3D Flash Memories by Rino Micheloni

Author:Rino Micheloni
Language: eng
Format: epub
Publisher: Springer Netherlands, Dordrecht

Wordline interference has also been observed in VG-type 3D NAND when programming adjacent wordlines in the same layer due to the tight pitch. The WL interference is associated to channel potential variations in the junction free channel of the selected cell, which are induced by the charge accumulated in adjacent pass cells. WL interference results in 400 mV VTH shift; this shift can be contained by proper algorithmic countermeasures. Programming algorithm with Pre-PV ( Pre-Program Verify ) level can be used to alleviate WL-interference. The algorithm consists of programming WL(n) to a pre-PV level lower than the target PV level, then programming WL(N + 1) to the pre-PV level, going back to program WL(n) to the final PV value, and so on.

Since all the layers are programmed simultaneously, as part of the same page in 3D-VG NAND, the interference and other effects caused by the programming pattern must be considered.

A program disturb effect has been studied. As shown in Fig. 7.23, when a certain layer is programmed, the related channel is kept at 0 V, while adjacent channels, if not programmed, are boosted to the inhibit voltage. Due to the capacitive coupling between layers, the resulting boosted value is lowered with respect to the case where all the channels are in the inhibit condition. The inhibit function is therefore degraded and a program disturb can be observed. The threshold shift due to Z-disturb can result in a shift of 0.6 V of the VTH distribution, because of the reduced inhibit efficiency [24]. Moreover, when different layers are programmed at different times, an impact of the first programmed cells on the following programming operations can be observed, due to the electrostatic potential change. This effect is substantially changing the VTH distribution.

Another Z-effect, called enhanced programming Z-disturb, can be observed in VG-type 3D NAND. When adjacent layers are programmed (biased to 0 V), the programming speed is reduced with respect to the case where adjacent layers are inhibited (boosted to inhibit high voltage). This effect is explained by the inversion electron density function at the polysilicon channel that is increased in the second case, while in the first case it limits the F-N tunneling current.

It is necessary to consider the impact of the Z-effects when the programming pattern changes during the ISSP steps. The described Z-disturbs will cause the enlargement of the programmed distribution and reduce the margin for multilevel operation.

Z-interference (electrostatic) effects are obviously dependent on the thickness of the buried oxide between layers; therefore, they are also controllable, to some extent, by augmenting the inter distance between layers [8]. Algorithmic methods can also be adopted to control the Z-disturbs. In the example shown in Fig. 7.24 the programming sequence is split into a sequence involving only certain layers at a time. In this example eight layers are divided into three groups and programming is accomplished in three programming steps. During each of the programming steps, each inhibited layer has only one adjacent layer (in Z-direction) which is programmed, thus limiting the interference to one neighbor only.

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