Metallic gate electrode

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A metallic gate electrode ( English metal gate (electrode) ) referred to in the field of semiconductor technology, one of the metal existing gate electrode ( control electrode ) of an insulated gate field effect transistor ( IGFET , z. B. MOSFET ). It differs from gate electrodes made of polysilicon (the polycrystalline form of silicon , also called silicon gate technology ), which have replaced the previously used gate electrodes made of aluminum ( aluminum gate technology ) since the late 1970s and have since been mainly used in integrated circuits (ICs) in CMOS technology.

history

Metals are characterized, among other things, by a high electrical conductivity (due to a high charge carrier concentration ). Therefore, they have always been used in electrical engineering for electrical connections and as electrode material. Even in the early days of microelectronics , metals (typically aluminum, which was vapor-deposited onto the wafer surface in a vacuum chamber ) were initially used as the material for the control electrode (gate) of field effect transistors. With increasing integration , that is to say downsizing, of microelectronic circuits, however, complications arose in the manufacture and operation of transistors manufactured in this way (relatively high threshold voltage, low thermal budget, etc.). Therefore, in the late 1960s and early 1970s, the semiconductor industry abandoned metal as the gate material in the layer stack of the metal-oxide-semiconductor field effect transistors (MOSFET) that are mainly used . Instead, highly doped (to reduce electrical resistance ) polysilicon was replaced (see silicon gate technology). This had various causes:

  1. Metals generally have a tendency to spread at high or very high temperatures, that is, to partially diffuse into adjacent materials or even to form alloys (e.g. silicides ) with them, which is partially exploited in production. In the heat treatment steps used in various production stages, however, there is always the risk that the metal will diffuse into the silicon substrate and negatively affect its electrical properties. The diffusion of metal ions, e.g. B. sodium impurities in aluminum, for example, can lead to a change in the threshold voltage of the transistor. The positive gate voltages used in NMOS and CMOS technology support this process, as the positive ions are drawn through the dielectric in the direction of the channel by the electric field. Early PMOS techniques were less sensitive to this effect, as negative voltages are applied to the gate, which counteract the drift of the positive ions towards the channel. Therefore, in IC production, special emphasis is still placed on purity, especially with regard to metallic impurities. At that time, however, such standards were difficult to achieve and associated with high costs. Also polysilicon is generally subject to this effect by the use of low concentrations of gaseous hydrogen chloride , however, is (HCl) during subsequent high temperature steps sodium in the form of sodium chloride (NaCl) bound ( getter ) and can be removed with the gas stream. This enabled a largely sodium-free gate structure to be achieved, which significantly improved the reliability of the semiconductor components.
  2. The electrode material used at that time, aluminum, leads to pitting corrosion in the silicon when subjected to higher or longer thermal treatments . The reason for this lies in the good solubility of silicon in aluminum. This means that the silicon diffuses into the aluminum, especially at higher temperatures, and leaves empty spaces on the surface. Due to the low melting temperature (660 ° C), aluminum in turn diffuses into these empty spaces and fills them. Together, both processes lead to the formation of pyramid-shaped "thorns" made of aluminum in the silicon substrate, which can reach a depth of a few micrometers. They are therefore sufficiently deep to influence the barrier layers even with the transistor sizes of that time , which can lead to short circuits between source and drain or other irreparable damage to the electrical circuit. To limit this effect, aluminum can be alloyed with approx. 0.5% to 1% silicon or a diffusion barrier can be built in between silicon and aluminum.
  3. The alternative material polysilicon can on the one hand be deposited very easily with the aid of chemical vapor deposition (CVD) and on the other hand it is significantly less sensitive than aluminum to high temperatures in subsequent production steps (in the range 900–1000 ° C.). Furthermore, it can be used robustly in the processes introduced at the time with self-aligning gate electrodes, which enable the production of an optimally aligned gate without additional photolithographic structuring and possible misalignment (see overlay ). The implantation or diffusion of the source and drain dopants can take place with the existing gate electrode, since this has already been doped to a significantly higher level with phosphorus and the additional dopants have hardly any effects on the electrical behavior.
  4. Polysilicon has a lower work function than aluminum, it favored the lowering of the threshold voltage, which has been necessary in the continuous reduction of the transistors in order to continue with comparable electric field strengths to work.

Polysilicon has a low specific resistance at the doping concentrations typically used , but this is still significantly higher than that of many metals. The higher electrical resistance of the gate worsens the electrical properties of the circuit with regard to the charging and discharging of the transistor gate capacitance , which results in slower switching times, cf. RC element .

Schematic cross-sections through an n-channel and a p-channel MOSFET in high-k + metal gate technology (in replacement metal gate technology) as introduced by Intel in 2007 with the Penryn processors in 45 nm technology .

With the 45 nm technology node, metallic gate electrodes are being increasingly used again. In addition to non-conductive materials with a high dielectric constant ( high-k material ), they form the second important part of the so-called high-k + metal gate technology , which Intel first used in industrial production. Since not only high electrical conductivity but also other electrical properties such as the work function are important in the choice of material, aluminum is only used here (if at all) as a secondary electrode material. Other metals are used as the primary electrode material, that is, in contact with the high-k material and facing the transistor channel. For the NMOS, among others, tantalum , tantalum nitride or niobium come into consideration, and for the PMOS, for example, a layer stack made of tungsten nitride and ruthenium (IV) oxide . The exact choice, however, depends on the integration chosen, i.e. production technology. In these cases, too, the gate electrode does not necessarily only consist of metals. Often, however, a thick layer of polysilicon, which has been silicided on the top, is over a complex stack of layers made of different metals.

Process sequence of the "aluminum gate technology" (1960s)

The following describes the production of p-channel field effect transistors and integrated circuits using planar technology with a metallic gate electrode, as was common in the 1960s, before the introduction of silicon gate technology. First, a thick silicon dioxide layer (called field oxide) was produced on an n-doped silicon single crystal wafer, for example by thermal oxidation of silicon . The source and drain regions were then doped with boron. The field oxide was then removed locally (photolithographic structuring and wet-chemical etching ) and a thin thermal oxide (gate oxide) was again generated in these regions under controlled conditions. Analogous to the gate area, the source and drain areas have now been defined (photolithographic structuring and wet chemical etching). In the last step, aluminum was deposited and structured to make contact with the three transistor electrodes.

Individual evidence

  1. ↑ Sami Franssila: Introduction to Microfabrication . John Wiley & Sons, 2004, ISBN 0-470-85105-8 , Chapter 25 CMOS Transistor Fabrication , p. 255 ff .
  2. ↑ Federico Faggin, Thomas Klein: A Faster Generation Of MOS Devices With Low Thresholds Is Riding The Crest Of The New Wave, Silicon-Gate IC’s . In: Electronics . tape 42 , no. 20 , 1969, p. 88 ( facsimile [accessed August 1, 2015]).