes. Put simply, during the process, an fluorinated compound input of one gas can be transformed into a different fluorinated compound which is exhausted to the atmosphere. For example, when either CHF3 or C2F6 are used in either cleaning or etching CF4 is generated and emitted as a process by-product. In the C2F6 case, the amount of CF4 produced can be 10% to 30% of the C2F6 input volume depending on the cleaning process. In the case of CHF3, reports indicate that approximately 10% (volume) of the input CHF3 is converted to CF4 and smaller yields of C2F6 have also been measured.



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Each layer of metal (wiring) and dielectric (insulating) material that is deposited and patterned upon the wafer requires specific fluorinated compound process steps. Thus, the complexity of a semiconductor product qualitatively correlates with the required number of CVD and etch (i.e., fluorinated compound-utilizing) process steps. Generally, as semiconductor devices increase in complexity, so does the quantity of fluorinated compounds (per square centimeter of silicon) required for manufacture.

3. Fluorinated Compounds and Environmental Effects 3.1. Perfluoromethane

Carbon–fluorine bonds are the strongest in organic chemistry. Additionally, they strengthen as more carbon–fluorine bonds are added to the same carbon. The carbon–fluorine bonds are strongest in Tetrafluoromethane. This effect is due to the increased coulombic attractions between the fluorine atoms and the carbon because the carbon has a positive partial charge of 0.76.

CF4

Tetrafluoromethane can be prepared by the reaction of silicon carbide with fluorine.

SiC + 2 F2 → CF4 + Si

It can also be prepared by the fluorination of carbon dioxide, carbon monoxide or phosgene with sulfur tetrafluoride. Commercially it is manufactured by the reaction of fluorine with dichlorodifluoromethane or chlorotrifluoromethane; it is also produced during the electrolysis of metal fluorides MF, MF2 using a carbon electrode. In semiconductor manufacturing, CF4 in mixture with oxygen is used for plasma cleaning of CVD reactors. Halocarbon 14 is a selective etching agent for many metals, metal silicides and oxides.

Tetrafluoromethane is the most persistent greenhouse gas. But, depending on the concentration, inhalation of tetrafluoromethane can cause headaches, nausea, dizziness and damage to the cardiovascular system(mainly the heart). Long-term exposure can cause severe heart damage.



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Due to its density, tetrafluoromethane can displace air, creating an asphyxiation hazard in inadequately ventilated areas.

3.2. Perfluoroethane Hexafluoroethane is a particularly versatile etchant for many substrates in semiconductor manufacturing. Halocarbon 116 can be used for selectively etching metal silicides and oxides versus their metal substrates.

With oxygen, hexafluoroethane strips photoresist. C2F6 is also used for selective etching of silicon dioxide (SiO2) over silicon.

Due to the high energy of C-F bonds, it is very inert and thus acts as an extremely stable greenhouse gas.

A main industrial emission of hexafluoroethane in semiconductor manufacturing is Tetrafluoromethane.

3.3. Perfluoropropane Octafluoropropane is used in mixture with oxygen in semiconductor applications as an etching material for SiO2layers. Oxides are selectively etched versus their metal substrates.

Major hazard : Suffocation

Toxicity: None Established Flammability limits in air: Non-flammable



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Odor : Faintly Sweet

3.4. Trifluoromethane

Fluoroform is the chemical compound with the formula CHF3. It is one of the "haloforms", a class of compounds with the formula CHX3 (X = halogen). Fluoroform is used in diverse niche applications and is produced as a by-product of the manufacture of Teflon. It is also generated biologically in small amounts apparently by decarboxylation of trifluoroacetic acid.

CHF3 is used in the semiconductor industry in plasma etching of silicon oxide and silicon nitride.

CHF3 is a potent greenhouse gas. The secretariat of the Clean Development Mechanism estimates that a ton of HFC-23 in the atmosphere has the same effect as 11,700 tons of carbon dioxide.

3.5. Nitrogen Trifluoride

Nitrogen trifluoride is the inorganic compound with the formula NF3. This nitrogen fluorine compound is a colorless, toxic, odorless, nonflammable gas. It finds increasing use as an etchant in microelectronics.

Nitrogen trifluoride is used in the plasma etching of silicon wafers. Today nitrogen trifluoride is predominantly employed in the cleaning of the PECVD chambers in the high volume production of liquid crystal displays and silicon-based thin film solar cells. In these applications NF3 is initially broken down in situ, by plasma. The resulting fluorine atoms are the active cleaning agents that attack the polysilicon, silicon nitride and silicon oxide. Nitrogen trifluoride can be used as well with tungsten silicide, and tungsten produced by CVD. NF3 has been considered as



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an environmentally preferable substitute for sulfur hexafluoride or perfluorocarbons such as hexafluoroethane.

It is prepared both by direct reaction (electric discharge) of ammonia and fluorine and by a variation of Ruff's method (prepared nitrogen trifluoride by the electrolysis of a molten mixture of ammonium fluoride and hydrogen fluoride).

It oxidizes hydrogen chloride to chlorine:

2 NF3 + 6 HCl → 6 HF + N2 + 3 Cl2

It converts to tetrafluorohydrazine upon contact with metals, but only at high temperatures: 2 NF3 + Cu → N2F4 + CuF2

NF3 reacts with fluorine and antimony pentafluoride to give the tetrafluoroammonium salt: NF3 + F2 + SbF5 → NF+ 4SbF−6

NF3 is a greenhouse gas, with a global warming potential (GWP) 17,200 times greater than that of CO2 when compared over a 100 year period

3.6. Sulfur Hexafluoride In semiconductor manufacturing, sulfur hexafluoride is a fluorine source for high density plasma etching without generating carbon by-products. SF6 can be used for etching metal silicides (specially tungsten etch-back), nitrides and oxides versus their metal substrates. It is an inorganic, colorless, odorless, non-toxic and non-flammable greenhouse gas. SF6 has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in nonpolar organic solvents. It is generally transported as a liquefied compressed gas.

According to the Intergovernmental Panel on Climate Change, SF6 is the most potent greenhouse gas that it has evaluated, with a global warming potential of 22,800 times that of CO2 when compared over a 100-year period.



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Appendix

Gas Properties of Fluorinated Compounds

Tetrafluoromethane

Molecular Weight Molecular weight : 88.01 g/mol

Solid phase Melting point : -184 °C Latent heat of fusion (1,013 bar, at triple point) : 8.09 kJ/kg

Liquid phase Liquid density (1.013 bar at boiling point) : 1603 kg/m3 Boiling point (1.013 bar) : -128 °C Latent heat of vaporization (1.013 bar at boiling point) : 135.7 kJ/kg

Critical point Critical temperature : -45.5 °C Critical pressure : 37.43 bar

Gaseous phase Gas density (1.013 bar and 15 °C (59 °F)) : 3.72 kg/m3 Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.9981 Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 3.038 Specific volume (1.013 bar and 21 °C (70 °F)) : 0.275 m3/kg Heat capacity at constant pressure (Cp) (1.013 bar and 30 °C (86 °F)) : 0.058 kJ/(mol.K) Heat capacity at constant volume (Cv) (1.013 bar and 30 °C (86 °F)) : 0.049 kJ/(mol.K) Ratio of specific heats (Gamma:Cp/Cv) (1.013 bar and 30 °C (86 °F)) : 1.178571 Viscosity (1.013 bar and 0 °C (32 °F)) : 0.000161 Poise Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 15.03 mW/(m.K)

Miscellaneous Solubility in water (1.013 bar and 20 °C (68 °F)) : 0.005 vol/vol Solubility in water (1.013 bar and 25 °C (77 °F)) : 0.0038 vol/vol

Hexafluoroethane Molecular Weight

Molecular weight : 138.02 g/mol

Solid phase Melting point : -100.6 °C



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Liquid phase Liquid density (1.013 bar at boiling point) : 1608 kg/m3 Liquid/gas equivalent (1.013 bar and 15 °C (59 °F)) : 276 vol/vol Boiling point (1.013 bar) : -78.2 °C Latent heat of vaporization (1.013 bar at boiling point) : 117.04 kJ/kg Vapor pressure (at 20 °C or 68 °F) : 27.5 bar

Critical point Critical temperature : 19.7 °C Critical pressure : 29.8 bar

Gaseous phase Gas density (1.013 bar at boiling point) : 8.86 kg/m3 Gas density (1.013 bar and 15 °C (59 °F)) : 5.84 kg/m3 Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.9875 Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 4.773 Specific volume (1.013 bar and 21 °C (70 °F)) : 0.175 m3/kg Heat capacity at constant pressure (Cp) (1.013 bar and 25 °C (77 °F)) : 0.105 kJ/(mol.K) Viscosity (1.013 bar and 0 °C (32 °F)) : 0.0001364 Poise

Thermal conductivity (1.013 bar and 0 °C (32 °F)): 13.47 mW/(m.K)

Octafluoropropane Molecular Weight


Liquid phase Liquid density (1.013 bar at boiling point) : 1601 kg/m3 Liquid/gas equivalent (1.013 bar and 15 °C (59 °F)) : 196 vol/vol Boiling point (1.013 bar) : -36.7 °C Latent heat of vaporization (1.013 bar at boiling point) : 104.25 kJ/kg


Gaseous phase Gas density (1.013 bar at boiling point) : 10.3 kg/m3 Gas density (1.013 bar and 15 °C (59 °F)) : 8.17 kg/m3 Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.975 Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 6.683 Specific volume (1.013 bar and 21 °C (70 °F)) : 0.125 m3/kg Heat capacity at constant pressure (Cp) (1.013 bar and 25 °C (77 °F)) : 0.149 kJ/(mol.K)



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Viscosity (1.013 bar and 0 °C (32 °F)) : 0.000125 Poise Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 12.728 mW/(m.K)

Sulfur Hexafluoride Molecular Weight


Solid phase Latent heat of fusion (1,013 bar, at triple point) : 39.75 kJ/kg

Liquid phase Liquid density (at triple point) : 1880 kg/m3 Boiling point (Sublimation) : -63.9 °C Latent heat of vaporization (1.013 bar at boiling point) : 162.2 kJ/kg Vapor pressure (at 21 °C or 70 °F) : 21.5 bar


Triple point Triple point temperature : -49.4 °C Triple point pressure : 2.26 bar

Gaseous phase Gas density (1.013 bar and 15 °C (59 °F)) : 6.27 kg/m3 Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.9884 Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 5.114 Specific volume (1.013 bar and 21 °C (70 °F)) : 0.156 m3/kg Heat capacity at constant pressure (Cp) (1.013 bar and 21 °C (70 °F)) : 0.097 kJ/(mol.K) Viscosity (1.013 bar and 0 °C (32 °F)) : 0.000142 Poise Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 12.058 mW/(m.K)

Miscellaneous Solubility in water (20 °C and 1 bar) : 0.007 vol/vol



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Nitrogen trifluoride



Liquid phase Liquid density (1.013 bar at boiling point) : 1540 kg/m3 Liquid/gas equivalent (1.013 bar and 15 °C (59 °F)) : 520 vol/vol Boiling point (1.013 bar) : -129 °C Latent heat of vaporization (1.013 bar at boiling point) : 163.02 kJ/kg

Critical point Critical temperature : -39.2 °C Critical pressure : 45.28 bar

Gaseous phase Gas density (1.013 bar and 15 °C (59 °F)) : 3.003 kg/m3 Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.9976 Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 2.46 Specific volume (1.013 bar and 21 °C (70 °F)) : 0.34 m3/kg Heat capacity at constant pressure (Cp) (1 bar and 25 °C (77 °F)) : 0.053 kJ/(mol.K)

Miscellaneous Solubility in water (20 °C and 1 bar) : 0.021 vol/vol

Trifluoromethane



Liquid phase Liquid density (1.013 bar at boiling point) : 1431 kg/m3 Liquid/gas equivalent (1.013 bar and 15 °C (59 °F)) : 488 vol/vol Boiling point (1.013 bar) : -82.1 °C Latent heat of vaporization (1.013 bar at boiling point) : 257.91 kJ/kg Vapor pressure (at 20 °C or 68 °F) : 43.8 bar



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Gaseous phase Gas density (1.013 bar at boiling point) : 4.57 kg/m3 Gas density (1.013 bar and 15 °C (59 °F)) : 2.99 kg/m3 Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.9913 Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 2.43 Specific volume (1.013 bar and 21 °C (70 °F)) : 0.343 m3/kg

Miscellaneous Solubility in water (20 °C and 1 bar) : 1 vol/vol



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Bibliography / References

Handbook of Chemicals and Gases for the SEMICONDUCTOR INDUSTRY, Misra, Hogan, Chorush.

Introduction to Microfabrication, Sami Franssila, Director of Microelectronics, Centre, Helsinki University of Technology, Finland.

United Nations Framework Convention on Climate change (UNFCCC),

Wikipedia, the free encyclopedia,

Other related websites.

l Fluoride in Semiconductor Application

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