Development Of 8Kb NOR Function Si Nanocrystals Floating Gate Memory Chip And Investigation Of Novel SiN_x Based Resistive Random Access Memory

With the development of semiconductor technology,the feature size of the memory continues to scale down,and the storage capacity doubled.With the increasing the density integration,the traditional polysilicon floating gate memory faces many challenges,such as the thickness of tunneling oxide layer is reduced which result in the loss of stored information due to electrons stored in the floating gate layer tunneling back to the channel.After the memory device undergo a long alternate write/erase operation,tunneling oxide layer is broken.In the traditional polysilicon floating gate memory is based continuous charge storage,even one weak spot in the tunnel oxide is sufficient to create a fatal discharge path,compromising long-term nonvolatility.The above problem can be resolved by the Si nanocrystals floating gate memory based on discrete charge storage.Because of the beneficial effects of Coulomb blockade and charge confinement,the thinner tunneling oxide layer is used without sacrificing retention characteristic,and the thinner tunneling oxide can significantly reduce the operating voltage.In addition,Si nanocrystals floating memory process is compatible with Flash technology.The morphology of Si nanocrystals can be directly observed by AFM and TEM.Then,the size and uniformity of Si nanocrystals can be adjusted by optimizing the growth parameters for obtaining high performance device.Thus,the Si nanocrystals floating gate memory has the good application prospects.As storage capacity continuation increasing and the process size further scaling down,the floating gate memory is expected to run into physical limits in the near future.To overcome the limit of traditional non-volatile memory,a variety of novel storage technology based on physical state changes have been proposed.Resistive random access memory(RRAM)has become one of promising candidates devices for next generation non-volatile memory due to its simple structure,better scalability,fast write/erase speed and easy realization of three-dimensional integration.SiN_x-based RRAM have attracted considerable attention because of ultra-low power consumption,faster resistance switching speed and multi-level storage.In addition,SiN_x as the resistive switching material is compatible with traditional Si-based technology,which is easy to integrate in the traditional process.In this paper,we show the storage characteristic of Si nanocrystals floating gate memory.Then,we have successfully fabricated the 8Kb NOR function Si nanocrystals floating gate memory chip,which can write/erase and read information.Moreover,many aspects of the novel SiN_x-based RRAM is systematically analyzed and researched,such as resistive switching characteristics,resistive switching physical mechanism and optimize device structure.The content of this article can be divided into five parts as follows:In the first part,we have successfully fabricated the Si nanocrystals floating gate memory cell,4×4 memory array and 8Kb NOR function memory chip.Memory window over 0.5V can be obtained under low pulsing voltage(±6V/1?s for writing and erasing).When the pulsing voltage increasing to±7V/1ms,the memory window increase to 2V,which is large enough for application.The Si nanocrystals floating gate memory has good retention characteristics.The charge loss rate of the memory is less than 30%even undergoing 10~7 write/erase cycles,which has good immunity for stress induced leakage current.In addition,the cross talk problem is resolved by optimizing the operating voltage in 4×4 memory array.Finally,we successfully realized the functional demonstration of 8Kb NOR function memory chip.In the second part,the Al/SiN_x/p~+-Sub device is fabricated and the based resistive switching characteristics is researched.The device shows the typical bipolar resistive switching behavior.When the compliance current is set to 100?A,the ratio of Ron/Roff reach to 10~3.No obvious degradation is observed in both HRS and LRS at 80°C and25°C after 10~5 s.In addition,the conductive mechanism of forming process is researched.The initial state current of Al/SiN_x/p~+-Sub origin from P-F current and F-N current.Under the electric field effect,the electrons inject into SiN_x film conduction band by P-F emission mechanism and F-N tunneling mechanism.The electrons in the conduction band are accelerated and bombard the Si-N bonds which results the broken Si-N bonds.Finally,nitride vacancies and the motion nitride ions are formed.The tunneling barrier of Fowler-Nordheim is reduced due to nitride vacancies generation during the forming process.The lower tunneling barrier result in the lower set voltage in subsequent resistive switching operation.We observed that the top electrode is destroyed by nitrogen bubbles in our experiment,which indirectly proved the existence of nitrogen ions migration.The similar resistive switching characteristics is observed in Al/SiN_x/p~+-Sub device by the magnetron sputtering fabrication.Therefore,the Si-N broken is rather than other chemical bonds during the forming process.Moreover,we realize forming-free in Al/SiN_x/p~+-Sub device by inducing the oxygen into SiN_x film which results increasing the defect density in the initial state and reducing the tunneling barrier of Fowler-Nordheim.In the third part,the conductive mechanism in high resistance state(HRS)and low resistance state(LRS)is researched in Al/SiN_x/p~+-Sub device.The conductive mechanism of HRS and LRS is due to Fowler-Nordheim and trap-assisted tunneling.Combined with the previous analysis of the forming process,we proposed the resistive switching mechanism is due to form/rupture conductive filament composed of nitride vacancies.The location of form/rupture conductive filament is located near the Al(anode)electrode.In addition,the LRS change from linear to nonlinear when the compliance current decreasing.This is due to the lower concentration of nitride vacancies in conductive filament under the lower compliance current.The trap-assisted tunneling barrier is higher when the lower concentration of nitride vacancies in conductive filament.In the fourth part,we study the effect of different electrode in SiN_x based RRAM.The performance of SiN_x based RRAM is attempted to optimize by changing electrode materials.In the Ag/SiN_x/p~+-Sub device,the endurance over 10~4 write/erase cycle is obtained.The multi-level storage phenomenon is observed.The resistive switching mechanism in Ag/SiN_x/p~+-Sub device is due to form/rupture chain conductive filament composed of Ag nanocrystal.In the Pt/SiN_x/p~+-Sub device,the lower LRS current is observed which stem from the lower nitride vacancies concentration in conductive filament.Because of high work function with Pt,the Schottky barrier is formed at Pt/SiN_x interface.During the forming process,a partial voltage drop at the Pt/SiN_xinterface and the effective voltage for nitrogen vacancies defects is reduced.In the Pt/SiN_x/TaN device,we obtain the opposite polarity bipolar resistive switching behavior comparing with Ag,Al,Pt as electrode materials SiN_x-based RRAM.The voltage polarity of resistive switching depends on the ability of electrode gettering nitride.Moreover,we obtain forming-free characteristic and lower operating voltage in the Pt/SiN_x/TaN device.Comparing with Pt/SiN_x/p~+-Sub device,Pt/SiN_x/TaN device provides better endurance characteristics,which proves that TaN electrode has better gettering nitride ability.