During the manufacture of integrated circuits such as memory products substrates, especially semiconductor wafers, are processed in high-temperature ovens, called reactors, in or- der to deposit layers of isolating, semiconducting or conduc- ting material. These reactors can be suited for the proces- sing of a plurality of wafers at one time. The wafers are placed on a wafer support inside the reactor. The deposition reactor and, thus, the wafers are heated to a desired tempe- rature. Typically, reactant gases are passed over the heated wafer, causing the chemical vapor deposition of a thin layer of the reactant material on the wafer. Alternatively, reac- tant gases passing over the heated wafer will immediately re- act with the substrate material, as is the case in thermal oxidation.

Figure 1 shows an exemplary deposition reactor which is sui- ted for low pressure chemical vapour deposition processes. A large number of wafers (typically at least 100) is carried by a wafer carrier, for example a slotted quartz boat, so that the gas flowing direction, which is defined by the line con- necting gas inlet and gas outlet and which is parallel to the longitudinal axis of the reactor, is orthogonal to the wafer surfaces. Heating means are provided in order to heat the re- actor to a predetermined temperature. As soon as the prede- termined temperature is reached, the reactant gases are in- troduced into the deposition reactor in order to effect the deposition reaction. According to the prior art method, the temperature of the deposition reactor is maintained constant during deposition.

In order to deposit silicon dioxide, for example TEOS, (Si (OC2H5) 4) is reacted at a temperature of 700°C and a pres- sure of 40 Pa. Silicon nitride layers can be generated by re- acting SiH2Cl2 and NH3 at a temperature of 750°C and a pressu- re of 30 Pa.

As is generally known, the deposition rate depends on the de- position temperature and the pressure inside the deposition reactor. More specifically, a higher deposition temperature results in a higher deposition rate. Accordingly, usually a temperature gradient is applied in a direction parallel to the gas flowing direction in order to compensate for the de- pletion of the reactant gases in that direction. As a conse- quence, the temperature at the reactant gases outlet is hig- her than the temperature at the reactant gases inlet. By the- se measures, it is possible to deposit a homogenous layer thickness onto all wafers which are simultaneously processed.

However, it is not possible to achieve a sufficient in plane uniformity of the layer thickness. More specifically, the layers on the wafers close to the gas outlet tend to assume a bowl shape in which the layer thickness at the edge of the wafer is greater than the layer thickness in the middle of the wafer. Typically, the difference between layer thickness at the edge and layer thickness in the middle is approximate- ly 10 nm at a mean value of the layer thickness of 200 nm. On the other side, the layers on the wafer close to the gas in- let tend to assume a pillow shape in which the layer thick- ness at the edge of the wafer is smaller than the layer thickness in the middle of the wafer.

It is an object of the present invention to provide a heating system and a method for heating an atmospheric reactor by which the in plane uniformity of the deposited or oxidized layer thickness is improved.

According to the present invention, the above object is achieved by a heating system for heating an atmospheric reac- tor in which a plurality of wafers is held perpendicularly to the reactant gas flowing direction which is parallel to the longitudinal axis of the reactor, so as to enable a depositi- on process or an oxidation process, wherein said heating sy- stem is adapted to change the reactor temperature during the process.

Moreover, the above object is achieved by a method for hea- ting an atmospheric reactor in which a plurality of wafers is held perpendicularly to the reactant gas flowing direction which is parallel to the longitudinal axis of the reactor, so as to enable a deposition reaction or an oxidation reaction, wherein the reactor temperature is changed during the process.

As the inventors of the present invention found out, the in plane uniformity of the deposited layers can be largely im- proved by changing the reactor temperature during the deposi- tion process. Accordingly, the reactor temperature is no lon- ger held at a constant value but it is changed. For example, the temperature can be lowered, raised or changed in an arbi- trary manner. Examplary temperature profiles which are applied in all the zones of the reactor are illustrated in Figures 2 and 3. As is shown in Figure 2, the temperature is ramped down by 40 K, whereas in Figure 3, during deposition which starts at point A end ends at point B, the temperature is first ramped up by 60 K and then again ramped down by 60 K. The time is depicted in abitrary units (a. u.). It is to be noted that depostion and oxidation are exchangeable in this invention. A feature of the invention which is described for a deposition reaction can equally be used for an oxidation reaction.

According to a preferred embodiment, the deposition (or oxi- dation) reactor is divided into a plurality (usually five) of

zones along the reactant gas flowing direction. The heating system is divided into heating elements and each of the hea- ting elements is seperately controlled so as to provide a different temperature profile indicating the temperature of the specific temperature element versus time as is shown schematically in Figure 4. The number of heating elements can be the same as the number of zones. As can be seen from Figu- re 4, in zone 1, which is close to the gas outlet, the tempe- rature is ramped down from 790°C to 710°C, in zone 2 the tem- perature is ramped down from 770°C to 730°C, in zone 3 the temperature is maintained constant at 750°C, and in zone 4, which is close to the gas inlet, the temperature is ramped up from 720°C to 780°C.

Generally stated, the temperature is ramped down in the two thirds of the reactor which are closer to the gas outlet. It is preferred that the difference between deposition starting temperature and deposition end temperature is greater in a zone closer to the gas outlet than in a zone closer to the gas inlet. Moreover, the temperature is ramped up in the third of the reactor which is closest to the gas inlet. In the zone forming the boundary between these regions, the tem- perature is maintained constant during deposition. The tempe- rature profiles during a deposition process within each zone differ from each other. The temperature profile in at least one of the zones is varying over desposition time according to a redefined profile or rate. The temperature profile can be constant in at least one of the zones, and must be varying in at least another one of the zones over deposition time.

Preferably, the temperature profiles of two ones of the zones do not behave in parallel to each other.

By these measures, it is possible to adjust the optimum depo- sition temperature in accordance with the deposition conditi- ons which vary in dependence from the location of the speci- fic reactor zone. In particular, the reactant gases are de- pleted along the reactant gas flowing direction. Moreover, in

a zone closer to the gas outlet the reactant gases are as well depleted in a direction parallel to the wafer surface so that the reactant gases are most depleted in the middle of the wafers. In the zones close to the gas inlet, in particu- lar, in the zones located in the lower third of the reactor, this effect is less important since in these zones the effect of the depletion of the reactant gases along the reactant gas flowing direction is not so strong.

Another relevant parameter is the heat flow in a direction of the wafer surface. Generally, heat is supplied by means of a heating spiral or heating lamp which is situated at the reac- tor walls. Accordingly, a certain temperature of the zone of the deposition reactor refers to the temperature at the wafer edge. In addition, in most commonly used deposition reactors, at a position closest to the gas inlet, redundant heating elements are provided at a position where no wafer is placed.

Accordingly, in the zone closest to the gas inlet, the hea- ting is not only effected from the wafer edge but also from the middle of the wafer. Therefore, dependent from the speci- fic location of the zone, different heating conditions will prevail.

More specifically, in the zones which are not closest to the gas inlet, the temperature at the wafer edge is different from the temperature in the middle of the wafer. Accordingly, by lowering the temperature of the reactor, a uniform heating amount can be achieved along the wafer surface.

On the other hand, in the zone closest to the gas inlet, the heating is not only effected from the edge as explained abo- ve. As a consequence, by raising the temperature of the reac- tor during the deposition, a uniform heating amount can be achieved along the wafer surface.

The effects of the present invention can be further improved if the temperature profiles of the zones are properly set so

that the temperature profiles of neighbouring zones do not cross each other during the deposition process. Stated more concretely, the elevation of the temperature of one zone should be avoided, if at the same time the temperature of a neighbouring zone is lowered in order to minimize a detrimen- tal heat flow between the zones.

The detrimental heat flow can be best suppressed if the depo- sition process ends at the same temperature in all zones.

Since different deposition reactors require different heating conditions, a calibration can be performed, as soon as a new batch of wafers has been processed. To this end, after the end of the deposition, the wafers of each zone are evaluated, for example using an ellipsometer. Thereafter, on the basis of the measurement results obtained, the heating conditions for the zones of the reactor are set for the next deposition processes. If the deposited layer has assumed a bowl shape, the difference between deposition start temperature and depo- sition end temperature must be increased in that specific zo- ne. On the contrary, if the deposited layer assumes a pillow shape, the difference between deposition start temperature and deposition end temperature must be decreased in that spe- cific zone.

In order to deposit the same layer thickness onto the wafers in all the zones, it is preferred that the mean value of the temperature taken over time in each of the zones is decrea- sing from the zone closest to the gas outlet to the zone clo- sest to the gas inlet. For example, zone 1 assumes a mean temperature of 800°C, zone 2 assumes a mean temperature of 790°C, zone 3 assumes a mean temperature of 780°C, zone 4 as- sumes a mean temperature of 770°C, and zone 5 assumes a mean temperature of 760°C. This is preferred when the temperature is equally changed in all the zones, for example, lowered by a certain amount, raised by a certain amount or changed in an

arbitrary manner, or when the temperature profile of every zone is changed in a different manner.

In summary, the present invention provides the following advantages: - The in plane uniformity of the deposited layers is largely improved. In particular, a difference between the layer thickness at the edge and the layer thickness in the middle of the wafer amounts to 4 nm at most if the mean value of the layer thickness amounts to 200 nm.

- The present invention can easily be implemented to existing deposition reactors.

- As is generally known,. by raising the pressure inside the deposition reactor, the deposition rate can be raised. Howe- ver, a high pressure will also result in very inhomogenous layers. By additionally regulating the temperature in accor- dance with the present invention, the homogenity of the lay- ers will be improved. Accordingly, when the present invention is applied to a deposition process which is performed at an elevated pressure, the deposition rate is raised and, simul- taneously, the quality of the layers in terms of their homo- genity is maintained.

- The present invention can be applied to all low pressure chemical vapour deposition processes. It is particularly applicable to the deposition of silicon nitride, silicon di- oxide (TEOS process and thermal oxidation), arsene oxide (TEAS process) and polysilicon layers. The advantageous ef- fects of the present invention become especially apparent at a layer thickness of at least 30 nm. If the layer thickness is smaller, the advantages become less apparent.

The present invention will be explained in more detail with reference to the accompanying drawings in conjunction with

deposition, although the invention also includes oxidation processes.

Figure 1 illustrates a CVD reactor which can be used for im- plementing the present invention; Figures 2,3 and 4 show examplary temperature profiles applied to the deposition reactor ; and Figure 5 shows the measurement results representing the uni- formity of layers deposited in accordance with the examples and the comparative example.

In Figure 1, reference numeral 1 denotes a deposition reactor in which the low pressure chemical vapour deposition takes place and which is implemented as a batch furnace. Reference numeral 2 denotes gas inlet for feeding one or more reactant gases to the deposition reactor, and reference numeral 3 de- notes a gas outlet for exhausting the reactant gases. As is obvious, the reactant gas flowing direction is parallel to the longitudinal axis of the reactor. Reference numeral 4 de- notes a wafer carrier for carrying a plurality (usually bet- ween 100 and 150) of wafers, and reference numeral 5 denotes a heating system for heating the deposition reactor.

The reactor may be divided into 5 zones, zone 1 to zone 5, wherein zone 1 is the zone closest to the gas outlet, whereas zone 5 is the zone closest to the gas inlet. In Figure 1, re- ference numeral 6 denotes zone 1 and reference numeral 7 de- notes zone 5.

In the present examples, a pad nitride layer is to be deposi- ted onto silicon wafers. After that, trenches for defining storage capacitors for DRAM cells are to be etched into these wafers.

After introducing the wafers into the deposition reactor, the reactor is evacuated, and the temperature thereof is raised.

Usually, the reactor is held at a standby temperature of ap- proximately 650°C so that the temperature is to be increased by about 100 to 250°C depending on the chosen reaction condi- tions. As soon as a desired vacuum degree is reached, a first reactant gas is fed into the reactor. In the present case, NH3 at a flow rate of 480 sccm (standard cubic centimeters per second) is fed into the reactor. As soon as the desired deposition temperature is reached, a second reactant gas, which is SiH2Cl2 at a flow rate of 120 sccm, is fed into the reactor so that the deposition reaction will start. A typical pressure inside the deposition reactor amounts to 14,63 Pa (110 mTorr).

The temperatures at which the deposition starts and the tem- perature profiles during deposition are varied in accordance with the following examples and the comparative example. Sin- ce the temperature profiles are selected so that the mean temperature amounts to 800°C in zone 1,790°C in zone 2, 780°C in zone 3,770°C in zone 4, and 760°C in zone 5, the deposition rate amounts to 2 nm/min.

The layers are deposited at a mean value of the thickness of 200 nm within a time period of 100 minutes.

Example 1 The reactor is brought to a temperature of 820°C in zone 1, 810°C in zone 2,800°C in zone 3,790°C in zone 4, and 780°C in zone 5. During deposition, the reactor temperature is ram- ped down by 40 K in all the zones.

Example 2 The reactor is brought to a temperature of 840°C in zone 1, 830°C in zone 2,820°C in zone 3,810°C in zone 4, and 800°C

in zone 5. During deposition, the reactor temperature is ram- ped down by 80 K in all the zones.

Example 3 The reactor is brought to a temperature of 840°C in zone 1, 830°C in zone 2,820°C in zone 3, a temperature of 790°C in zone 4 and a temperature of 760°C in zone 5. During depositi- on, the temperature is ramped down by 80 K in zones 1 to 3, the temperature is ramped down by 40 K in zone 4, and it is held constant in zone 5.

Example 4 The reactor is brought to a temperature of 840°C in zone 1, 830°C in zone 2,810°C in zone 3,790°C in zone 4 and 750°C in zone 5. During deposition, the temperature is ramped down by 80 K in zones 1 and 2, the temperature is ramped down by 60 K in zone 3, the temperature is ramped down by 40 K in zo- ne 4, and the temperature is ramped up by 20 K in zone 5.

Example 5 The reactor is brought to a temperature of 840°C in zone 1, 830°C in zone 2,820°C in zone 3,785°C in zone 4 and 740°C in zone 5. During deposition, the temperature is ramped down by 80 K in zones 1 to 3, the temperature is ramped down by 30 K in zone 4, and the temperature is ramped up by 40 K in zone 5.

Example 6 The reactor is brought to a temperature of 841°C in zone 1, 832°C in zone 2,820°C in zone 3,790°C in zone 4 and 734°C in zone 5. During deposition, the temperature is ramped down by 82 K in zone 1, the temperature is ramped down by 84 K in zone 2, the temperature is ramped down by 80 K in zone 3, the

temperature is ramped down by 40 K in zone 4, and the tempe- rature is ramped up by 52 K in zone 5.

Comparative example The reactor is brought to a temperature of 800°C in zone 1, 790°C in zone 2,780°C in zone 3,770°C in zone 4, and 760°C in zone 5. During deposition, the temperature is held con- stant in all zones.

When the deposition reaction is finished, the flow of the re- actant gases is interrupted and the reactor is rinsed with an inert gas such as nitrogen.

Thereafter the quality of the deposited layers is evaluated as follows. The standard deviation from the mean value of the layer thickness based on 13 measurement points on the wafer surface is determined for each of the examples and the compa- rative example, and the results represented by uniformity sigma % are given in the following table: Example zone 1 zone 2 zone 3 zone 4 zone 5 [%] [%] [%] [%] [%] 1 1, 14 0,92 0, 77 0, 37 1,07 2 0, 45 0,61 0, 54 0, 41 1,78 3 0, 49-0, 59 0, 22 0, 17 0, 95 4 0, 64 0,74 0, 67 0, 31 0,85 5 0, 58 0,73 0, 47 0, 32 0, 69 6 0, 71 0,73 0, 57 0, 29 0,66 comparative 1, 78 1,38 1, 19 1, 03 0,69

The measurement results for examples 1,2,5 and the compara- tive example are illustrated in Figure 5.

As can be seen from the table, all of the examples provide a layer thickness having an improved in plane uniformity in zo- nes 1 to 4, whereas only examples 5 and 6 provide an improved uniformity in zone 5.

However, since in normally used deposition reactors the posi- tions of the wafer carrier of zone 5 closest to the gas inlet and the positions of the wafer carrier of zone 1 closest to the gas outlet are occupied by dummy wafers which are not used for chip production, the deteriorated in plane uniformi- ty in zone 5 is of minor relevance for the chip production.

In summary, the present invention provides improved results in examples 1 to 4 and excellent results in examples 5 and 6.

List of Reference Numerals 1 deposition reactor 2 gas inlet 3 gas outlet 4 wafer carrier 5 heating system 6 first zone 7 fifth zone