Metallization Process

To know about the different IC fabrication techniques, click on the link below.

TAKE A LOOK : IC FABRICATION TECHNIQUES

Metallization is the final step in the wafer processing sequence. Metallization is the process by which the components of IC’s are interconnected by aluminium conductor. This process produces a thin-film metal layer that will serve as the required conductor pattern for the interconnection of the various components on the chip. Another use of metallization is to produce metalized areas called bonding pads around the periphery of the chip to produce metalized areas for the bonding of wire leads from the package to the chip. The bonding wires are typically 25 micro meters diameter gold wires, and the bonding pads are usually made to be around 100×100 micro meters square to accommodate fully the flattened ends of the bonding wires and to allow for some registration errors in the placement of the wires on the pads.

Aluminium

Aluminium (At) is the most commonly used material for the metallization of most IC’s, discrete diodes, and transistors. The film thickness is as about 1 micro meters and conductor widths of about 2 to 25 micro meters are commonly used. The use of aluminium offers the following advantages:

• It has as relatively good conductivity.
• It is easy to deposit thin films of Al by vacuum evaporation.
• It has good adherence to the silicon dioxide surface.
• Aluminium forms good mechanical bonds with silicon by sintering at about 500°C or by alloying at the eutectic temperature of 577°C.
• Aluminium forms low-resistance, non-rectifying (that is, ohmic) contacts with p-type silicon and with heavily doped n-type silicon.
• It can be applied and patterned with a single deposition and etching process.

Aluminium has certain limitations:

1. During packaging operation if temperature goes too high, say 600°C, or if there is overheating due to current surge, Al can fuse and can penetrate through the oxide to the silicon and may cause short circuit in the connection. By providing, adequate process control and testing, such failures can be minimized.
2. The silicon chip is usually mounted in the package by a gold perform or die backing that alloys with the silicon. Gold lead wires have been bonded to the aluminium film bonding pads on the chip, since package lead are usually gold plated. At elevated temperatures, a reaction between the metal of such systems causes formation of intermetallic compounds, known as the purple plague. Purple plague is one of six phases that can occur when gold and aluminium inter-diffuse. Because of dissimilar rate of diffusion of gold and aluminium, voids normally occur in the form of the purple plague. These voids may result in weakened bonds, resistive bonds or catastrophic failure. The problem is generally solved by using aluminium lead wires, or another metal system, in circuits that will be subjected so elevated temperatures. One method is to deposit gold over an under layer of chromium. The chromium acts as a diffusion barrier to the gold and also adheres well to both oxide and gold. Gold has poor adhesion to oxide because it does not oxide itself. However, the chromium-gold process is comparatively expensive, and it has an uncontrollable reaction with silicon during alloying.
3. Aluminium suffers from electromigration which can cause considerable material transport in metals. It occurs because of the enhanced and directional mobility of atoms caused by the direct influence of the electric field and the collision of electrons with atoms, which leads to momentum transfer. In thin-film conductors that carry sufficient current density during device operations, the mode of material transport can occur at much lower temperature (compared to bulk metals) because of the presence of grain boundaries, dislocations and point defects that aid the material transport. Eecctromigration-induced failure is the most important mode of failure in Al lines.

In general the desired properties of the metallization for IC can be listed as follows.

• Low resistivity.
• Easy to form.
• Easy to etch for pattern generation.
• Should be stable in oxidizing ambient , oxidizable.
• Mechanical stability; good adherence, low stress.
• Surface smoothness.
• Stability throughout processing including high temperature sinter, dry or wet oxidation, gettering, phosphorous glass (or any other material) passivation, metallization.
• No reaction with final metal, aluminium.
• Should not contaminate device, wafers, or working apparatus.
• Good device characteristics and life times.
• For window contacts-low contact resistance, minimum junction penetration, low electromigration.

Metallization Application in VLSI

For VLSI, metallization applications can be divided into three groups:

1. Gates for MOSFET
2. Contacts, and
3. Interconnects.

Interconnection metallization interconnects thousands of MOSFETs or bipolar devices using fine-line metal patterns. It is also same as gate metallization for MOSFET. All metallization directly in contact with semiconductor is called contact metallization. Polysilicon film is employed in the form of metallization used for gate and interconnection of MOS devices. Aluminium is used as the contact metal, on devices and as the second-level inter-connection to the outside world. Several new schemes for metallization have been suggested to produce ohmic contacts to a semiconductor. In several cases a multiple-layer structure involving a diffusion barrier has been recommended. Platinum silicide (PtSi) has been used as a Schottky barrier contact and also simply as an ohmic contact for deep junction. Titanium/platinum/gold or titanium/palladium/gold beam lead technology has been successful in providing high-reliability connection to the outside world. The applicability of any metallization scheme in VLSI depends on several requirements. However, the important requirements are the stability of the metallization throughout the IC fabrication process and its reliability during the actual use of the devices.

Ohmic contacts

When a metal is deposited on the semiconductor a good ohmic contact should be formed. This is possible, if the deposition metal does not perturb device characteristics. Also die contact should be stable both electrically and mechanically.

Other important application of metallization is the top-level metal that provides a connection to the outside world. To reduce interconnection resistance and save area on a chip, multilevel metallization, as discussed in this section is also used. Metallization is also used to produce rectifying (Schottky barrier) contacts, guard rings, and diffusion barriers between reacting metallic films.

We have already stated the desired properties of metallization for ICs. None of the metals satisfies all the desired characteristics. Even Al, which has most of the desired properties suffers from a low melting point-limitation and electromigration as discussed above.

Poly-silicon has been used for gate metallization, for MOS devices. Recently, poly-silicon/refractory metal silicide bi-layers have replaced poly-silicon so that lower resistance an be achieved at the gale and interconnection level. By preserving the use of polysilicon as the “metal” in contact with the gate oxide, well known device characteristics and processes have been unaltered. The silicides of molybdenum (MoSi₂), tantalum (TaSi₂) and tungsten (WSi₂) have been used in the production of microprocessors and random-access memories. TiSi₂ and CoSi₂ have been suggested to replace MoSi₂, TaSi₂, and WSi2. Aluminium and refractory metals tungsten and Mo are also being considered for the gate metal.

For contacts, Al has been the preferred metal for VLSI. However, for VLSI applications, several special factors such as shallower junctions, step coverage, electromigration (at higher current densities), and contact resistance can no longer be ignored. Therefore, several possible solutions to the contact problems in VLSI have been considered. These include use of

• Dilute Si-Ai alloy.
• Polysilicon layers between source, drain, or gate and top-level Al.
• Selectively deposited tungsten, that is, deposited by CVD methods so that metal is deposited only on silicon and not on oxide.
• A diffusion barrier layer between silicon and Al, using a silicide, nitride, carbide, or their combination.

Use of self-aligned silicide, such as, PtSi, guarantees extremely good metallurgical contact between silicon and silicide. Silicides are also recommended in processes where shallow junctions and contacts are formed at the same time. The most important requirement of an effective metallization scheme in VLSI is that metal must adhere to the silicon in the windows and to the oxide that defines die window. In this respect, metals such as, Al, Ti, Ta, etc., that form oxides with a heat of formation higher than that of Si0₂ are the best. This is why titanium is the most commonly used adhesion promoter.

Although silicides are used for contact metallization, diffusion barrier is required to protect from interaction with Al which is used as the top metal. Aluminium interacts with most silicides in the temperature range of 200-500 degree Celsius. Hence transition metal nitrides, carbides, and borides are used as a diffusion barrier between silicide (or Si) and Al due to their high chemical stability.

Metallization Processes

Metallisation process can be classified info two types:

1. CVD and
2. Physical Vapour Deposition

To know about CVD click on the link below.

TAKE A LOOK : CHEMICAL VAPOUR DEPOSITION (CVD)

CVD offers three important advantages. They are

• Excellent step coverage
• Large throughput
• Low-temperature processing
• The basic physical vapour deposition methods are
• Evaporation
• Sputtering

Both these methods have three identical steps.

• Converting the condensed phase (generally a solid) into a gaseous or vapour phase.
• Transporting the gaseous phase from the source to the substrate, and
• Condensing the gaseous source on the substrate.

In both methods the substrate is away from the source.

In cases where a compound, such as silicide, nitride, or carbide, is deposited one of the components is as gas and the deposition process is termed reactive evaporation or sputtering.

Deposition Methods

In the evaporation method, which is the simplest, a film is deposited by the condensation of the vapour on the substrate. The substrate is maintained at a lower temperature than that of the vapour. All metals vaporize when heated to sufficiently high temperatures. Several methods of heating are employed to attain these temperatures. For AI deposition, resistive, inductive (RF), electron bombardment [electron-gun] or laser heating can be employed. For refractory metals, electron-gun is very common. Resistive heating provides low throughput. Electron-gun cause radiation damage, but by heat treatment it can be annealed out. This method is advantageous because the evaporations take place at pressure considerably lower than sputtering pressure. This makes the gas entrapment in the negligible. RF heating of the evaporating source could prove to be the best compromise in providing large throughput, clean environment, and minimal levels of radiation damage.

In sputtering deposition method, the target material is bombarded by energetic ions to release some atoms. These atoms are then condensed on the substrate to form a film. Sputtering, unlike evaporation is very well controlled and is generally applicable to all materials metals, alloys, semiconductors and insulators. RF-dc and dc-magnetron sputtering can be used for metal deposition. Alloy-film deposition by sputtering from an alloy target is possible because the composition of the film is locked to the composition of the target. This is true even when there is considerable difference between the sputtering rates of the alloy components. Alloys can also be deposited with excellent control of composition by use of individual component targets. In certain cases, the compounds can be deposited by sputtering the metal in a reactive environment. Thus gases such as methane, ammonia, or nitrogen, and diborane can be used in the sputtering chamber to deposit carbide, nitride, and boride, respectively. This technique is called reactive sputtering. Sputtering is carried out at relatively high pressures (0.1 to 1 pascal or Pa). Because gas ions are the bombarding species, the films usually end up including small amount of gas. The trapped gases cause stress changes. Sputtering is a physical process in which the deposited film is also exposed to ion bombardment. Such ion bombardment causes sputtering damage, which leads to unwanted charges and internal electric fields that affect device proxies. However such damages can be annealed out at relatively low temperatures (<500°C), unless the damage is so severe as to cause an irreversible breakdown of the gate dielectric.

Deposition Apparatus

The metallization is usually done in vacuum chambers. A mechanical pump can reduce the pressure to about 10 to 0.1 Pa. Such pressure may be sufficient for LPCVD. An oil-diffusion pump can bring the pressure down to 10^-5 Pa and with the help of a liquid nitrogen trap as low as 10^-7 Pa. A turbomolecular pump, can bring the pressure down to 10^-8-10^-9 Pa. Such pumps are oil-free and are useful HI molecular-beam epitaxy where oil contamination must be avoided. Besides the pumping system, pressure gauges and controls, residual gas analyzers, temperature sensors, ability to clean the surface of the wafers by backsputtering, contamination control, and gas manifolds, and the use of automation should be evaluated.

As typical high-vacuum evaporation apparatus is shown in the figure below.

The apparatus consists of a hell jar, a stainless-steel cylindrical vessel closed at the top and sealed at the base by a gasket. Beginning at atmospheric pressure the jar is evacuated by a roughing pump, such as a mechanical rotary-van pump reducing pressure to about 20 Pa or a combination mechanical pump and liquid-nitrogen-cooled molecular pump (reducing pressure lo about 0.5 Pa). At the appropriate pressure, the jar is opened to a high-vacuum pumping system that continues to reduce the pressure. The high-vacuum, pumping system may consist of a liquid nitrogen-cooled trap and an oil-diffusion pump, a trap and a turbomolecular pump, or a trap and a closed-cycle helium refrigerator cryopump. The cryopump acts as a trap and must be regenerated periodically, the turbomolecular and diffusion pumps act  as transfer  pumps, expelling their gas t a forepump. The high vacuum pumping system brings the jar to a low pressure that is tolerable for the deposition process.

All components in the chamber are chemically cleaned and dried. Freedom from sodium contamination is vital when coating MOS devices.

The sputtering system operates with about 1 Pa of argon pressure during film deposition. For sputtering, a throttle valve should be placed between the trap and the high-vacuum pumping system. The argon gas pressure can to be maintained by reducing the effective pumping speed of the high-vacuum pump, while the full pumping speed of the trap for water vapour is utilized. Water vapour and oxygen are detrimental to film quality at background pressures of about 10^-2 Pa.

The use of thickness monitors is common in evaporation and sputtering deposition. This is necessary for controlling the thickness of the film, because thinner film can cause excess current density and excessive thickness can lead to difficulties in etching.

Metallization Patterning

Once the thin-film metallization has been done the film must be patterned to produce the required interconnection and bonding pad configuration. This is done by a photolithographic process of the same type that is used for producing patterns in Si0₂ layers. Aluminium can be etched by a number of acid and base solutions including HCl, H₃PO₄, KOH, and NaOH. The most commonly used aluminium etchant is phosphoric acid with the addition of small amounts of HN0₃ (nitric acid) and acetic acid, to result a moderate etch rate of about 1 micro meter per minute at 50°C. Plasma etching can also be used with aluminium.

Lift-off Process

The lift-off process is an alternative metallization patterning technique. In this process a positive photoresist is spun on the wafer and patterned using the standard photolithographic process. Then the metallization thin film is deposited on top of the remaining photoresist. The wafers are then immersed in suitable solvent such as acetone and at the same time subjected to ultrasonic agitation. This causes swelling and dissolution of the photoresist. As the photoresist comes off it lifts off the metallization on top of it, for the lift-off process to work, the metallization film thickness must generally be somewhat less than the photoresist thickness. This process can, however produce a very fine line-width metallization pattern, even with metallization thickness that are greater than the line width.

Pattering for VLSI Applications

VLSI applications require anisotropic etching techniques for metallization patterning because of the requirements of tight control on metallization dimensions. Therefore dry-etching techniques are most suitable. Reactive-ion etching (RIE) is anisotropic. Hence it is preferred. For RIE, reactive gases such as, Cl₂ and CCI₃F are used, hence the name reactive ion etching.