Gate oxides

The carriers in tire channel of an enhancement mode device exhibit unusually high mobility, particularly at low temperatures, a subject of considerable interest. The source-drain current is carried by electrons attracted to tire interface. The ionized dopant atoms, which act as fixed charges and limit tire carriers mobility, are left behind, away from tire interface. In a sense, tire source-drain current is carried by tire two-dimensional (2D) electron gas at tire Si-gate oxide interface. 

Gate oxide dielectrics are a cmcial element in the down-scaling of n- and -channel metal-oxide semiconductor field-effect transistors (MOSEETs) in CMOS technology. Ultrathin dielectric films are required, and the 12.0-nm thick layers are expected to shrink to 6.0 nm by the year 2000 (2). Gate dielectrics have been made by growing thermal oxides, whereas development has turned to the use of oxide/nitride/oxide (ONO) sandwich stmctures, or to oxynitrides, SiO N. Oxynitrides are formed by growing thermal oxides in the presence of a nitrogen source such as ammonia or nitrous oxide, N2O. Oxidation and nitridation are also performed in rapid thermal processors (RTP), which reduce the temperature exposure of a substrate. 

Step 5. The remaining Si N, pad oxide, and resist are stripped away and a thin, precisely controlled Si02 gate oxide layer is thermally grown. 

Step 6. The doping concentration of the "typ substrate under the gate oxide is adjusted by another boron implantation. Boron passes through the thin gate oxide. This provides the threshold voltage adjustment for the final device. 

Step 8. The -type source and drain regions are created by As ion implantation. The As can penetrate the thin gate oxide, but not the thick field oxide or the polysihcon gate. The formation of the source and gate does not require a separate resist pattern, thus this technique is called self-aligning. 

Fig. 9. Fabrication sequence for an oxide-isolated -weU CMOS process, where is boron and X is arsenic. See text, (a) Formation of blanket pod oxide and Si N layer resist patterning (mask 1) ion implantation of channel stoppers (chanstop) (steps 1—3). (b) Growth of isolation field oxide removal of resist, Si N, and pod oxide growth of thin (<200 nm) Si02 gate oxide layer (steps 4—6). (c) Deposition and patterning of polysihcon gate formation of -source and drain (steps 7,8). (d) Deposition of thick Si02 blanket layer etch to form contact windows down to source, drain, and gate (step 9). (e) Metallisation of contact windows with W blanket deposition of Al patterning of metal (steps 10,11). The deposition of intermetal dielectric or final...

When Uq3 > Up the MOSEET conducts. The conduction current is deterrnined by 1 where Q is the amount of charge in the inversion layer and t is the transit time for electrons to travel from source to drain. Q = C LW (U g — Up) where C = is the gate oxide capacitance per unit area... 

It has been observed that the H-Si(lll) and H -Si(lOO) surfaces develop a submonolayer oxide when exposed to an average sitting time of 1 h prior to gate oxidation in typical room air and room lighting conditions. Furthermore, the oxidation of the H-Si(lll) and H3.-Si(100) surfaces occurs when they are exposed to UV light in the presence of dry air or humid air. Analogous experiments are reported for the photo-oxidation of pSi. ... 

Sources" are formed in the first step and then "Gates" are formed. A silicide is then used to lower the contact resistance between the silicon of the gate, source and drain and the contacting plug. The thickness of gate oxides is only about 40-50 A. They are thermally grown rather than... 

Gate Oxide Tunnel Oxide f SiH4/N20 PECVD Microwave Plasma Anodisation Laser CVD... 

G.. Xuan, J. Kolodzey, V. Kapoor, and G.. Gonye, Characteristics of field-effect devices with gate oxide modification by DNA. Appl. Phys. Lett. 87, 103903-1-3 (2005). 

Robertson, J. 2006. High dielectric constant gate oxides for metal oxide Si transistors. Rep. Prog. Phys. 69 327-396. 

Relevant for our discussion is the genesis of the sensitivity behaviour in a class of devices all generated from the well known MOSFET structure (ISFET and GASFET). In particular, the influence of charges into the gate oxide on the threshold voltage and MOSFET behaviour under shrinking conditions will be discussed. 

From the transconductance expressions we see that the first (eq. 15) is linear with Vds and the second (eq. 16) is related to both MOSFET gate voltage Vgs and its threshold voltage Vt- In both cases Cox plays an important role. In fact in order to increase the sensitivity, the gate oxide thickness should be as thin as possible. gm,SAT, according to equation 16, depends on Vt and it is known that Vt depends on Vfb-, the flat band voltage, according to ... 

In principle the ISFET is derived from a MOSFET, where the metal is replaced by the couple solution-reference electrode and where a CIM (Chemically Interactive Material) is deposited on the S1O2, the gate oxide. 

Contamination of silicon wafers by heavy metals is a major cause of low yields in the manufacture of electronic devices. Concentrations in the order of 1011 cm-3 [Ha2] are sufficient to affect the device performance, because impurity atoms constitute recombination centers for minority carriers and thereby reduce their lifetime [Scl7]. In addition, precipitates caused by contaminants may affect gate oxide quality. Note that a contamination of 1011 cnT3 corresponds to a pinhead of iron (1 mm3) dissolved in a swimming pool of silicon (850 m3). Such minute contamination levels are far below the detection limit of the standard analytical techniques used in chemistry. The best way to detect such traces of contaminants is to measure the induced change in electronic properties itself, such as the oxide defect density or the minority carrier lifetime, respectively diffusion length. 

Materials with better properties are also of interest as possible replacements for aSiO for gate oxide applications, since the ultrathin SiO layers required for low-voltage, high-drive logic is inherently too leaky to be employed. Instead, a thicker layer of a material with a higher dielectric constant is needed. As in capacitor appli-... 

The saturation current is denoted /sd,sat> the effective mobihty and the capacitance of the gate oxide Cox- The effective geometrical parameters of the transistor include its overall gate width, W g, and the effective gate length, Leff. 

In this equation UA, UB, and UC are a set of parameters from the foundry for the respective CMOS technology. V"sbx denotes the potential difference between source and bulk, which is zero in the case of the MOSFET-heater. Finally Cx represents the thickness of the gate oxide. 

In addition to requiring high dielectric films for DRAM capacitors (dynamic random access memories) and for the active memory elements in FRAMs, the microelectronics industry has a stated demand for a replacement for Si02 gate oxides very soon. The leading candidate is hafnia (Hf02), and there are significant opportunities for the ferroelectrics community to contribute to the solution of this problem. 

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