Semiconductor device manufacturing: process – Making field effect device having pair of active regions...
Reexamination Certificate
1999-06-17
2001-12-25
Christianson, Keith (Department: 2813)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Reexamination Certificate
active
06333214
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor memory, and more particularly, to a memory of a multilevel quantum dot structure and a method for fabricating the same.
2. Background of the Related Art
It can be foreseen that the use of a MOS structure as a basic switching device will reach its limit as device packing density increases. In a case of the MOS structure with a device packing density in a range of 4 giga DRAM, a switching operation using a gate voltage in accordance with the principles of a MOS device operation will be impossible, because the distance between a source and a drain is reduced to approx. 0.13 &mgr;m (S. Wolf. “Silicon Processing; for the VLSI Era’, V2, chap.8). Due to device malfunctions caused by tunnelling between the source and drain and tunnelling through a gate oxide film when no gate voltage is applied, the limit of an integration using the MOS structure will be approx. 4 giga DRAM. Therefore, in order to fabricate a device having a large density such as giga class, or the like a device other than the current MOS structure should be employed. The form of device suggested by many research groups is the SET (Single Electron Transistor) (see, K. K. Likharev, IBM J. Res. Develop. 32(1) p144(1988)). The SET is a device employing the so called Coulomb blockade effect pertaining to quantum effects exhibited by the interaction between electrons having a nano-scale dimension, whereby any further tunnelling of charges is suppressed during the process of tunnelling charge carriers such as (electrons or holes) through an insulating film, such that the individual flow of discrete electrons, can be controlled.
The principle of the Coulomb blockade effect caused by an SET tunnelling is as follows. (M. H. Devoret and H. Grabert, in “Single Charging Tunnelling”, edited by H. Grabert and M. H. Devoret (Plenum N.Y., 1992) p1).
If the total capacitance caused by a region into which electrons enter through tunnelling is very small, a charge effect of the discrete electrons can be observed. If a charge energy e
2
/2C of the discrete electron charge is greater than an energy k
B
T of a thermal vibration (M. H. Devoret and H. Grabert, in “Single Charging Tunnelling”, edited by H. Grabert and M. H. Devoret Plenum N.Y., 1992) p1), and there is no voltage increase applied externally when the temperature remains constant, an electron can not have the energy required for charging a capacitor by tunnelling. Accordingly, there is no further tunnelling occurred once one electron is charged. That is, since an electron previously tunnelled, and charged in a capacitor causes a low voltage having a level of at least a voltage drop at the capacitor, is applied to the next electron, the next electron does not achieve the level of energy required for charging by tunnelling, and thus no further tunnelling occurs. This effect of suppression of further tunnelling due to electrons that are already tunnelled called a Coulomb blockade effect. When an effective voltage applied to an electron in a double junction structure receiving an external voltage is V, the electron will have an energy of eV, and when the electron is charged in a capacitor by tunnelling, an energy loss amounting to the charged energy occurs. The effective voltage that this charge applies to the next charge is
V
-
e
2
C
such that the electron is to have an energy of
e
V
-
e
2
2
C
.
Because a subsequent electron to tunnel at a later time can not have the energy required for charging due to this energy difference occurs (even if an identical voltage is applied), further tunnelling occurs.
It is a basic concept of the SET that individual electrons are controlled to make a discrete movement using this Coulomb blockade effect. If thermal energy can complement the energy difference, the tunnelling of electrons can also occur without any increase in the applied external voltage. In order to suppress this undesirable tunnelling caused by thermal vibrations, an energy loss of
e
2
2
C
caused by the charging should be greater than the thermal energy k
B
T. Therefore, in order to satisfy this condition, an overall capacitance of the system should be, for example, as small as 3 aF. Most of the SET devices reported currently have a large quantum dot formed such that a capacitance of greater than 3 aF is achieved, resulting in an operation temperature being maintained very low due to a thermal smearing effect. In order to cause the Coulomb blockade effect, in addition to the suppression of the thermal smearing by having the charge energy greater than the thermal energy, the following condition should also be satisfied (M. H. Devoret and H. Grabert, in “Single Charging Tunnelling”, edited by H. Grabert and M. H. Devoret (Plenum N.Y., 1992) p1).
Rt>>Rk,
where Rt is a tunnelling resistance, and Rk is a resistance quantum (=25.8 k&OHgr;). The above condition, coming from the theory of uncertainty, implies that the above condition should be satisfied for preventing an occurrence of tunnelling, because tunnelling can occur without any increase of external voltage if the range of energy variation of an electron is greater than the charged energy.
Δ
E
Δ
t
≥
h
2
(
Theory
of
uncertainty
)
Two types of the SET structures using the Coulomb blockade effect have been known. One type, like a MOS structure, has a source, a drain, and a gate, with a channel having conductive quantum dots for facilitating a discrete electron flow. Therefore, the channel consists of an insulating material and the conductive quantum dots, facilitating an electron flow by using discrete tunnelling. That is, the channel has quantum dots contained in the insulating material. (K. Nakazato, T. J. Thornton, J. White, and H. Ahmed, Appl. Phys. Lett. 61(26), 3145(1992); D. J. Paul, J. R. A. Cleaver, H. Ahmed, and T. E. Whall, Appl. Phys. Lett. 63(5), 631(1993); D. Ali and H. Ahmed, Appl. Phys. Lett. 64(16), 2119(1994); E. Leobandung, L. Guo, Y. Wang, and S. Y. Chou, Appl. Phys. Lett. 67(7), 938(1995); K. Nakazato, R. J. Blankie, and H. Ahmed, J. Appl. Phys. 75(10), 5123(1992); Y. Takahashi, M. Nagase, H. Namatsu, K. Kurihara, K. Iwdate, Y. Iwadate, Y. Nakajima, S. Horiguchi, K. Murase, and M. Tabe, IEDM 1994, p938; E. Leobandung, L. Guo, and S. Y. Chou, IEDM 1995, p367; O. I. Micic, J. Sprague, Z. Lu, and A. J. Nozik, Appl. Phys. Lett. 68(22), 3150(1996)). In the formation of the quantum dots, a variety of methods can be employed. The quantum dots may be formed in an insulating material by EBL(Electron Beam Lithography) and RIE(Reactive Ion Etching)(Nakazato, T. J. Thornton, J. White, and H. Ahmed, Appl. Phys. Lett. 61(26), 3145(1992); D. J. Paul, J. R. A. Cleaver, H. Almed, and T. E. Whall, Appl. Phys. Lett. 63(5), 631(1993); D. Ali and H. Ahmed, Appl. Phys. Lett. 64(16), 2119(1994); E. Leobandung, L. Guo, Y. Wang, and S. Y. Chou, Appl. Phys. Lett. 67(7), 938(1995); K Nakazato, R. J. Blankie, and H. Ahmed, J. Appl. Phys. 75(10), 5123(1992); Y. Takahashi, M. Nagase, H. Namatsu, K. Kurihara, K. Iwadate, Y. Takahashi, M. Nagase, H. Murase, and M. Tabe, IEDM 1994, p938; E. Leobandung, L. Guo, and S. Y. Chou, IEDM 1995, p367), or by applying an electric field to a doped substrate to form local conductive regions (H. Matsuoka and S. Kimura, Appl. Phys. Lett 66(5), 613(1995)). Nano scale quantum dots may be formed by chemical synthesis (N. Uyeda, J. Colloidal Interface Science 43, 264(1973); A. A. Guzelian, U. Banin, A. V. Kadavanich, X. Peng, and A. P. Alivisatos, Appl. Phys. Lett 69(10), 1432(1996)), or by cluster beam deposition (W. Chen, H. Ahmed, and K. Nakazato, Appl. Phys. Lett 66(24), 83(1995)).
In the aforementioned form of devices, a current-to-voltage curve has an ideal stepped form due to electrons passed through the channel by tunnelling and blocking the next incoming electrons. These different currents are used to provide memory capabilities. Although the
Kim Ki Bum
Kwon Jang Yeon
Yoon Tae Sik
Christianson Keith
Hynix / Semiconductor Inc.
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