Plasma Etch Chemistries for Deprocessing Integrated Circuits

The five most commonly used materials, which are etched while deprocessing ICs are silicon dioxide, silicon nitride, polyimide, polysilicon and aluminum. This article provides a detailed recipes for deprocessing each of these materials and reasons for using these specific materials.

Commonly Used Materials

Table I shows the commonly used materials and the common etch chemistries for all these materials.

Table 1. Common materials and etch processes

Polyimide

There are different types of polyimides presently available. They have a number of solids content, curing properties, etc. But they are all hydrocarbons and easily etch in oxygen plasmas. This reaction is a simple oxidation of the organics and described by the following equation:

C_xH_y (s) + O₂ (g) + plasma --------> CO (g) + H₂O (g)

The key challenge in removing polyimide is to ensure that the polyimide does not get overheated in the process. In case this occurs, carbonization of the polyimide takes place resulting in a grass-like residue, which is almost impossible to remove. This problem can be solved by minimizing the ion bombardment, running minimal power levels, sample floating in the sample or by using a hybrid reactor. An appropriate starting recipe for removal of polyimide is shown in Table II

Table 2. Polyimide etch recipe

Silicon Nitride

Silicon nitride normally consists of the final passivation layer of an IC. It etches easily in plasmas that contain a large amount of free fluorine such as CF₄/O₂ or SF₆/O₂ or plasmas. The SF₆ is isotropic by nature. In this particular case, this characteristic is beneficial in removing the nitride sidewalls surrounding the top metal. The equation for this reaction is given below:

Si₃N₄ (s) + SF₆ + plasma --------> SiF₄ (g) + SF₂ (g) + N₂ (g)

A good starting recipe for nitride is given in Table III.

Table 3. Nitride etch Recipe

Silicon Dioxide

There are several varieties of silicon dioxide used today. The chemistry of etching is the same for all of them, but the etch rates and recipes differ a little with the type. The etching process is normally quicker for highly doped oxides and etching done with oxides having high carbon content is not very fine. The chemical reaction for this process is shown below:

SiO₂ (s) + CF₄ (g) + plasma --------> SiF₄ (g) + CO (g)

Silicon dioxide etching is essentially anisotropic because the strong chemical bond between oxygen and silicon and oxygen needs bombardment of ions in order to break.

A staring recipe for upper layer SiO₂ is provided in Table IV-a.

Table 4-a. SiO₂ etch recipe

In case an inter-layer dielectric (ILD) etch is preferred where the ILD is SiO₂ another technique is preferred. This process depends on extremely low pressures to prevent grass formation. When the process is conducted under lower pressures the mean free path (MFP) and the surface of the IC are bombarded with a highly reactive species that possesses more energy. Low RIE power or voltage is used to maintain the IC’s functionality and allows less heat build up on it. ICP power is used to maintain a relatively high etch rate by creating a HDP. This process is shown in Table IV-b.

Table 4-b. ILD etch recipe

Aluminum

Pure aluminum etches easily in a Cl₂ plasma by itself. But, a native oxide layer covers all aluminum films. Pure Cl₂ does not etch this oxide, so BCl₃ is added to increase the sputtering amount and to scavenge the oxygen in the aluminum oxide layer.

In most advanced integrated circuits, aluminum is alloyed with tiny amounts (0.5% to 2%) of silicon and copper. It is especially difficult to etch copper by RIE, so BCl₃ is also useful for increasing the amount of physical sputtering which removes the copper. Aluminum itself is removed very rapidly, but completely removing other metal residues need a longer etch.

A range of wet chemical etchants can also be utilized to remove the metallization, but the chemicals will enter the vias and isotropically undercut the next lower metal line. It is important that aluminum etching be done in a separate reactor or one that has been cleaned thoroughly, as opposed to a reactor used for silicon compound etching. This is because the chemistries may actually “poison” each other. Polymer residues containing fluorine when etched with oxide will react with aluminum to form aluminum fluoride on the metal surface, which is inert to chlorine etch. By-products of aluminum chloride left behind from the aluminum etch, form an aluminum fluoride powder when exposed to fluorinated plasma. This results in the powder subsequently falling and contaminating the sample surface. Another important consideration in aluminum etching is contamination of moisture. It is highly recommended that a vacuum load-lock is used for this application. Aluminum etching is one of the most difficult processes. When done properly, one can expect excellent etch results. A good starting recipe for a more uniform but isotropic etching of the Al is given in Table V.

Table 5. Aluminum Etch Recipe

Polysilicon

Polysilicon can be etched anisotropically & isotropically in chlorine gas, and it is also very selective to oxidize. The table below outlines an isotropic & anisotropic etch recipes using chlorine chemistry.

Table 6. Polysilicon Etch Recipe

This information has been sourced, reviewed and adapted from materials provided by Trion Technology.

For more information on this source, please visit Trion Technology.