Proximity Effect in the Low Pressure Chemical Vapor Deposition of Tungsten.
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Proximity Effect in the Low Pressure Chemical Vapor Deposition of Tungsten. N. Lifshitz AT&T Bell Laboratories Murray Hill, New Jersey 07974 ABSTRACT The self-limiting effect during Low Pressure Chemical Vapor Deposition of tungsten is manifested by a sudden interruption of the reduction of tungsten hexafluoride WF 6 by silicon, so that only very thin (selflimited) films of silicon-reduced tungsten can be grown. It has been shown that the self-limiting effect is caused by formation of the non-volatile subfluornde WF 4 . The temperature dependence of the self-limiting thickness of the tungsten films grown in a hot-wall reactor exhibits a characteristic maximum at temperatures near 350"C, which indicates that at this temperature the rate of formation of WF 4 is lower than at temperatures above and below. In the present paper we attempt to explain this peculiar dependence. We suggest a mechanism responsible for formation of the blocking agent WF 4 . We demonstrate that the presence of a hot tungsten surface is essential for the strong self-limiting effect. This leads us to the discussion of the proper selection of the reactor type (hot-wall vs. cold-wall) for different process requirements.
INTRODUCTION The self-limiting effect during Low Pressure Chemical Vapor Deposition (LPCVD) of tungsten has puzzled the scientific community for several years. Deposition of tungsten via reduction of tungsten hexafluoride WF 6 by hydrogen is described by the reaction: WF 6 + 3H 2 -+ W + 6HF
(1)
When a silicon surface is exposed, reduction of WF 6 by silicon can also occur. 2WF 6 + 3Si -* 2W + 3SiF4
(2)
In a neutral argon atmosphere only reaction (2) takes place. The self-limiting effect is manifested by a sudden interruption of reaction (2) at the very early stages of the process. In the present work we will be mainly concerned with the process of WF6 reduction by Si. Later in the paper we will relate our findings with the process of tungsten deposition that involve both reactions (1) and (2). Recently a model was proposed to explain the self-limiting effect by formation of a non-volatile subfluoride of tungsten, most likely tungsten tetrafluoride WF4 [1]. This subfluoride is found in large amounts in siliconreduced tungsten films. It is solid in the deposition temperature range, with a three-dimensional polymeric structure [2], and may block (chemically or physically) further interaction between WF 6 and silicon, thus causing the self-limiting effect. The exact nature of the blocking action still needs further investigation. Interestingly, analogous chemistry employing the reduction of molybdenum hexafluoride, MoF6 , by silicon
does not exhibit the self-limiting effect, so that silicon-reduced Mo films grow linearly with time [3]. This is expected because all subfluorides of molybdenum are volatile [4]. The extent of Si reduction before the onset of the self-limiting mechanism is an important parameter of the deposition process because it determines the extent of damage inflicted on the Si device by exposure to WF6 [5]. It i
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