The condition to be satisfied for satisfactory heat removal from electronic structures is that the number of phase boundaries (interfaces) present in the ceramic-metal joint should be limited. The number of continuous interfaces can be reduced by using the technique of direct bonding of the ceramic to copper. This process, known as the Copper Direct Bonding (CDB), has well been known in joining alumina ceramics. Compared to alumina, AlN-Cu substrates have however much more advantageous properties (e.g. better heat conductivity) and are more often used for the fabrication of heat sinks of electronic devices. This paper describes studies on the surface modification of the aluminium nitride ceramic intended for joining with copper by the CDB technique. Prior to the joining, the AlN surface was modified by isothermal oxidation or by oxygen ion implantation. The effects of the oxidation process temperature, within the range from 673K (400°C) to 1373K (1100°C), and of the doses and energies of the implanted ions upon the microstructure of the ceramic material were analyzed. In the oxidized samples, the increase of the sample mass was determined using the differential thermal analysis (DTA). In the implanted samples, the depth profiles of the elements were examined by the RBS technique. The direct bonding of aluminium nitride with copper was conducted at a temperature of about 1353K (1080°C) in a nitrogen atmosphere. After the bonding process, the microstructure, phase changes and shear strength of the joints were determined.