E L S E V I E R Thin Solid Films 276 (1996) 257-260

Formation of porous silicon on patterned substrates

M. Krtiger a, R. Arens-Fischer a, M. Thrnissen a, H. Miinder a, M.G. Berger a, H. Liith a, S. Hilbrich b, W. Theiss b

a lnstitutfiir Schicht- und lonentechnik (ISi), Forschungszentrum Jiilich GmbH, D-52425 Jiilich, Germany b L Physikalisches lnstitut, RWTH Aachen, B-52056 Aachen, Germany

Abstract

Application of porous silicon in device structures requires the formation of micron-size porous areas. Therefore, selective area anodization on photolithographically patterned p-doped substrates was investigated. As shown in this work, porosity and layer thickness vary from the edge to the middle of the structures. This inhomogeneity strongly depends on the doping level of the substrate aud the lateral size of the structure. When organic photoresists are used, an anisotropic undercutting of up to several 10 p,m occurs at the edge of the structures. This can largely be reduced by using thermally treated Si3N4 deposited by plasma-enhanced chemical vapour deposition as a masking layer. In this case an isotropic undercutting of the masking layer is observed permitting fabrication of porous silicon structures in the Ixm range by photolithography.

Keywords: Electrochemistry; Etching; Silicon

1. Introduction

Most experimental studies of porous silicon (PS) layers have been performed on layers with lateral sizes of several cm 2, which is given by the dimensions of the electrochemical cells. However, the use of porous silicon in optoelectronic devices either as an active (e.g. light emi~:er) or passive (e.g. waveguide) component or as a sacrificial layer for micro- machining requires the formation of micron-size porous areas on a silicon chip in combination with other silicon integrated circuits. The lateral dimensions required for the porous silicon structures range from several 100 p.m down to a few Ixm depending on the application. The formation of PS on the whole area of the chip and patterning of the porous layer after formation is incompatible with the requirements of other silicon circuits on the chip (e.g. therma!!y grown SiO2 layers). Therefore, we studied the selective anodization on patterned substrates.

In this case two other problems arise [ 1 ]. First, an under- cutdng of the masking layer occurs (up to several 10 p.m in some cases), which makes the formation of structures sepa- rated by less than this distance impossible. Second, inhom- ogeneous porosity and depth profiles of the layers are observed. The dependence of these effects on doping level, structure size and current density was investigated.

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2. Homogeneity of layer thickness and porosity

To study the influence of the selective area formation of PS on the layer properties a series of different masks were investigated. All the masks consist of parallel stripes with a length of 10 ram. The width of the stripes was varied between 2 000 and 10 p.m. The distance between the stripes was equal to the width of the stripes in all cases. The number of stripes was increased with decreasing width of the stripes to keep the total anodization area equal to 0.2 cm 2 in all cases. As masking material a commercially available photoresist (AZ 5214; 1.4 p.m) was used. After the photolithography process the photoresist was dried for 5 min at 130 *(2 to increase its resistance to the ethanoic HF solution. The anodization was performed using an electrolyte composition of H20:HF: ethanol 1"1:2. After formation of the porous layer the pho- toresist was removed with pure ethanol. The thickness of the porous layer was measured with a surface profiler after removal of the PS with NaOH.

Fig. 1 shows a cross-section of the depth profile for differ- ent doping levels of the substrate. For this a mask with four parallel stripes of 500 p.m width was chosen. In all cases the thickness of the porous layer increases from the centre to the border of the whole structure. In addition, each stripe itself shows the same behaviour. The same bending with even larger differences in depth is observed along the stripes. As

258 M. Kriiger et al. /Thin Solid Films 276 (1996) 257-260

0

! ov-7: !t I:

I

"~ -3 t

1,10~9cm.3

a) b)

~ l* lO:'cm "3

2,lO~Scm.~

-0.5 1"10 ~40m "~

-!.0

-1.5 ' 0 1000 2 0 0 0 3 0 0 0 4000 5000

x-position [pm]

Fig. 1. Dependence of the depth profile on doping level. The overall bending increases with decreasing doping level. The current density used for the highly doped substrates ( ! X 10:9 cm "3) was 113 mA cm-2 and 38 mAcm -2 for the other substrates.

measurements on medium-doped substrates ( 1 x 1017 c m - 3) with etching times from 1 to 10 min show, the bending is independent of etching time. With increasing doping level the inhomogeneity of the layer thickness decreases: while the depth varies from 1.2 to 4.5 I~m along the single stripes for low-doped substrates (2 × l0 Is cm-3) the inhomogeneity is almost absent for highly doped substrates (1 × 1019 cm-3). Because of the different doping levels different anodization conditions had to be chosen in order to obtain comparable porosities for all samples. However, the different bending of the interface is not related to the anodization current density. This was checked by the application of the anodization cur- rent density used for low-doped substrates (38 mA cm -2) on highly doped substrates, too. Both current densities resulted in the same bending.

The reason for the higher etch rate at the border of the structures is probably a higher local current density due to an inhomogeneous distribution of the electrical field in the sub- strate. First calculations of the field distribution assuming static conditions are in good agreement with the experimental results. The higher current density should then not only result in an increased etch rate but also cause a higher porosity at the border of the structures. In order to estimate the local porosity values IR reflectance measurements have been per- formed [2,3]. The spot size of the IR microscope was 30 I~m. A fit of the interference spectra, using the Looyenga effective medium theory and the measurement of the local layer thickness with the surface profiler, allows the extraction of a local porosity value. For substratcs with a doping con- centration of I x 10 t7 cm- 3 a porosity of 70% has been found at position (a) (see Fig. 1) and 68% at position (b). For low-doped substrates (2 x 10 ts cm-3) porosities of 72% and 67% are found at positions (a) and (b) respectively. These

values are in good agreement with a porosity of 68% expected from gravimetric measurements at large area samples [4].

If the inhomogeneous layer thickness is indeed an effect of the distribution of the current flow in the substrate one should be able to influence the depth profile by an additional current flow through the masking layer. For this purpose the masking layer has to be conductive and has to form a homo- geneous ohmic contact to the substrate. A Cr/Au layer system with thicknesses of 50 and 400 nm has been used on top of a low-doped substrate (2 × 1015 cm -3) (Fig. 2). To achieve a low resistive, homogeneous ohmic contact the sample has been annealed at 400 °C for 60 s. The metal masking layer was insulated from the electrolyte by a photoresist layer on top of the metal. During anodization, the current density through the interface substrate-metal layer was twice the anodization current density. Fig. 2 shows the depth profile which is nearly perfectly homogeneous in contrast to the depth profile of the same substrate shown in Fig. 1. With this active masking technique it should be possible not only to achieve homogeneous profiles in etching depth and porosity but also to invert the profile or to increase the bending by choosing suitable combinations of current density values.

In addition the dependence of the lateral homogeneity of the layer thickness on the structure size was investigated. While the bending of the whole structure remains the same for all stripe widths from 2 000 ~m down to 10 I~m, the bonding of the single stripes themselves vanishes for widths of 100 ~m or less. This is shown in Fig. 3, where a strong undercutting of the photoresist layer also becomes obvious. One might argue that this undercutting is the reason for the disappearance of the bending because part of the high current density at the edge is necessary to dissolve the silicon in the undercutting ar~a. This could bo ruled out by using a Si3N 4

electrolyte

,.., -0.5

lO

r ' - - - - I T ' - j 1000 2000 3000 4000

x-position [pm]

Fig. 2. Depth profile of the same structure as in Fig. 1, obtained by using a biased metal masking layer. The doping level is 2 × 10:5 cm-3. The upper part shows the configuration during anodization. The anodization current density was 38 mA cm -2.

M. Kriiger et al. / Thin Solid Fihns 276 (1996) 257-260 259

o K -1

"~ O

0 50 100 150 200

x-position [ttrn] Fig. 3. Dependence of the depth profile on structure size. While the overall bending of the profile remains the same when the structure size is reduced, the bending of the single stripe itself vanishes if its width is 100 itm or less. At the edge of the single stripe the strong undercutting of the photoresist mask becomes obvious.

masking layer which strongly reduces undercutting (see below) and which resulted in the same depth profile as in Fig. 3. The reason for the dependence on structure size has not become obvious yet and is still under investigation.

3. Undercutting

In Fig. 3 the main problem of PS formation on patterned substrates can be seen. The porous layer extends far below the etch mask. This strong undercutting, which we observe for organic photoresists as masking material, is anisotropic in nature and extends up to several 10 ~m below the mask for porous layers of only 3 ta,m thickness (Fig. 4). This imposes strong limitations upon the width and distance of porous structures formed by this technique.

The shape of the undercutting profile suggests that the photoresist is removed from the substrate during anodization. This would cause the anodization to start later in regions further away from the border of the mask and therefore result in a reduced layer thickness. The removal of the photoresist mask is not only caused by chemical dissolution of the resist by the ethanoic HF solution. This can be seen from the fact that storing the sample in the electrolyte for 1 h before begin- ning the anodization results in the same undercutting. Fur- thermore, the undercutting is stronger for higher anodization current densities. A possible explanation is that the resist is lifted by hydrogen emerging during anodization. The amount of the undercutting decreases slightly when thicker photores- ist (AZ4562; 6.2 ~m) is used and increases with thinner resist (AZ5206; 0.6 IJ, m). This can be explained by an increase of the mechanical strength of the photoresist with increasing thickness. Removal of the native oxide by HF and usage of an adhesion promoter before spinning on the resist does not result in a decrease of undercutting.

In order to overcome the problem of undercutting, poly- imide was used as an alternative masking layer. Its use for devices is limited by the high drying temperature of 450 °C

Fig. 4. Edges of porous silicon stripes fabricated using photoresist (a) and Si3N4 (b) masking layers.

which is not compatible with other device elements, e.g. gate structures. The width of undercutting is roughly the same as observed for the photoresist so that polyimide is not an useful alternative.

On the contrary, a strong reduction of undercutting is pos- sible by using Si3N4 masking layers [5 ]. We used plasma- enhanced chemical vapour deposition (PECVD) Si3N4 layers of 100 nm thickness which were deposited at 300 °C from silane and ammonia sources. The desired structures were transferred to the SiaN4 by photolithography and reac- tive-ion etching (RIE). The PECVD Si3N4 turned out to be rapidly dissolved in the hydrofluoric electrolyte, as opposed to the low-pressure CVD (LPCVD) SiaN4 reported in Ref. [ 5 ]. The reason for this is probably the high oxygen content of the SiaN4 built in during the PECVD process. Therefore, the Si3N4 was heated in an RTA oven for 10 rain which resulted in a strong decrease of the Si3N4 etch rate in the electrolyte used for PS formation. The etch rate depends strongly on annealing temperature and decreases from 60 nm min-i for 800 °C to 7 nm rain-~ for 1 100 °C. In this way anodization times longer than 10 min can be achieved before the masking layer is dissolved. For device application the high temperatures of 800-1100 °C mean that annealing of the Si3N 4 has to be the first preparation step. Then the other device

260 M, Kritger et al. / Thin Solid Films 276 (1996) 257-260

4. Conclusion

Fig ', fabricated using a Si3N4 masking layer. Anodization starts at the edges of the stripes because of the stre,ss in the Si substrate induced by the Si3N4.

In this work problems of porous silicon formation on pat- terned p-type substrates were investigated and some solutions were given. Inhomogeneities in etching depth and porosity strongly depend on doping level and structure size while they are independent of etching time and current density. These inhomogeneities can be prevented by using a new active masking technique. Undercutting of organic photoresists turned otit to limit structure size and distance of porous silicon areas. These limits can be reduced by using PECVD Si3N 4 masking layers which have to be annealed to increase their resistance to the electrolyte.

Acknowledgements

structures have to be fabricated and protected against the etching solution (e.g. by photoresist) during anodization.

As can be seen from Fig. 4, the undercutting becomes isotropic in the case of Si3N4 masking layers. The lateral extension of the undercutting is equal to the thickness of the PS layer. This is not the effect of a removal of the masking layer but is caused by the direction of the current flow in the substrate.

Isotropic undercutting allows the formation of structures in the p,m range (Fig. 5), which is not possible when pho-

toresist is used. Nevertheless this current-induced isotropic undercutting is a fundamental limitation that prevents fabri- cation of small structures with rectangular cross-sections, e.g. for waveguide applications. Moreover, it is not possible to obtain structures with widths smaller than twice the layer thickness. Another problem which is specific for Si3N4 mask- ing layers is the mechanical stress induced in the crystalline silicon [5,6] which causes the anodization to proceed faster near the border of the mask (Fig. 5).

We would like to thank H.-P. Bochem for the SEM pictures and H.D. Drescber for the deposition of the Si3N 4 layers. This work was supported by the Bundesministerium fiir Bildung und Forschung under contract number 01 BM 401/3.

References

[ 1 ] P. Steiner and W. Lang, Thin Solid Films, 255 (1995) 52-58. [2] W. Theiss, IR spectroscopy of porous silicon, in J.-C. Vial and J. Derrien

(eds.), Porous Silicon Science And Technology, Springer Verlag, Berlin, 1995.

[3] P. Grosse, L. Ktipper and W. Theiss, SPIE Vol., 1575 ( 1991 ) 269. [4] M.G. Berger, St. Frohnhoff, W. Theiss, U. Rossow and H. MUnder,

Porous silicon: from single porous layers to porosity superlattices, in J.- C. Vial and J. Derrien (eds.), Porous Silicon Science And Technology, Springer Verlag, Berlin, 1995.

[5] A.G. Nassiopoulos, S. Grigoropoulos, L. Canham, A. Halimaoui, l. Berbezier, E. Gogolides and D. Papadimitriou, Thin Solid Films, 255 (1995) 329-333.

[6] I. De Wolf, H. Norstr6m and H.E. Maes, J. Appl. Phys., 74(7) (1993) 4490-4500.