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Application of etching in plasma cleaning machine and introduction of new magnetic memory

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23 Dec,2020

Application of etching in plasma cleaning machine and introduction of new magnetic memory:

 

Magnetic Random Access Memory (MRAM) is a type of Memory with Magnetic Tunnel Junction (MTJ) as its core component. The magnetic tunnel junction is a sandwich structure of ferromagnetic Layer/tunnel barrier Layer (metal oxide, such as MgO)/ ferromagnetic Layer. One Layer of ferromagnetic material is called the Reference Layer, whose magnetization direction is fixed, while the other Layer is called Free Layer, whose magnetization direction can be changed by the external magnetic field or Polarized Current. When the magnetization directions in the fixed layer and the free layer are the same, the MRT junction presents low electrical positive. When the magnetization directions are different, the MRT junction presents high electrical resistance. This phenomenon is called the tunneling magnetoresistance effect.

 

Conventional magnetic memory, which uses an external current to generate a circular magnetic field to change the direction of free-layer magnetization, has been replaced by Spin Transfer Torque (STT) magnetic memory, which has large storage units and no read-write speed advantages over other memory. The so-called spin transfer moment refers to the change in the magnetic moment of atoms in the ferromagnetic layer when the spin polarization current passes through the nano-sized ferromagnetic layer. This means that the magnetic tunnel junction can be driven directly by the current. After the electron spin polarization, a torque is generated on the ferromagnetic atom to change the magnetization direction in the ferromagnetic layer to realize the change of resistance. Therefore, the area and performance of the memory can be improved. The 1T1M (One One MTJ) magnetic memory consists of a magnetic memory in which a magnetic tunnel junction is visualized using a few bytes or bytes in the middle of the Transistor.

 

Spin transfer torque of magnetic memory are made in standard CMOS logic circuit after a period of metal connection layer embedded storage unit (magnetic tunnel junction), integrated with the spin transfer torque of the magnetic tunnel junction logic after a period of electric and magnetic tunnel junction of general process, obviously, the magnetic tunnel junction etching on device performance is very important. Ion Beam Etching (IBE), inductively coupled plasma Etching (ICP), REACTIVE Ion Etching (RIE) and other systems are the main Etching technologies used at present. It is worth noting that the shape of magnetic tunnel junction not only affects device performance, but also significantly affects the etching process of plasma cleaning machine. For example, the etching of cylinder or ring pattern is relatively simple.

 

At present, it has been reported that the materials used in magnetic tunneling junction contain Fe, Co, Ni, Pt, Ir, Mn, Mg and other metal elements and are generally made of 5-10 layers of single-layer materials (alloy or metal oxide) at the order of 1nm. Therefore, the challenges of plasma etching in the plasma cleaning machine of magnetic memory include:

1. Traditional reactive plasma (RIE) is confronted with the non-volatile problem of metal etching by-products;

2. The super-thin stacked structure of single layer materials has a high requirement for etching selection ratio and directivity;

3. The halogen gas commonly used in metal etching can corrode ultra-thin metal material layers easily. In particular, the barrier is mainly metal oxide and its thickness in the vertical magnetic tunnel junction is less than 3nm, which is prone to corrosion and therefore affects the electrical isolation of the fixed layer and the free layer (Electrically Isolated).

4. Process temperature limit, for example, the magnetism of most metal materials will decrease after exceeding 200°C. This temperature limitation is not only manifested in the shrinkage of the temperature window of the corresponding material etching formula, but also in the low temperature forming of the hard mask material etching resistance is generally lower.

 

Therefore, IBE as the representative of ion milling technology without corrosive side effect always occupies a place in the etching of magnetic tunnel junction plasma cleaning machine. The problem it faces is that the metal material stripped in the etching process may be redeposited in the side wall, and the subsequent cleaning process is difficult to remove, which will greatly affect the performance of the device. If it is deposited on the side wall of the barrier layer, it will directly cause short circuit. In addition, the shadow effect of secondary sediments will cause the etching shape to become more and more oblique with time. The overall tilt and rotation of the wafer can ameliorate this problem, but it also severely restricts its productivity. The uniformity and orientation of the ion beam at the 300mm wafer level also remain to be resolved.

Compared with the plasma cleaning machine RIE and ICP etching, the formation of lateral wall sediments can be controlled more effectively. The etching selection between different materials is of great significance to the graphics transfer precision and the etching shape control. Plasma cleaning machine metal etching usually use halogen gases (mainly containing Br, Cl, F gas), if used in magnetic storage graphical, bring the side effects of etching is volatile by-products metal corrosion problems caused by the residual, ultra-thin layer material in magnetic core memory unit performance more obvious etching. Although the etch by-products can be activated at temperatures above 350°C, the associated magnetic properties are significantly degraded. Generally, it is improved through etching treatment of post-plasma cleaning machine (such as He/H2 post-etching treatment), optimization of wet cleaning process and multi-process integration machine (film deposition, etching and cleaning modules are placed on the same platform, and vacuum environment is maintained all the time).

 

The alternative of halogen gas is non-corrosive etching gas and magnetic tunnel junction etching mainly by physical bombardment. Inductively coupled plasma with higher plasma density is generally used in plasma cleaning machine. Currently, CO/NH3 mixtures are studied more, and the etching by-products Fe(CO)5 and Ni(CO)4 formed in plasma etching are volatile, which can effectively alleviate the need for etching treatment after etching. However, the plasma dissociation rate of the mixture is much lower than that of halogens, the etching rate is lower, and the control of the etching shape is weak. CH3OH (methano, also written me-OH) plasma overcomes this problem, and it can be seen that the range of etching rate of the PLASMA in the ME-OH plasma cleaning machine is even beyond that of the halogen plasma). Meanwhile, due to the extremely high selection ratio of hard mask (usually tantalum (Ta)), a relatively straight etching shape can be achieved by enough etching. The relatively large hysteresis loop migration of Ar/Cl plasma indicates that the underlying fixed layer is seriously corroded, while the smaller hysteresis loop migration of CH3OH than Ar ICP also indicates the existence of chemical reaction in plasma etching of CH3OH plasma cleaning machine. The carbon-bearing film formed by this chemical reaction can absorb the energy of the incident ion, thus reducing the plasma damage (PID). It is found that the decrease of magnetic resistivity caused by the inevitable magnetic degradation of materials caused by reactive ion etching can be improved by optimizing the CH3OH/Ar ratio.

 

In addition to the optimization of the shape control by gas selection, the introduction of pulse power technology has brought further improvements.

Besides IBE and ICP, NBE, Neutral Beam Etch is also an important candidate technology. In the NBE scheme, a metal oxide layer is first formed on the surface of transition metal elements (Ru, Pt, etc.) through low temperature (-30℃)O2NBE, and then the oxide layer is removed by chemical reaction with EtOH/Ar/O2NBE. Due to the absence of physical bombardment and corrosive etching by-products, the problems of secondary wall stacking and plasma damage in the plasma cleaning machine can be avoided. The shape of Ru etching is even close to vertical. The measured hysteresis curve also indicates the ability of NBE to solve the magnetic material damage.

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