Memories - Memory Faults - Part 4

As memories grow larger, with more memory cells packed into an ever-shrinking die area, the cost to manufacture a die remains fairly constant, while the time it takes to apply test programs increases exponentially. It is estimated that the cost to test a memory chip runs from 50% to 70% of the total cost of the finished product. The first steps in reducing the cost of memory test is to understand what fault mechanisms are most likely to occur and then develop test programs that target those faults. With this approach, the manufacturer and the end-user can determine their priorities, balancing cost versus DPM (defects per million) that they can tolerate in their applications.

A number of different failure types can occur in semiconductor memories, affecting memory cell contents, cell addressing, and the time required to read out data. Some of the more common failures include the following:

Cell opens or shorts
Address non uniqueness
Cell/column/row disturb sensitivity
Sense amplifier interaction
Slow access time
Slow write recovery
Data sensitivity
Refresh sensitivity
Static data losses

Opens and shorts within semiconductor memory cells may occur because of faulty processing, including misaligned masks or imperfect metallization. These failures are characterized by a general randomness in their nature. Opens and shorts may occur at the chip connections to a printed circuit board. In a km × n memory system containing km words of n bits each, and made up of memory chips of size m × 1, a fault that occurs in bit position i of m consecutive bits is indicative of either a totally failed chip or one in which an open or short exists between the chip and the PCB on which it is mounted.

Address non-uniqueness results from address decoder failures that may either cause the same memory cell to be accessed by several different addresses or several cells may be addressed during a single access. These failures often cause some cells to be physically inaccessible. An effective test must insure that each read or write operation accesses one, and only one, memory cell.

Disturb sensitivity between adjacent cells or between cells in the same row or column can result from capacitive coupling. Slow access time can be caused by slow decoders, overloaded sense amplifiers, or an excessive capacitive charge on output circuits. Slow write recovery may indicate a saturated sense amplifier that cannot recover from a write operation in time to perform a subsequent read operation.

A memory cell can be affected by the contents of neighboring cells. Worse still, the cell may be affected only by particular combination's on neighboring cells. This problem grows more serious as the distance between neighboring cells shrinks. Refresh sensitivity in dynamic RAMs may be induced by a combination of data sensitivity and temperature or voltage fluctuations. Static RAM cells are normally able to retain their state indefinitely. However, data may become lost due to leakage current or opens in resistors or feedback paths.

By contrast, when we look at faults in random logic, that fault models other than the stuck-at model were examined. The one trait these models had in common was a susceptibility to combinatorial explosion. For very small circuits, the number of faults grew so quickly that it was simply not feasible to consider them. Memory circuits, because of their density and the close proximity of cells to one another, exhibit this problem of combinatorial explosion to a far greater degree. Hence, it becomes necessary to restrict consideration to faults that are most likely to occur.

The first step is to group the faults into three broad categories: address decoder faults, memory array faults, and read/write logic faults. From there we use the fact, that faults in memory addressing and read/write logic, which includes sense amplifiers, write drivers, and other supporting logic, can be mapped onto functionally equivalent faults in the memory array. This makes it possible to concentrate on faults in the memory array and to develop tests addressed at the functionality of the memory array.

First consider faults in the address decode logic. A fault may cause multiple cells to be accessed, or no cell may be accessed, or the wrong cell may be addressed. In the case of multiple cells being addressed, the fault may be viewed as a coupling fault between cells. If no cell is addressed, then, depending on the logic, the response from the read logic may appear as a stuck-at-1 or a stuck-at-0. If the wrong cell is addressed, then, given the presence of the opposite value in that cell, it appears as a stuck-at fault.

A fault in the read/write logic may cause an output line to be stuck-at-0 or stuckat-1. In either case, the corresponding cell may be considered to be stuck-at-0 or stuck-at-1. If there are shorts or capacitive coupling between data input or data output lines, these faults can be regarded as coupling between memory cells.

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Article submitted by:
Murali of IBM