Low-k dielectrics
For the fabrication of 0.18µm circuits, dielectric layers are required that correspond to a silicon dioxide layer thickness of only 4nm. For 0.13µm structures, layers are even needed that correspond to a 2nm SiO₂ layer. For further scaling down, the use of SiO₂ layers becomes increasingly critical. In modern semiconductor technologies, especially at very small feature sizes, SiO₂ layers as dielectrics are no longer always sufficient: Here, low-k and even ultra-low-k materials are increasingly being used. Modern production lines utilize materials with a significantly lower permittivity (k-value) than SiO₂, such as porous silicates or carbon-doped oxides. In addition, further optimized dielectrics are being researched, such as those with air gaps or innovative composite materials, to further improve the performance and energy efficiency of chips with extremely fine structure sizes.
Low-k material can be used in modern semiconductor factories for multilayer interconnections, for interlayer dielectrics (ILDs), passivation layers, shallow trench isolation, and deep trench masking. Low-k material can be employed either with traditional aluminum wiring or with modern dual-damascene copper processes.
Low-k materials can be divided into the following groups: doped oxides, organic materials, highly fluorinated materials, and porous materials. The group of porous materials combines micropores with the other listed materials. Conventionally used silicon dioxide has a dielectric constant of 4.2. Air is considered an ideal insulator and has a dielectric constant of 1. Porous materials can therefore generally achieve lower dielectric constants than the corresponding solid substances. These new materials are being developed for ICs with structures <0.10µm. Dielectric constants of 2–3 can be achieved. Oxides, doped oxides, organic materials, and highly fluorinated materials will all in the future be combined with micropores.
Deposition methods
Low-k materials can be deposited either by spin-coating or by CVD processes. For porous materials, the spin-coating method is predominantly used. Special attention must be paid to controlled evaporation of solvents. The subsequent required curing usually takes place in a batch furnace. Today, vertical furnaces are mostly used for this application to achieve the necessary high temperature uniformity. The cure temperature, however, is typically in the 350°C – 400°C range, which places high demands on a conventional vertical furnace.
The curing process
JTEKT Thermo Systems (formerly Koyo Thermo Systems) has long experience with a similar SOG curing process and has developed a special furnace using an LGO heating element with particularly low thermal mass for this purpose. Now JTEKT Thermo Systems has again developed a furnace for curing low-k dielectrics. The LGO heating element used has already been successfully applied for years in JTEKT Thermo Systems furnaces for other low-temperature processes such as polyimide curing and hydrogen baking.
Low oxygen levels
For the production of good low-k dielectrics an oxygen content below 20 ppm during the process is indispensable. In our JTEKT furnace oxygen levels below 8 ppm can be achieved. These low oxygen levels nevertheless do not require vacuum technology! JTEKT Thermo Systems has developed an (considerably more economical) atmospheric furnace that meets the needs of polyimide curing and delivers excellent results. The graphic shows the reduction of the oxygen level in the furnace over time (from right to left, with multiple changing concentration scales).
Furnaces
JTEKT Thermo Systems (formerly Koyo Thermo Systems) manufactures vertical furnaces for curing low-k dielectrics for batch sizes of 100–150 wafers. The 150 mm and 200 mm versions are available with carousel loading (VF5100) and optionally with an integrated cassette storage (VF5300). For processing 300 mm wafers JTEKT Thermo Systems has developed a large FOUP-loaded version (VF5900). A mini-batch furnace is also available (VF5700). For pilot lines and research institutes there is the manually loaded VF1000 for wafer sizes from 150 mm to 300 mm.
Please contact us if you need more information about low-k cure or our related furnaces. Trial runs with our furnaces can be carried out in our applications laboratory in Tenri.