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What is a defect in microelectronic?

It is not a seecret that micrtoelectronic chips are fabricated in "clean rooms". The reason for this is very simply: to avoid defects which could compromise the functionality of the chips. 

However, what is a defect? That’s a simple question, but very difficult to answer!

Test structures on SDRAM wafers

A SDRAM chip contains a lot of structures like pn-junctions (diodes), transistors, resistors, P/N-walls etc. In order for a chip to function well the structures enumerated before should fulfill some criteria specified by the designers of the chip. Thus, methods are needed to measure the properties of “transistor & Co” in order to see if the manufactured structures fulfill the specified criteria.

Front Opening Unified Pod / FOUP: boxes for wafers

A FOUP (Front Opening Unified Pod) is a closed and transportation safe box for wafers within a semiconductor factory. The main purpose of a FOUP is to isolate the wafers from the clean room environment. The FOUP concept allows to save investments needed for a class 1 clean room. Due to the fact that inside the FOUP a special microclimate/micro-clean-room is generated/maintained and the wafers are isolated from the clean room atmosphere, the class of the clean room itself can be higher(=worse), e.g. 1000, as would be necessary without the FOUP concept. Investments and maintenance for a class 1000 clean room are much lower as compared with a class 1 clean room. Due to this reasons, FOUPs are indispensable accessories in modern 300mm microelectronic factories. Examples of FOUPs can be seen in Figure 1 and 2.

SDRAM Factory Schematics: Back End and Front End Factories

As illustrated in Figure 1, the manufacturing process of SDRAMs can be divided in two parts:

  • Front End (FE)
  • and Back End (BE).

The FE is the part of the manufacturing where the work is done on the wafer level, whereas in BE the work is done at chip and module levels. According to this classification also the so called FE and BE factories can be distinguished. FE and BE factories can be on the same or on different locations.

SDRAM cross section view: stack vs. trench capacitor concepts

Technologically two DRAM concepts dominated the market till 2009:

  1. Stack capacitor concept (see a cross section in Figure 1)
  2. Trench capacitor concept (see Figure 2);

The last microelectronic companies which produced, but not producing anymore, DRAM chips based on the trench capacitor concept were Infineon Technologies, namely its daughter company Qimonda, Inotera Memories, Nanya Technologies and Winbond. The “rest of the DRAM world” (Samsung, Micron, Elpida etc.) produced and are still producing DRAMs based on the stack capacitor concept.

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