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Influence of plasma etching on NbTi in plasma equipment

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07 Jan,2021

Influence of plasma etching on NbTi in plasma equipment:

Negative bias temperature instability (NBTI) refers to the instability of device parameters such as VTH, GM and IDSAT of PMOS when operating at high gate negative bias and temperature. If it is NMOS device, corresponding to PBTI, positive bias temperature instability. The NBTI effect was discovered in 1961. Plasma etching effect on NBTI plasma equipment or pretty big, NBTI effect is the main reason, plus negative on the PMOS transistor gate bias, after a certain time of negative grid bias and temperature stress, the PMOS transistor in the Si/SiO2 interface, a new interface interface potential increase, because the hole capture the interface state and fixed charge are positively charged, the threshold voltage drift in the direction of the negative. In contrast, NMOS is much less affected by PbTi, because its interface states and fixed charge polarities cancel each other out. In the new technology node, with the reduction of the feature size of integrated circuit, the increase of gate electric field and the increase of the operating temperature of integrated circuit, NBTI becomes one of the key failure phenomena of the reliability of integrated circuit devices.

Reaction-diffusion model can well explain the phenomenon of VTH drift and NbTi recoverability caused by the increase of interface states in NbTi effect. PMOS is negative gate bias, and the direction of electric field in the SiO2 layer is the direction away from the interface. If the Si-H bond is broken during the operation of the device, an H+ ion will be released, leaving a positively charged interface state. The drift direction of H+ is away from the Si/SiO2 interface. The concentration of H+ ions in SiO2 begins to increase, resulting in oxide layer traps. These interface states and traps lead to the change of semiconductor device parameters. With the increase of H+ ion concentration in SiO2 dielectric layer, H+ will diffuse towards the interface. In fact, if the stress effect is stopped, i.e., the electric field drops to 0, the H+ will generate backflow, resulting in partial recovery of the device. However, complete recovery is not possible because some of the H+ ions will undergo a reduction reaction in the SiO2 gate dielectric layer without reflux. According to the model, the failure time of NBTI can be expressed as

F=A0exp(-ϒE)exp(Ea/kBT)        (7-13)

A0=[1/C(△Vth/Vth)crit]1/m        (7-14)

Where, C0 is a constant proportional to the concentration of Si-H bond at the Si/SiO2 interface; M is the power-law index of time T. In general, M =0.15~ 0.35. It can be seen from Equation (7-14) that the pre-failure time factor A0 is positively dependent on the parameter degradation critical value, and negatively dependent on the Si-H bond concentration at the Si/SiO2 interface. If C0 goes to 0, then NBTI invalidates for an infinite time. The saturation phenomenon of NbTi degradation can also be explained by the reaction-diffusion model, because the number of Si-H bonds is limited. With the increase of time, the number of unbroken Si-H bonds decreases, and the degradation rate caused by S-H fracture also decreases continuously, approaching zero.

Si - the formation of SiO2 interface state is a major factor in the production of NBTI effect, while the hydrogen and water vapor is the cause of NBTI two main substances, they occur on the interface of the electrochemical reaction, induced by the donor interface state type Nit threshold voltage drift, in the device operation process of the oxide trap charge Not will also make the threshold voltage drift. In order to reduce the NbTi effect, the initial defect density at the Si-SiO2 interface must be reduced and water does not appear in the oxide layer. Injecting deuterium into the Si-SiO2 interface to form the Si-D bond is an effective method to improve NbTi.

The NbTi properties of devices subjected to plasma damage (PID) degrade because charge damage results in higher interfacial state densities. These higher initial interfacial state densities result in higher NbTi degradation, although they may be passivated during subsequent annealing. NBTI can be used as an effective means to detect plasma damage (PID) of potential plasma equipment.

Jin et al. studied the influence of various annealing processes on NbTi, and found that pure H2 annealing in plasma equipment helped more to improve NbTi than N2/H2 mixture, which was explained as pure H2 had higher H2 content, more H reached the Si-SiO2 interface, and passivation of suspension bond was more obvious. However, the annealing time has obvious saturation effect, when the annealing time is more than 0.5h, the extension of annealing time can not further increase the failure time of NbTi. According to the conclusion of Lee et al., excessive H in plasma equipment is closely related to the formation of interface states. Therefore, Jin believes that when too much H drifts to the Si-SiO2 interface, it will combine with the H in the passivated Si-H bond to form H2, leaving a new suspension bond, thus degrading the performance of NbTi. So the way H is introduced, the amount of H introduced is very important.

In the pseudo - gate removal process of plasma equipment, HBR gas is generally used to achieve a high selection ratio of etching work function metals, and the active H ions generated by the plasma equipment disassembly will damage the gate dielectric and affect NbTi. The dissociation rate of HBR can be reduced and the NbTi can be improved obviously by using plasma equipment to synchronize the isohigh daughters without affecting other properties. In the light removal process after pseudo gate removal, the N2/H2 with higher H2 content can increase the failure time of NbTi by an order of magnitude compared to the N2/H2 ash process with lower H2 content in the plasma equipment.

 plasma equipment

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