4D-technologies for production of three-dimensional integrated circuits
Traditionally, there are two types of complex three-dimensional objects in the production of TEU:
•quasi-three-dimensional shapes created from individual 2D modules based on rigid or flexible substrates with use of screen printing, hot stamping, thermal transfer etc. ;
•rigid three-dimensional (3D) shapes that are divided into two kinds: created on the basis of rigid substrates with use of traditional industrial methods (casting, stamping) and formed layer by layer using 3D printers [2–6].
All of these types of traditional 2D (quasi-3D) and the 3D technology for creation of three-dimensional electronic devices (table.1) have the general disadvantage, which is the inability to obtain multilayer complex 3D objects with use of all faces, edges and walls as functional elements.
The first step in the development of technologies for production of a full-fledged 4D TEU was the creation of a quasi 4D-objects. Corresponding group of technologies is called 3D-MID (Molded Interconnected Devices) or technologies for the manufacture of 3D IC on molded base [8–10]. Since there is no strict classification of 3D MID, from our point of view, it is reasonable to arrange these technologies in order of the discretization of the structures (table.2):
◦V 3D MID (V – volume) or molding technologies (insert molding);
◦S-3D MID (S – surface) or surface treatment technologies;
•hybrid (multi-level) 1G 3D MID technologies, where 1G is the generation of miniaturization.
In turn, the surface S-3D MID-technologies (Fig.1) can be classified into: liquid (LS-3D MID) and dry (DS-3D MID) or gas-phase. The first group includes the following technologies:
•2S-technologie (two step or two shot molding);
•laser direct structuring, LDS;
•aerosol jet, maskless, mesoscale or material deposition (M3D);
Dry (gas-phase) technologies include gas-plasma metallization (Flame con) and cold plasma deposition (Plasma dust).
From our point of view, it is necessary to separate the concepts of functional and accessory materials when considering the 3D MID-technologies. If the first is the primary material of the structural element, then the accessory one is not included in the functional material, but may participate in its shaping, fabrication or operation.
1. V-3D MID TECHNOLOGIES
The injection moulding with pre-shaped embedded parts is one of the most commonly used casting techniques (or moulding technologies). It includes several types of technologies [11–13]:
•in-mold decoration (IMD);
•in-mould labelling (IML);
•film insert molding (FIM);
Variations of film insert molding can be considered as a separate technologies of fabrication of quasi-4D-objects, and as methods of making molded bases (MID), for example, for S-3D MID technology (table.2).
Traditionally, the main idea of the insert molding is the production of multilayer multicomponent parts with fusion in the standard mold of two coctolabile bases with a layer between them (Fig.2a). Technically this operation is similar to the process of double-sided lamination of documents. Almost any materials – metal plates, various polymers, rigid or flexible composites, etc. – can be used as the intermediate layer. Thermoplastics that have different optical, electrical, mechanical and other characteristics, or completely different materials are used as bases.
Insert molding allows the finishing fusion of thin packages-bases (panels), using as standard structural insert the flat functional module (Fig.2b) with the switching system or the assembly, consisting of three-dimensional switching system and different components (conductors, resistors, capacitors, LEDs, etc.), for example, on the basis of the flexible printed circuit board (Fig.3).
The advantages of insert molding:
•low cost of equipment;
•the shortest production cycle per unit of production;
•use of enclosures as a substrate (circuit board) for the TEU;
•ability to use structural and functional modules as an insert.
Disadvantages of insert molding:
•limited range of materials;
•the inability to change the configuration of the TEU in the manufacturing process (use of finished insert);
•restrictions of the configuration for the non-thermoplastic bases (preparation of vias, large alignment errors for separate parts, etc.);
•ability to produce TEU only for the millimeter range using ready modules. ■
To be continued in the next issue