DE10317208A1 - Plasma deposition process - Google Patents

Abstract

A method for depositing a layer on a wafer by means of plasma-induced gas-phase reactions is proposed, in which the layer is deposited by high-frequency excitation of the plasma by means of high-frequency power. The high-frequency power is modulated at low frequency, in particular clocked. The proposed method opens up a continuous change in the excitation between high-frequency character and low-frequency character, as a result of which layer stress minimization can be carried out without changing the carrier frequency and without a matching network (6) connected between a high-frequency generator (7) and an upper showerhead electrode (2) ) have to change, and allows higher process pressures without additional effort.

Description

The The invention relates to a method for depositing a layer a wafer by plasma-induced gas phase reactions, in particular one in which the layer is excited by high frequency excitation of the Plasmas is deposited by means of a high-frequency power.

State of technology

Plasma deposition processes usually use reactive plasmas made of silanes, German or other gases either alone or in a mixture with one another (e.g. SiH ₄ and GeH ₄ for SiGe deposition) or in conjunction with reactants such as N ₂ O, NH ₃ , CH ₄ , C ₂ H ₅ , ... etc. or in conjunction with doping gases such as diborane or phosphine ... etc. in order to deposit layers on wafers by means of plasma-induced gas phase reactions (α-Si, α-Ge, α-SiGe, poly-Ge, poly -SiGe, SiC, SiN, SiO, ...). The wafers are usually placed on a grounded and heated to a certain temperature, eg T = 300 ... 600 ° C substrate electrode. The process gases are usually introduced into the plasma chamber via an upper showerhead electrode, which is arranged parallel to the mostly grounded substrate electrode and which is fed with a high-frequency and / or low-frequency power. There is also the case of feeding the substrate electrode or the case of feeding both electrodes in parallel, ie showerhead and substrate electrodes in a so-called triode arrangement.

For the properties the deposited layers, especially their intrinsic stress properties, the frequency of the plasma excitation plays a crucial role. So it is e.g. possible, by means of high frequency excitation e.g. 13.56 MHz SiO layers with to remove compressive intrinsic stress. Using low frequency excitation from e.g. 380 kHz or e.g. 100 kHz it is possible to use these SiO layers with tensile Eliminate stress. Frequently low frequency and high frequency plasma excitation is combined, to separate stress-free or at least stress-free layers. Here, by choosing and optimizing the strengths of the high-frequency and low-frequency portions of the plasma excitation minimizes stress reached. This can e.g. done that over elaborate Matching networks both the high frequency as well as the Low frequency applied to the powered electrode (s) and adjusted can be. In this case, the strengths of the low frequency and high-frequency plasma excitation components simply by the set Performance of the individual components determined. Another alternative looks before, the plasma alternating with high frequency alternating or to operate with low frequency. For a certain period of time high-frequency excitation is selected, low-frequency excitation for a subsequent period etc. The weights of the two alternating parts can be both over their Time spans as well the chosen one Performance of each component can be controlled. Both for the high frequency generator for as well the low frequency generator is a separate matching network to the power Electrode needed which ensures the impedance matching in each case. With alternating However, the two networks do not need HF or LF operation decouple both generators from each other since they are not simultaneously are operated and therefore do not interfere with each other. In this respect, the adaptation networks are then less complex than in in the event that both generators are to be operated simultaneously.

The High frequency and low frequency generator requirements and the associated matching networks for high and low frequency in any case represents a considerable effort In addition, low-frequency plasmas are generally unstable, ignite badly and can also be poorly adapted. This statement therefore applies also for Processes that are stronger are determined by low frequency as well as high frequency excitation, i.e. in which the properties of high frequency excitation versus those the low frequency excitation due to the selected weights in the background to step. The problems stir known dimensions hence that with low frequency excitation relatively low ion densities and currents generated and therefore high voltages are built up on the powered electrode Need to become, which in turn is great Quality requirements of the adaptation networks used. Electric loss paths occur on, so those for the ignition and the operation of the plasma no longer requires voltages reached, causing instability to the point of extinction of the plasma occur. The adjustment settings depend on critical of the Frequency of low frequency excitation; e.g. they change also through that inevitably occurring growth of dielectric, i.e. electrically more or less insulating, layers on the powered electrode while of the deposition process sometimes very strong, which in the process another uncertainty in the form of possible temporal drift phenomena occurs.

Low-frequency plasmas are often combined with ignition aids, such as a high-power switch-on pulse, to ignite the plasma and only afterwards to the actual desired power value cut back. This also leads to great effort.

On Another physical disadvantage of low-frequency plasmas is the fact that the energy of the ions generated is not sharp is defined, but rather a wide range of energy from 0 to a peak value, which depends on the selected power. This broad energy distribution of the ions makes process optimization difficult, especially a model-based one Optimization. One would be desirable preferably sharply defined energy distribution of the ions generated in the plasma, as is the case with high-frequency excitation e.g. at 13.56 MHz or even better at 27.12 MHz or even higher Multiples of 13.56 MHz, e.g. 40.68 MHz, the case is.

Advantages of invention

The inventive method for depositing a layer on a wafer by plasma-induced Gas phase reactions in which the layer by high frequency excitation the plasma is deposited by means of a high-frequency power, is characterized in that the high frequency power is low frequency is modulated.

The overcomes the inventive method the disadvantages of the prior art described above. Instead of a combined low frequency and high frequency excitation of the plasma, a single high frequency generator of e.g. 13.56 MHz and a single matching network for e.g. 13.56 MHz on the powered Electrode inserted. about the low frequency modulation can make the character of the excitation continuous be changed between high-frequency and low-frequency situations, making everyone desired Intermediate state of the excitation character set and thus e.g. shift stress minimization can be performed without the carrier frequency to change and change without the fitting network to have to. Since the plasma power is on a desired average over time Power value can be kept constant changes through the variation nothing between high-frequency and low-frequency situations the chemistry of the plasma as for the decomposition of the process gases above all the average performance value is crucial in plasma. The variation according to the invention between high-frequency and low-frequency situation only affects those with Ion exposure controlled physical effects on the wafer surface, this but clearly.

As particular advantage of the method according to the invention is to be emphasized, that the ion energy distribution even with the low frequency character the excitation due to the high carrier frequency remains, provided the high-frequency power has sufficiently fast clock edges having. This is e.g. process optimization towards minimal Shift stress relieved.

at the inventive method the high-frequency power is preferably at a frequency greater than 1 kHz, in particular greater than 10 kHz, furthermore in particular greater than 10 kHz and less than 1 MHz, even more particularly greater than 10 kHz and less than 500 kHz, even more especially 50 kHz and less than 200 kHz, even more particularly of 100 kHz low frequency modulated.

Also is in the inventive method preferably a pulse to period ratio, i.e. the duty cycle d, the modulation of the high-frequency power freely selectable, in particular from a range between 1% and 100%, further particularly from a range between 5% and 100%, still further in particular a range between 10% and 90%.

For d = 1 then there is a high-frequency character in which the ion energies are relatively low and the ion currents and densities in the plasma are relative are high. For small values of d, e.g. d = 0.2 or d = 0.1 or d = 0.05 a low frequency character in which the ion energies are relative are high and the average ion currents and densities are relatively low are. The setting of a desired one Intermediate state for implementation shift stress minimization takes place e.g. with medium d values. The duty cycle parameter d also influences the previously specified Ion exposure correlated physical effects clearly on the wafer surface, without changing the chemistry of the plasma because the plasma power constant over time at a desired performance value is held. Even if through the duty cycle parameter d low-frequency character is set, nothing on the sharp definition of the excitation of the ion energy distribution changed.

To the inventive method the high-frequency power preferably has a frequency of 1 up to 50 MHz, in particular 13.56 MHz or 27.12 MHz. The high frequency power preferably according to the invention a peak pulse power of up to 1000 watts, more preferably up to 2000 watts.

Preferably the high-frequency power according to the invention by means of a pulse signal modulated low-frequency, which further preferably approximately rectangular Has clock edges that have a rise time below 1 μs preferably between 0.1 μs and 0.3 μs exhibit. According to the invention, the low-frequency modulation preferably brings about periodic switching off and switching on the high-frequency power.

According to the invention the wafer is preferably on a grounded substrate electrode Device for depositing a layer on a wafer and process gases are about an upper showerhead electrode arranged parallel to the substrate electrode into a plasma chamber of the device for depositing a layer introduced on a wafer. According to the invention, the high-frequency power is preferably provided by a high-frequency generator generated and over a matching network to a powered electrode, preferably the upper showerhead electrode, a device for separating a Layer coupled onto a wafer.

following becomes the excitation mechanism according to the invention described. Let P be the mean coupled plasma power, the for one certain process should be kept constant, d the freely selected pulse to period ratio, i.e. the duty cycle parameter, p the peak pulse power resulting from P and d, u the voltage at the showerhead electrode corresponding to the energy the ions generated in the plasma, i the pulsed ion current and I the time average of the ion current. The pulse rate must be sufficient be high to allow the ion current to be averaged over time, i.e. the physical effects of the current pulses on a surface, e.g. on the wafer surface, have to correspond to the effect of a constant current flow over time. This is for a carrier frequency of 13.56 MHz at frequencies above 1 kHz, with certainty at frequencies above 10 kHz. Are preferred in the method according to the invention Pulse repetition rates of 100 kHz are used.

Assuming that the plasma impedance X changes only slightly with the plasma power, i.e. that Ohm's law approximately applies, the following applies:

und es erniedrigt sich der mittlere Strom I ∝ √d. Bei ein- und demselben Leistungseintrag kann also über den Duty Cycle-Parameter d gewählt werden, ob eine hohe Ionenenergie mit entsprechend geringem mittleren Ionenstrom oder eine niedrige Ionenenergie mit entsprechend hohem mittleren Ionenstrom gewünscht wird. Die zeitliche Mittelung, wie sie für den Ionenstrom für Pulsfrequenzen von > 10 kHz zulässig ist und die für solche Pulsfrequenzen die physikalische Realität auf den betroffenen Oberflächen korrekt wiedergibt, ist für die Ionenenergie ohne Bedeutung, da die physikalischen Effekte der Ioneneinwirkung nicht mit dem zeitlichen Mittelwert, sondern mit der Energie jedes eintreffenden Ions korrelieren. Eine Pause ohne Ioneneinwirkung bzw. einer Einwirkung von Ionen von sehr geringer Energie ist ergebnisneutral, entscheidend ist vielmehr die Ionenenergie in den Prozessphasen, in denen tatsächlich eine Einwirkung von Ionen hoher Energie stattfindet, d.h. während der Pulsphasen. Mit dem Duty Cycle-Parameter d ist also ein Freiheitsgrad geschaffen, der in seiner Wirkung einer Einstellbarkeit einer zu einer "Plasmaimpedanz" analogen Größe entspricht, die sich aus der Relation der Ionenenergie zum mittleren Ionenstrom ergibt. Der Duty Cycle-Parameter d variiert also nicht wirklich die Plasmaimpedanz, sondern eine zu einer "Impedanz" ähnliche Relation "Spitzenspannung bzw. Ionenenergie/mittlerer Strom". Die tatsächliche Plasmaimpedanz X wird wesentlich bestimmt von der Trägerfrequenz, z.B. 13,56 MHz, und bleibt bei Variationen des Duty Cycle-Parameters d, wie vorstehend angenommen, näherungsweise fest, da die Trägerfrequenz als die vorwiegend die Impedanz bestimmende Größe fest bleibt.If the pulse to period ratio d is reduced and the peak pulse power p is scaled accordingly at a constant mean power P, the energy u of the ions generated in the plasma increases

and the average current I ∝ √ decreases d , With one and the same power input, the duty cycle parameter d can be used to select whether a high ion energy with a correspondingly low average ion current or a low ion energy with a correspondingly high average ion current is desired. The temporal averaging, as is permissible for the ion current for pulse frequencies of> 10 kHz and which correctly reflects the physical reality on the affected surfaces for such pulse frequencies, is irrelevant for the ion energy, since the physical effects of the ion influence do not match the temporal average but correlate with the energy of each incoming ion. A pause without ionic action or an action of ions of very low energy is neutral in terms of results, rather the decisive factor is the ionic energy in the process phases in which an action of ions of high energy actually takes place, ie during the pulse phases. The duty cycle parameter d thus creates a degree of freedom which, in its effect, corresponds to an adjustability of a quantity analogous to a "plasma impedance", which results from the relation of the ion energy to the average ion current. The duty cycle parameter d therefore does not really vary the plasma impedance, but rather a relation "peak voltage or ion energy / average current" that is similar to an "impedance". The actual plasma impedance X is essentially determined by the carrier frequency, for example 13.56 MHz, and remains approximately fixed in the case of variations in the duty cycle parameter d, as assumed above, since the carrier frequency remains fixed as the variable which predominantly determines the impedance.

In order to the character of the plasma excitation can be determined via the duty cycle parameter d freely choose between "high frequency" and "low frequency", without other parameters like e.g. the adaptation by the adaptation network can be changed. In particular, plasma excitation occurs even when the frequency is low no ignition or stability problems insulation layers deposited in the plasma, on the powered electrode are thanks to the high carrier frequency not or only slightly disturbing and the generated ion energy is also for the low frequency case sharply defined. about small values of the duty cycle parameter According to the invention, d becomes the Case of low frequency excitation, taking advantage of Low frequency excitation can be obtained without the disadvantages in To have to buy.

Another advantage of pulse operation with a particularly low duty cycle parameter d and ho The peak power p in the pulse peaks is the fact of improved plasma stability, especially at high process pressures, especially with regard to an undesired occurrence of current channels or sparks between the electrodes, so-called "arcing". For many processes, e.g. deposition of poly-SiGe (silicon germanium poly crystalline) using a process gas mixture of silane SiH ₄ and German GeH _9, possibly in conjunction with doping gases such as B ₂ H ₆ or PH ₃ , e.g. at a wafer temperature of 500 ° C to 600 ° C., in particular 550 ° C., a high pressure of, for example, 10 to 500 mbar, in particular approximately 100 mbar, is advantageous for a high deposition rate, as a "near-atmospheric" plasma. Unfortunately, due to the "Paschen curve", such high-pressure plasmas tend to spark at the narrow electrode spacings required, which makes plasma support for SiGe deposition considerably more difficult or even impossible. Although the deposition at 550 ° C under atmospheric conditions is also purely thermally induced and does not necessarily require plasma support, plasma support has great advantages in controlling and improving the deposition rate and the layer quality of the deposited poly-SiGe layers. The pulsation of the plasma, in particular with a low duty cycle d and high peak pulse powers p, effectively prevents arcing and thereby significantly increases the permissible working pressure of a plasma, which in turn has advantages in terms of achievable deposition rates. This expansion of the working pressure range upwards into the vicinity of atmospheric conditions is achieved without additional measures that go beyond the pulsing already described, and therefore does not constitute any additional effort. Without effort, another advantage in the form of higher possible process pressures can thus be achieved from the technology according to the invention.

drawing

Further Advantages of the invention result from the following description of the drawings. In the drawing is an embodiment the invention is exemplified. The drawing, the description and the claims contain numerous features in combination or as a preferred embodiment referred to the invention. Those skilled in the art will find the features convenient also consider individually and / or for a meaningful further combination sum up.

It shows:

1 a schematic diagram of the plasma deposition method according to the invention.

description of the embodiment

1 shows in the upper part a basically known plasma deposition system, in which SiH ₄ , GeH ₄ , SiH ₄ + GeH ₄ , SiH ₄ + GeH ₄ + B ₂ H ₆ , SiH ₄ + GeH ₄ + PH ₃ , ..., SiH ₄ + N ₂ O, SiH ₄ + NH ₃ , SiH ₄ + CH ₄ , Si (OC ₂ H ₅ ) ₄ + ..., Si (C ₂ H ₅ ) ₄ + ..., etc., to one upper showerhead electrode 2 fed and via this into a plasma chamber 3 be introduced. The top showerhead electrode 2 is parallel to a grounded substrate electrode 4 arranged on which a wafer 5 rests.

The high-frequency power generated according to the invention is via a customary matching network 6 eg of the "L-type" for eg 13.56 MHz from an RF generator 7 , which generates a high-frequency carrier signal of 13.56 MHz, for example, to the upper showerhead electrode 2 created. The RF generator 7 allows a modulation of its high-frequency carrier signal of, for example, a 13.56 MHz carrier frequency with approximately rectangular clock edges. A low-frequency LF pulse generator is used to modulate the high-frequency carrier signal 8th , here with a freely selectable frequency of, for example, 1 kHz to 1 MHz, in particular 100 kHz, and an adjustable duty cycle of, for example, 5% to 100%. The high frequency generator 7 and the low-frequency pulse generator 8th can also be integrated in one device. The modulation of the high-frequency carrier can be a periodic switching on and off or clocking of the high-frequency generator 7 thanks to the low-frequency pulse generator 8th to be viewed as.

The rise time of the clock edges should be significantly less than 1 μs. Commercially available generators can achieve values from 0.1 to 0.3 μs for a carrier frequency of 13.56 MHz. According to the invention, the pulse repetition rate should be between 1 kHz and 500 kHz, in particular between 50 kHz and 200 kHz, preferably at 100 kHz. With the low-frequency pulse generator according to the invention 8th the pulse-to-period ratio can be set over a wide range, for example between 1% and 100% or, as in 1 shown, between 5% and 100%, or at least between 10% and 90%. The maximum pulse peak power of the high frequency generator 7 should range up to 1000 watts, preferably up to 2000 watts, and should also be freely selectable up to this value.

The middle and lower part of 1 shows a pulse train as generated according to the invention and to the powered showerhead electrode 2 the plasma deposition system 1 can be created. Pulses of the high-frequency carrier signal of eg 13.56 MHz are generated with a repetition rate of eg 100 kHz and a duty cycle d = 0.05 to 1 and a constant average power that corresponds to the plasma power required in the process. In the example given, the average power is 80 watts. If the duty cycle parameter d is selected to be 10%, the required peak pulse power is 800 watts. The RF generator 7 generates a high-frequency carrier signal 9 with a frequency of, for example, 13.56 MHz and a voltage amplitude that corresponds to a power of, for example, 800 watts, as described in 1 is shown below. The clocking of the carrier signal of the high-frequency generator 7 thanks to the low-frequency pulse generator 8th takes place with 100 kHz and a pulse to period ratio of 10%, whereby the carrier signal for the low frequency modulation is periodically switched on and off or clocked, as in 1 shown in the middle.

Claims (13)

Method for depositing a layer on a wafer by plasma-induced gas phase reactions, in which the layer is deposited by high-frequency excitation of the plasma by means of high-frequency power, characterized in that the high-frequency power is modulated at low frequency. A method according to claim 1, characterized in that the high-frequency power with a frequency greater than 1 kHz, in particular greater than 10 kHz, furthermore in particular greater than 10 kHz and less than 1 MHz, even more particularly greater than 10 kHz and less than 500 kHz, even more especially 50 kHz and less than 200 kHz, even more particularly of 100 kHz low frequency is modulated. A method according to claim 1 or 2, characterized in that that a pulse-to-period ratio (d) the modulation of the radio frequency power freely, in particular from a range between 1% and 100%, further particularly from one Range between 5% and 100%, even more particularly from one Range between 10% and 90 selected can be. Method according to one of the preceding claims, characterized characterized in that the high frequency power is a frequency of 1 to 50 MHz, in particular 13.56 MHz or 27.12 MHz or 40.68 MHz having. Method according to one of the preceding claims, characterized characterized in that the high frequency power is a peak pulse power up to 1000 watts, preferably up to 2000 watts. Method according to one of the preceding claims, characterized characterized in that the high-frequency power by means of a pulse signal is modulated at low frequency. A method according to claim 6, characterized in that the pulse signal is approximately has rectangular clock edges that have a rise time of less than 1 μs, preferably between 0.1 μs and 0.3 μs exhibit. Method according to one of the preceding claims, characterized characterized in that the low-frequency modulation is a periodic Switching off and switching on the high-frequency power causes. Method according to one of the preceding claims, characterized characterized in that the wafer is on a grounded substrate electrode a device for depositing a layer on a wafer and process gases over an upper showerhead electrode arranged parallel to the substrate electrode into a plasma chamber of the device for depositing a layer introduced on a wafer become. Method according to one of the preceding claims, characterized characterized in that the wafer lies on an RF-fed substrate electrode, which faces a grounded showerhead electrode as a gas inlet. Method according to one of the preceding claims, characterized characterized in that the wafer lies on an RF-fed substrate electrode, the one also HF-fed Showerhead electrode faces as a gas inlet. Method according to one of the preceding claims, characterized in that the high-frequency power is generated by a high-frequency generator and via a matching network to a powered electrode, preferably the upper showerhead electrode, of a device for depositing a layer is coupled onto a wafer. Method according to one of the preceding claims, characterized in that the wafer temperature is 500 ° C to 600 ° C, in particular 520 ° C - 560 ° C, a gas mixture of at least SiH ₄ and GeH _{4 is} supplied and the process pressure at 10 to 500 mbar , in particular 10 to 100 mbar.