(Best Student Presentation Award) Reduced Pressure – Chemical Vapor Deposition of Monocrystalline and Polycrystalline Si(:B) and SiGe(:B) Layers on Blanket Wafers
Authors: Lespiaux Justine , Deprat Fabien , Vives Jérémy , Duru Romain , Bicer Mehmet , Gauthier Alexis , Brezza Edoardo , Juhel Marc , Chenevas-Paule Florence , Baron Alain , Valery Alexia , Hartmann Jean-Michel 
Published: November 2022

Monocrystalline SiGe alloys can be used instead of monocrystalline Si in order to increase the performances of devices such as heterojunction bipolar transistors (HBTs) [1]. The epitaxial growth of blanket, monocrystalline SiGe layers on Si with a dichlorosilane + germane + hydrochloric acid chemistry was extensively investigated in the literature [2,3]. Similarly, a switch from polycrystalline Si to polycrystalline SiGe can be favorable, for instance, in Complementary-MOS (C-MOS) devices [4,5]. However, the blanket growth by Reduced-Pressure Vapor Deposition (RP-CVD) of polycrystalline Si and SiGe has not been systematically investigated. We have thus performed a one-to-one comparison in terms of growth kinetics and electrically active dopant incorporation between blanket monocrystalline and polycrystalline Si(:B) and SiGe(:B) layers.

Epitaxies or depositions were performed, in 300 mm RP-CVD chambers, on two types of templates (Fig.1): i) N-type Si substrates (Fig.1(a)) and ii) poly-Si/oxide/ P-type Si substrates (Fig.1(b)). The poly-Si layer in stack 1.(b) was obtained thanks to amorphous Si deposition followed by annealing at 750 °C, resulting in a poly-Si film. SiH4+HCl+H2 (named Process 1) and SiH2Cl2 (DCS)+HCl+GeH4+H2 (named Process 2) chemistries were used to deposit Si and SiGe layers between 675-750 °C and 10-20 Torr.

Monocrystalline and polycrystalline SiGe growth kinetics for two process conditions: i) 750 °C, 10 Torr with a HCl partial pressure of 0.05 Torr and ii) 725 °C, 10 Torr, with a HCl partial pressure of 0.06 Torr, were similar to that in the literature [1-3, 6-8]. As already shown for monocrystalline layers, there was an increase of the intrinsic SiGe growth with the germane partial pressure and the temperature. Meanwhile, it was reduced when adding HCl (Fig.2). Very similar growth rates and evolutions were however obtained whatever the crystalline state of the layers.

When diborane was added the gaseous mixture, different behaviors were evidenced depending on i) the crystallinity and ii) the stoichiometry. The Si:B and SiGe:B growth rates increased with the B2H6 flow whatever the chemistry (Fig.3). This was due to a hydrogen desorption increase on boron surface sites [5,10-11]. Growth rates were otherwise higher for monocrystalline than for polycrystalline layers. The electrical resistivities of c-Si:B and c-SiGe:B layers were one decade lower than those of poly-Si:B and poly-SiGe:B layers, however (Fig.4). Resistivity values were otherwise higher for c-Si:B than for c-SiGe:B. This was likely due to i) a better incorporation of B atoms into SiGe than in Si [3,8,9] and ii) a better hole mobility in SiGe [10]. An opposite behavior was observed for polycrystalline layers, with higher resistivities for poly-SiGe:B than for poly-Si:B. We otherwise had similar resistivity evolutions as the partial pressure of diborane Pp(B2H6) increased for all types of layers. A decrease was observed first, followed by a resistivity plateau then some resistivity re-increase for really high diborane flows. Such a phenomenon was already observed for monocrystalline layers and was explained by a crystalline quality degradation [3,8,9] that reduced the hole mobility. A similar behavior was assumed for poly-layers, with a morphological transition to the amorphous phase [11,12]. Top-view Scanning Electron Microscopy (SEM) images (Fig. 5) confirmed the morphological evolution of c-Si:B, poly-Si:B, c-SiGe:B and poly-SiGe:B as the diborane partial pressure increased.

New data points for c-SiGe and c-SiGe:B will also be provided for other process conditions. For intrinsic layers, we will show that Refs. [1,2] x2/(1-x) = n×(F(GeH4)/F(SiH2Cl2)) relationship between the Ge concentration x and the germane over dichlorosilane mass-flow ratio is also valid at 10 Torr, with n = 3 at 700 °C with a F(SiH2Cl2)/F(H2) Mass-Flow Ratio (MFR) of 0.005 and a F(HCl)/F(H2) MFR of 0.006. Various characterization techniques were also used to determine the concentration of substitutional Boron atoms in SiGe:B layers and the lattice contraction coefficient (β = 9.11×10-24 cm3). The value of the latter, which is most useful to assess the compressive strain compensation by small boron atoms in SiGe layers, will be compared to literature values.

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Figure 1